Science Primers, Introductory - free ebook by Thomas Henry Huxley

Science Primers. INTRODUCTORY.

Entered according to the Act of the Parliament of Canada, in the year One Thousand Eight Hundred and Eighty-one by Macmillan &amp; Co., London , in the Office of the Minister of Agriculture ....

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1. Sensations and Things.

All the time that we are awake we are learning by means of our senses something about the world in which we live and of which we form a part; we are constantly aware of feeling, or hearing, or smelling, and, unless we happen to be in the dark, of seeing; at intervals we taste. We call the information thus obtained sensation . When we have any of these sensations we commonly say that we feel, or hear, or smell, or see, or taste, something. A certain scent makes us say we smell onions; a certain f

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2. Causes and Effects.

Moreover, we say of all these things, or objects, that they are the causes of the sensations in question, and that the sensations are the effects of these causes. For example, if we hear a certain sound, we say it is caused by a carriage going along the road, or that it is the effect, or the consequence, of a carriage passing along. If there is a strong smell of burning, we believe it to be the effect of something on fire, and look about anxiously for the cause of the smell. If we see a tree, we

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3. The reason Why. Explanation.

In the case of the smell of burning, when we find on looking about, that something actually is on fire, we say indifferently either that we have found out the cause of the smell, or that we know the reason why we perceive that smell; or that we have explained it. So that to know the reason why of anything, or to explain it, is to know the cause of it. But that which is the cause of one thing is the effect of another. Thus, suppose we find some smouldering straw to be the cause of the smell of bu

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4. Properties and Powers.

When a thing is found always to cause a particular effect, we call that effect sometimes a property , sometimes a power of the thing. Thus the odor of onions is said to be a property of onions, because onions always cause that particular sensation of smell to arise, when they are brought near the nose; lead is said to have the property of heaviness, because it always causes us to have the feeling of weight when we handle it; a stream is said to have the power to turn a water-wheel, because it ca

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5. Artificial and Natural Objects. Nature.

A great many of the things brought to our knowledge by our senses, such as houses and furniture, carriages and machines, are termed artificial things or objects , because they have been shaped by the art of man; indeed, they are generally said to be made by man. But a far greater number of things owe nothing to the hand of man, and would be just what they are if mankind did not exist,—such as the sky and the clouds; the sun, moon and stars; the sea with its rocks and shingly or sandy shores; the

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6. Artificial Things are only Natural Things shaped and brought together or separated by Men.

Although this distinction between nature and art , between natural and artificial things, is very easily made and very convenient, it is needful to remember that, in the long run, we owe everything to nature; that even those artificial objects which we commonly say are made by men, are only natural objects shaped and moved by men; and that, in the sense of creating , that is to say, of causing something to exist which did not exist in some other shape before, man can make nothing whatever. Moreo

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7. Many Objects and Chains of Causes and Effects in Nature are out of our reach.

Among natural objects, as we have seen, there are some that we can get hold of and turn to account. But all the greatest things in nature and the links of cause and effect which connect them, are utterly beyond our reach. The sun rises and sets; the moon and the stars move through the sky; fine weather and storms, cold and heat, alternate. The sea changes from violent disturbance to glassy calm, as the winds sweep over it with varying strength or die away; innumerable plants and animals come in

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8. The Order of Nature: nothing happens by Accident, and there is no such thing as Chance.

But the first thing that men learned, as soon as they began to study nature carefully, was that some events take place in regular order and that some causes always give rise to the same effects. The sun always rises on one side and sets on the other side of the sky; the changes of the moon follow one another in the same order and with similar intervals; some stars never sink below the horizon of the place in which we live; the seasons are more or less regular; water always flows down-hill; fire

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9. Laws of Nature; Laws are not Causes.

When we have made out by careful and repeated observation that something is always the cause of a certain effect, or that certain events always take place in the same order, we speak of the truth thus discovered as a law of Nature . Thus it is a law of nature that anything heavy falls to the ground if it is unsupported; it is a law of nature that, under ordinary conditions, lead is soft and heavy, while flint is hard and brittle; because experience shows us that heavy things always do fall if th

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10. Knowledge of Nature is the Guide of Practical Conduct.

If nothing happens by chance, but everything in nature follows a definite order, and if the laws of nature embody that which we have been able to learn about the order of nature in accurate language, then it becomes very important for us to know as many as we can of these laws of nature, in order that we may guide our conduct by them. Any man who should attempt to live in a country without reference to the laws of that country would very soon find himself in trouble; and if he were fined, impris

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11. Science: the Knowledge of the Laws of Nature obtained by Observation, Experiment, and Reasoning.

No line can be drawn between common knowledge of things and scientific knowledge; nor between common reasoning and scientific reasoning. In strictness all accurate knowledge is Science ; and all exact reasoning is scientific reasoning. The method of observation and experiment by which such great results are obtained in science, is identically the same as that which is employed by every one, every day of his life, but refined and rendered precise. If a child acquires a new toy, he observes its ch

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12. The Natural Object Water.

One of the commonest of common natural objects is water ; everybody uses it in one way or another every day; and consequently everybody possesses a store of loose information—of common knowledge—about it. But, in all probability, a great deal of this knowledge has never been attended to by its possessor; and certainly, those who have never tried to learn how much may be known about water, will be ignorant of a great many of its powers and properties and of the laws of nature which it illustrates

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13. A Tumbler of Water.

Suppose we have a tumbler half-full of water. The tumbler is an artificial object (§ 5); that is to say, certain natural objects have been brought together and heated till they melted into glass, and this glass has been shaped by a workman. The water, on the other hand, is a natural object, which has come from some river, pond, or spring; or it may be from a water-butt into which the rain which has fallen on the roof of a house has flowed. Now the water has a vast number of peculiarities. For ex

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14. Water occupies Space; it offers Resistance; it has Weight; and is able to transfer Motion which it has acquired; it is therefore a form of Matter.

The water, we see, fills the cavity of the tumbler for half its height, therefore it occupies that much space , or has that bulk or volume . If you put the closed end of another tumbler of almost the same size into the first, you will find that when it reaches the water, the latter offers a resistance to its going down, and unless some of the water can get out, the end of the second tumbler will not go in. Any one who falls from a height into water will find that he receives a severe shock when

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15. Water is a liquid.

You will easily observe that, though water occupies space, it has no definite shape, but fits itself exactly to the figure of the vessel which holds it. If the tumbler is cylindrical, the contour of the surface of the water will be circular when the tumbler is held vertically, and will change, without the least break or interruption, to more and more of an oval when the tumbler is inclined, and whatever the shape of the vessel into which you pour it, the sides of the water always exactly fit aga

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16. Water is almost incompressible.

It has been seen that water, like every other material substance, resists the intrusion of other matter into the place which it occupies. But many things, though they resist, can be easily squeezed or compressed into a smaller volume. This, however, is not the case with water, which like other liquids, is almost incompressible ; that is to say, an immense pressure is needful to cause its volume to diminish to any appreciable extent. It may seem strange that anything so apparently yielding as wat

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17. The Meaning of Weight.

Let us next consider the property of weight. We say that anything has weight when, on trying to lift it from the ground, or on holding it in the hand, we have a feeling of effort. Or again, if anything which is supported at a certain height above the ground, falls when the support is taken away, we say that it has weight. Now the ground merely means the surface of the earth; and, as all bodies which possess weight fall directly towards the surface of the earth when they are not kept away from it

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18. Gravity and Gravitation.

The word gravity , when it was first used, had exactly the same meaning as weight; and a body which has weight is said to gravitate towards the center of the earth. But gravity has now acquired a much wider sense than weight. For an immense number of careful observations and experiments have established the general rule, or law of nature, that every material substance, tends to approach every other material substance, just in the same way as a drop of rain falls towards the earth; and, in fact,

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19. The cause of Weight: Attraction: Force.

We know nothing whatever of the reason why bodies possess weight. Bodies do not fall on account of the law of gravitation (§ 9); nor does their gravity explain why they fall. Gravity, as we have seen, is only a name for weight, and the law of gravitation is only a statement of how bodies approach one another, not why they do so. It is often said that gravitation is attraction , and that bodies fall to the earth because the earth attracts them. But the word “attract” simply means to “draw towards

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20. The Weight of Water is Proportioned to its Bulk.

We must next consider, not weight in general, but the weight of water. We say that a tumbler full of water is heavier than an empty tumbler, because the full tumbler gives us a greater feeling of effort when we lift it than the empty tumbler does. The more water there is in the tumbler the greater is the effort. A pail full of water requires still more effort, though the empty pail feels quite light; and, when we come to deal with a large tub full of water, we may be unable to stir it, though th

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21. The Measuring of Weights. The Balance.

Such an instrument is the balance or scales , which you may see in every grocer’s shop. It is composed of a beam which moves easily on a pivot fixed to its middle, and which has a scale-pan attached to each end. So long as both scale-pans are empty the beam is horizontal; but if you put anything which has weight into one, that one goes down and the other rises. If now you either pull or push the empty scale downwards, the beam may be brought into the horizontal position again, and the effort req

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22. The Weight of the same Bulk or Volume of Water is Constant under the same conditions. Mass. Density.

Now let two graduated thin glass measures be put into the two scales, and made to counterpoise one another exactly. Then, if even a single drop of water is put into the one measure the scale will descend, if the balance is a good one; showing that the drop has weight. If the measures are graduated accurately, then whatever volume of water is put into one, an exactly similar volume of the same water must be put into the other to make the beam level. This obviously means that the same volume of wa

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23. Equal Volumes of Different Things under the same circumstances, have Different Weights: the Density of Different Bodies is Different.

The important fact which has just been alluded to must be considered more fully. We have seen that an imperial pint measure gives us the space which is taken up by as much water as weighs a pound and a quarter; and this space is the bulk or volume of that weight of water. But if you take an ordinary pound weight and a quarter-pound weight, and put them into an imperial pint measure, you will find that instead of filling it, they take up only a very small portion of the space in its interior, or

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24. The Meaning of Heavy and Light—Specific Gravity.

We are in the habit of using the words heavy and light rather carelessly. We call things that are easily lifted light, and things that are hard to lift heavy. We say that sand, which is blown about by the wind, is light, and that a block of wood is heavy, and yet we have just seen that sand is heavier, bulk for bulk, than wood. In order to get rid of this double meaning, the weight of a volume of any liquid or solid, in proportion to the weight of the same volume of water at a known temperature

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25. Things of greater Specific Gravity than Water sink in Water; Things of less Specific Gravity float.

Here are two tumblers of water. Throw some sand into one and some sawdust into the other. What happens? The sand sinks to the bottom, the sawdust floats at the top. We may stir them up as we like, but the sand will tumble to the bottom and the sawdust, as obstinately, rise to the top. Thus that which is lighter than the water floats, and that which is heavier (bulk for bulk) sinks. So, if we pour some oil into the water, it floats, and if we pour some coloured spirit in carefully, it also floats

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26. A Body which Floats in Water always occupies as much Space beneath the level of the Surface of the Water as is equal to the Volume of Water which weighs as much as that Body; in other words, it displaces its own Weight of Water.

A cubic inch of water weighs about 252 grains and a half. Suppose that the tin box in the previous experiment was square, and had the bulk of 100 cubic inches, then the weight of a corresponding volume of water would be 25,250 grains. If the box weighed 8,416 grains, just a third of its bulk would be immersed; if 12,625 grains, half; if 16,832 grains, it would sink two-thirds of its volume, and so on. Or, if, when the box is floating, you make a mark upon its side at the exact level of the surfa

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27. Water presses in all Directions.

Thus water presses in all directions upon things which are immersed in it. If a long wooden or metal pipe, placed vertically, has its lower end stopped with a cork which does not fit very tightly, and water is poured into the top of the tube, the water will at first fill the part of the tube above the cork, and its weight will exert a certain pressure on the cork. In fact, if the end of the tube is stopped by applying the palm of the hand closely against it, the downward pressure of the water wi

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28. The Transference of Motion by Moving Water: the Momentum of Moving Water.

Suppose a wooden vat with a horizontal tap, the sectional area [3] of the tube of which is one square inch, inserted close to the bottom, to be filled with water up to 100 inches above the tap. Then supposing the tap to be shut, the pressure upon its sectional area will be 25,250 grains, or rather more than three pounds and a half—and there is the same pressure on every square inch of the bottom of the vat. 3 . The sectional area of a tube is the surface occupied by its cavity when it is cut acr

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29. The Energy of Moving Water.

If a short pipe bent at right angles like the letter L is fitted by one arm on to the end of the tap, while the other is turned vertically upwards, and the vat is full as before; when the tap is turned, the water will shoot up into the air, and after rising for a certain distance will stop, and then fall. In fact we shall have a fountain. Observe the difference between the vertical jet of water and the horizontal jet. If we leave the resistance of the air out of consideration, the water in the h

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30. The Properties of Water are Constant.

If, whenever there is a shower, you catch some rain-water, you will find that it possesses all the properties which have been described. It will be found to be an almost incompressible liquid, an imperial pint of which weighs about a pound and a quarter. It would make no difference if the rain-water were collected in Africa or in New Zealand; or if it had been obtained centuries ago and kept bottled up ever since. And there is every reason to believe that rain-water will have exactly the same pr

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31. Increase of Heat at first causes Water to Increase in Volume.

It has been seen that a certain weight of water always has the same volume under the same conditions. The most important of these conditions is the heat or cold to which it is exposed. Water which has stood for some time in a warm room becomes less in volume, or contracts , if it is taken into a cool place; while its volume increases, or it expands , if it is made hot. The same thing is true of quicksilver, of spirit, and of liquids in general. A thermometer is simply a small flask—the bulb—with

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32. Increase of Heat at length causes Water to become Steam.

Thus a change is effected in the properties of water by heating it ever so little. If it is more strongly heated a still greater change takes place. You know what happens when a saucepan containing water is put on the fire. The water gets hotter and hotter, then it begins to simmer, and finally, when it reaches 212°, it boils away into steam, which passes into the air and disappears. If the boiling is carried on long enough all the water vanishes. It looks at first as if the water had been destr

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33. The taking away of Heat from Steam causes the steam to change into Hot Water.

Now take a cold spoon, or a cold plate, and hold it against the jet of steam, for a moment or two. When you take it away, you will find that it is quite wet, being covered with drops of warm water, and, moreover, the cold spoon, or plate, has become warm. And if you fit a long cold metal pipe to the nozzle of the tea-kettle, you will find that no steam at all issues from the end of the pipe, but only water, while the pipe becomes warmed. Thus the heat passes from the fire into the saucepan, or k

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34. When Water is changed into Steam, its Volume becomes about 1,700 times greater that it was at first.

If you could measure and weigh the water in your kettle to begin with, and then measure and weigh all the steam into which the heat of the fire changes it, you would find that the bulk of the steam was nearly 1,700 times as great as the bulk of the water, though the weight of the steam would be exactly the same as that of the water. If you had a small square cup like a die, the inside measure of which was exactly one inch each way, it would hold one cubic inch of water. If this cup full of water

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35. Gases or Elastic Fluids. Air.

Here is a glass flask with a long neck and an open mouth. If we pour water in at the mouth until it rises to the lip we say that the flask is full of water. If we now pour the water out we say that the flask is empty. But is it empty? Press the flask mouth downwards into a glass jar full of water. If the flask were empty there would be no reason why the water should not enter the neck of the flask and stand at the same height inside the neck as it does outside. If you take an “empty” glass tube

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36. Steam is an Elastic Fluid or Gas.

In all the properties which have been mentioned water in the form of steam is an elastic fluid or gas like air. If a little water is placed in the flask mentioned in the preceding section all the “empty” part of the space will contain air. If the flask is now made hot the water will at length boil, bubbles of steam forming in the water and breaking at its surface. By degrees, the air, which at first lay above the water, will be driven out; and if the whole flask is kept hot, the “empty” part of

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37. Gases and Vapours.

Air is as much a gas in the coldest winter as it is in the hottest summer. But air can be liquefied by exposing it to a very low temperature, while, at the same time, it is subjected to an extremely great pressure. Thus, the difference between gases like air, which are condensed with extreme difficulty, and gases like steam, which are condensed easily, is only one of degree. Nevertheless there is a certain convenience in distinguishing those gases, which, like steam, are easily condensed as vapo

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38. The Evaporation of Water at ordinary Temperatures.

If some water is poured into a saucer and is allowed to stand even in a cool room or in the open air, you know that it sooner or later disappears. Wet clothes hung on a line soon dry—that is to say, the water clinging to them disappears or evaporates . The disappearance of the water under these circumstances results from the property just mentioned. In fact, it becomes gaseous water of the density appropriate to the temperature, and as such mixes with the air as any other gas would do. And as th

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39. When Hot Water is cooled, it Contracts to begin with, but after a time Expands.

We have now seen what a wonderful change is brought about by heating water. At first, it expands gradually and slightly; but, when it reaches the boiling-point, it suddenly expands enormously, and is no longer a liquid, but a gas. On the other hand, if warm water is allowed to cool, it gradually contracts till it reaches the ordinary temperature of the air in mild weather; but, if the weather is very cold, or if the water is cooled artificially, it goes on contracting only down to a certain temp

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40. Water cooled still further becomes the transparent brittle solid Ice.

Our tumbler of water, if put out of doors on a cold winter’s night, would gradually cool until it assumed a temperature of 39° throughout. Cooling below this temperature, the water so cooled would gradually accumulate at the surface by reason of its less density, and its temperature would fall till the thermometer placed in it marked 32°. As soon as this upper water cooled ever so little below 32°, a film like glass would form on its surface by the conversion of the coldest fluid water into soli

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41. Ice has less Specific Gravity than the Water from which it was formed.

But though the ice in the tumbler has the same weight as the water had, it has not the same volume. The expansion which began at 39° goes on, and when water passes into the solid state its volume is about 1 11 th greater than it was at 39°. Taking water at this temperature as 1·0, ice has a specific gravity of 0·916. But although water in freezing expands only to this small amount, it resembles steam in the tremendous force with which it expands. If you fill a hollow iron shell quite full of wat

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42. Hoar Frost is the Gaseous Water which exists in the Atmosphere, condensed and converted into Ice Crystals.

In the winter-time you often notice, on a clear sharp night, that the tops of the houses and the trees are covered with a white powder called hoar frost ; and, on the windows of the room when you wake up, you see most beautiful figures, like delicate plants. Take a little of the hoar-frost, or scrape off some of the stuff that makes the window look like ground glass, and you find that it melts in your hand and turns to water. It is in fact ice. And if you look at the figures on the window pane w

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43. When Ice is warmed it begins to change back into Water as soon as the Temperature reaches 32°.

A lump of ice brought out of the open air in very cold weather may have a temperature of 30°, or 20°, or lower. If such a lump is brought into a warm room it gradually becomes warmer, but remains unchanged otherwise, until it has risen to 32°. Then it begins to melt, and remains at 32° as long as it is melting; and the water which proceeds from it is at first also at 32°. If you were to throw a lump of ice into the middle of a hot fire, so long as a particle of ice remained as such, it would hav

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44. Ice the solid, Water the liquid, and Steam the gas, are three states of one natural object; the Condition of each State being a certain Amount of Heat.

Ice, liquid water, and steam, are three things as unlike as any three things can well be. What do we mean then by saying that they are states of one substance, water? What we really mean is that if we take a given quantity of water, say a cubic inch, and change it first into ice and then into steam, there is something which remains identically the same through all these changes. This something is, in the first place, the weight of the material substance. The water weighs 252½ grains, the ice int

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45. The Phenomena of Heat are the Effects of a rapid Motion of the Particles of Matter.

This much, however, is certain: that heat can be caused by motion. Every boy knows that a metal button may be made quite hot by rubbing it. A skilful smith will hammer a piece of iron red hot. The axles of wheels become red hot by rubbing against their bearings, if they are not properly lubricated; and even two pieces of ice may be melted by the heat evolved when they are rubbed together. And there are abundant other reasons, as you will find when you study physics, for the belief that the sensa

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46. The Structure of Water.

We have seen that pure water is perfectly clear and transparent. The naked eye can discern no difference between one part and another. In other words, it has no visible texture or structure . It does not follow that it really possesses none, however, for there are many things which seem to be the same throughout, or homogeneous , which yet show structure if they are examined with a magnifying glass. Thus the surface of a sheet of fine white paper looks perfectly even and smooth to the eye; but a

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47. Suppositions or Hypotheses; their Uses and their Value.

When our means of observation of any natural fact fail to carry us beyond a certain point, it is perfectly legitimate, and often extremely useful, to make a supposition as to what we should see, if we could carry direct observation a step further. A supposition of this kind is what is called a hypothesis , and the value of any hypothesis depends upon the extent to which reasoning upon the assumption that it is true, enables us to explain or account for the phenomena with which it is concerned. T

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48. The Hypothesis that Water is composed of Separate Particles (Molecules).

It has been pointed out that we cannot see, and indeed that there is not much hope of our ever being able to see, the separate particles of water, even if water is composed of such particles. But it is perfectly legitimate to suppose that water is made up of such particles, if that hypothesis will enable us to explain the properties of water. Let us suppose then that any portion of fluid water is really composed of a prodigious number of particles less (and probably much less) than a millionth o

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49. All Matter is probably made up either of Molecules or of Atoms.

The same reasons which lead to the adoption of the hypothesis that water is composed of separate particles justify its extension to all forms of matter whatever. The metal mercury or quicksilver , for instance, may be supposed to be made up of distinct particles of mercury of extreme minuteness, and according to the temperature, these associate themselves in the solid (frozen mercury), liquid (ordinary quicksilver), or gaseous form (vapour of mercury). To whatever treatment pure mercury may be s

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50. Elementary Bodies are neither destroyed nor is their Quantity increased in Nature.

It has been seen that when a cubic inch of water is dissipated by heat, it is not destroyed, but that it merely changes its form from the fluid to the gaseous state, while its weight remains unaltered. If the same cubic inch of water is decomposed into oxygen and hydrogen gases, the water is indeed destroyed, but the matter of which it consisted remains unchanged in weight. If the water weighed 252·5 grains, the oxygen gas will weigh 224·45 grains and the hydrogen gas will weigh 28·05 grains. An

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51. Simple Mixture.

In order to learn the manner in which water may be broken up into its elements or decomposed, you must turn to the Primer on Chemistry. But as a preliminary to the study of that science, it may be useful to consider some simple cases of composition and decomposition which are exemplified by water. If half a pint of water, coloured by putting a little ink into it, is added to the same quantity of clean water, the two will readily mingle; the total quantity of water will be a pint; and its colour

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52. Mixture followed by Increase of Density; Alcohol and Water.

Strong spirit, or alcohol , is a clear transparent fluid which looks like water, but is a very different substance. For example, it boils at a much lower temperature, it burns with a blue flame, it has intoxicating properties, and, like oil, it is very much lighter than water. Hence, if coloured spirit is poured gently upon the surface of water the spirit rests upon the water. Suppose, now, that we take a tall measure graduated into ten equal parts. Fill the lower five with water, and then, very

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53. Solution; Water Dissolves Salt.

If a spoonful of salt is put into a tumbler of cold water and the water is stirred, the salt swiftly vanishes from view; and, after a time, so far as our sense of vision goes, the water appears to be just what it was before. But if the water in the tumbler at first weighed five ounces and the salt weighed two ounces, the water in the tumbler will now weigh seven ounces; the water will now taste salt, the salt is said to be dissolved , and the solution is called brine . Moreover, the solution is

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54. Quicklime and Water: Plaster of Paris and Water: Combination.

Quicklime is a substance obtained by heating chalk or limestone to redness. When pure, it is a white hard solid which can be made to pass into the liquid and gaseous states only at enormously high temperatures. If a lump of fresh quicklime be placed in a saucer and about one-third of its weight of water poured upon it, there will be a great turmoil, heat will be evolved, the water will disappear, and the lime will crumble down into a soft white powder. This operation is what bricklayers call sla

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55. Mineral bodies may take on definite shapes and grow, or increase in size, by the addition of like parts.

Water and all the other natural bodies which have hitherto been mentioned, are what are called mineral bodies , although, in common use, the term mineral is usually restricted to ores and metals. Now we have repeatedly had occasion to remark that, under certain circumstances, not only water, but many other mineral bodies, assume regular shapes. The most familiar example is that of the beautiful imitation of leaves and foliage which is presented by the ice which forms on a window in winter. But w

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56. The Wheat Plant and the substances of which it is composed.

Every one has seen a cornfield. If you pluck up one of the innumerable wheat plants which are fixed in the soil of the field, about harvest time, you will find that it consists of a stem which ends in a root at one end and an ear at the other, and that blades or leaves are attached to the sides of the stem. The ear contains a multitude of oval grains which are the seeds of the wheat plant. You know that when these seeds are cleared from the husk or bran in which they are enveloped, they are grou

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57. The Common Fowl and the Substances of which it is Composed.

Everybody has seen a common fowl. It is an active creature which runs about and sometimes flies. It has a body covered with feathers, provided with two wings and two legs, and ending at one end in a neck terminated by a head with a beak, between the two parts of which the mouth is placed. The hen lays eggs , each of which is enclosed in a hard shell. If you break an egg the contents flow out and are seen to consist of the colourless glairy “white” and the yellow “yolk.” If the white is collected

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58. Certain Constituents of the Body are very similar in the Wheat Plant and in the Fowl.

The wheat plant contains neither horn, nor gelatin, and the fowl contains neither starch, nor cellulose; but the albumin of the plant is very similar to that of the animal, and the fibrin and syntonin of the animal are bodies closely allied to both albumin and gluten. That there is a close likeness between all these bodies is obvious from the fact that when any of them is strongly heated, or allowed to putrefy, it gives off the same sort of disagreeable smell; and careful chemical analysis has s

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59. Proteid Substances are met with in Nature only in Animals and Plants; and Animals and Plants always contain Proteids.

It is a very remarkable fact that not only are such substances as albumin, gluten, fibrin and syntonin, known exclusively as products of animal and vegetable bodies; but that every animal and every plant, at all periods of its existence, contains one or other of them, though, in other respects, the composition of living bodies may vary indefinitely. Thus, some plants contain neither starch nor cellulose, while these substances are found in some animals; while many animals contain no horny matter

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60. What is meant by the word Living?

The wheat plant in the field is said to be a living thing; the fowl running about the farm-yard is also said to be a living thing. If the plant is plucked up, and if the fowl is knocked on the head, they soon die and become dead things. Both the fowl and the wheat plant, as we have seen, are composed of the same elements as those which enter into the composition of mineral matter, though united into compounds which do not exist in the mineral world. Why then do we distinguish this matter when it

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61. The Living Plant increases in size, by adding to the Substances which compose its Body, like Substances; these, however, are not derived from without, but are manufactured within the Body of the Plant from simpler Materials.

In the spring, a wheat-field is covered with small green plants. These grow taller and taller until they attain many times the size which they had when they first appeared; and they produce the heads of flowers which eventually change into ears of corn. In so far as this is a process of growth, accompanied by the assumption of a definite form, it might be compared with the growth of a crystal of salt in brine: but, on closer examination, it turns out to be something very different. For the cryst

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62. The Living Plant, after it has grown up, detaches part of its Substance, which has the Power of developing into a similar Plant, as a Seed.

The grain of wheat is a part of the flower of the wheat plant, which, when it becomes ripe, is easily separated. It contains a minute and rudimentary plant; and, when it is sown, this gradually grows, or becomes developed into, the perfect plant, with its stem, roots, leaves and flowers, which again give rise to similar seeds. No mineral body runs through a regular series of changes of form and size and then gives off parts of its substance which take the same course. Mineral bodies present no s

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63. The Living Animal increases in Size by adding to the Substances which compose its Body, like Substances; these, however, are chiefly derived directly from other Animals or from Plants.

The fowl in the farm-yard is incessantly pecking about and swallowing now a grain of corn, and now a fly or a worm. In fact, it is feeding, and, as every one knows, would soon die if not supplied with food. It is also a matter of every day knowledge that it would not be of much use to give a fowl the soil of a cornfield, with plenty of air and water, to eat. In this respect, the fowl is like all other animals; it cannot manufacture the proteid materials of its body, but it has to take them ready

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64. The Living Animal, after it has grown up, detaches part of its Substance, which has the Power of growing into a similar Animal, as an Egg.

The fowl’s egg is formed in the body of the hen, and is, in fact, part of her body inclosed in a shell and detached. It contains a minute rudiment of a fowl; and when it is kept at a proper temperature by the hen’s sitting upon it, or otherwise, for three weeks, this rudiment grows or develops, at the expense of the materials contained in the yolk and the white, into a small bird, the chick, which is then hatched and grows into a fowl. The animal, therefore, is produced by the development of a g

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66. Mental Phenomena.

Material objects are all either not living, that is to say, mineral bodies, or they are living bodies. Everything which occupies space, offers resistance, has weight and transfers motion, belongs to one or other of these two great provinces of nature. The sciences of Astronomy, Mineralogy, Physics, and Chemistry deal with the former, while Biology, with its two divisions of Zoology and Botany, treats of the latter. But natural knowledge is not exhausted by this catalogue of its topics. In the ve

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67. The order of Mental Phenomena: Psychology.

A definite order obtains among mental phenomena, just as among material phenomena; and there is no more chance, nor any accident, nor uncaused event, in the one series than there is in the other. Moreover, there is a connection of cause and effect between certain material phenomena and certain mental phenomena. Thus, for example, certain sensations are always produced by the influence of particular material bodies on our organs of sense. The prick of a pin gives pain, feathers feel soft, chalk l

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