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foot high; and that, when these gases unite and the water is reproduced, this energy again becomes active. Two experiments will enforce the truth of this state

ment:

For the first, I have mixed together in this rubber bag oxygen and hydrogen in the exact proportions in which they unite to form water, and, with the gas, I will now blow up into froth the soap-suds contained in this iron mortar-thus confining the gas only by the thinnest possible envelope. I will now ask my assistant to inflame the mixture with his lighted taper, when a deafening explosion announces to us that the chemical union has taken place. But what has been the occasion of the development of such tremendous energy ? The formation of a single drop of water, so small that you could hold it on the point of a needle.

For the second experiment I will burn the same gas-mixture at a jet, and show you how great is the intensity of the heat which may be thus developed. This apparatus is the well-known compound blow-pipe invented by our countryman Dr. Hare. The oxygen and hydrogen flow through rubber hose from separate gas-holders into a very small chamber, where they mix before issuing from the jet. The same chemical union takes place here as before; the same product (water) is formed; the same amount of energy is developed; but, under these different conditions, the explosive gas burns with a quiet flame as it is gradually supplied from the jet, and the energy, instead of being expended in driving back the air, and thus determining that violent commotion in the atmosphere which caused the noise, is here manifested wholly as heat. And see how intense the heat is! . . . It is a steel file which is burning with such rapidity in this flame. As I have already

told you, heat is only heat is only a mode of energy, and, like any other manifestation of power, may be measured in footpounds. Hence, this brilliant experiment is an apt illustration of the amount of energy developed in the production of water. In witnessing the magnitude of the effects, we are surprised, as before, by the apparent inadequacy of the cause; for the amount of water, whose production was the occasion of all this display of power, is only a few drops.

Who could believe that such power was concealed in the familiar liquid which is so intimately connected with our daily life? Between the qualities of water and the qualities of these gases there is not the most distant resemblance. When the water is decomposed, the qualities of the water are wholly lost in the qualities of the two gases produced from it, and a certain amount of energy is absorbed. When the water is formed, the qualities of oxygen and hydrogen are wholly merged in those of the resulting liquid, while the same amount of energy is set free. Whether the oxygen and hydrogen exist, as such, in the water, or whether they are produced by some unknown and unconceived transformation of its substance, is a question about which we may speculate, but in regard to which we have no knowledge. All we know is, that the change of water into the two gases or of the two gases into water is attended with no change of weight, and hence we conclude that in the change the material is preserved, or, in other words, that water and the gases are the same material in different forms.

Now, the only theory which has as yet succeeded in giving an intelligible explanation of the facts, assumes that hydrogen and oxygen do exist as such in water, preserving each its individuality; that each molecule

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of water consists of three particles, two of hydrogen and one of oxygen; that, when the water is decomposed, the molecules are broken up, and that then the oxygen particles associate themselves together to form molecules of oxygen gas, and the hydrogen particles to form molecules of hydrogen gas; that, on the other hand, when the gases recombine, the reverse takes place, each particle of oxygen uniting to itself two particles of hydrogen to form a molecule of water.

These parts of molecules (these particles, into which the molecules break up under various chemical processes) are what we call atoms, and this theory is the famous atomic theory, which has played such a prominent part in modern chemistry. We shall find, as we proceed, that there is very strong evidence in its support. Indeed, without it a large part of the modern science would be wholly unintelligible; and, were I to confine my regards to purely chemical facts, I should regard the evidence in its favor as overwhelming. Still, I must confess that I am rather drawn to that view of Nature which has favor with many of the most eminent physicists of the present time, and which sees in the cosmos, besides mind, only two essentially distinct beings, namely, matter and energy, which regards all matter as one and all energy as one, and which refers the qualities of substances to the affections of the one substratum, modified by the varying play of forces. According to this view, the molecules of water are perfectly homogeneous, and the change, which takes place when water is decomposed, does not consist in the separation from its molecules of preexisting particles, but in imparting to the same material other affections.

I know that this language is very vague, but it is

no more vague than the idea it attempts to embody. Still, vague as it is, no one who has followed modern physical discussions can doubt that the tendency of physical thought is to refer the differences of substances to a dynamical cause. Nevertheless, as I said before, the atomic theory is the only one which, as yet, has given an intelligible explanation of the facts of modern chemistry, and I shall next proceed to develop its fundamental principles. I wish, however, before I begin, to declare my belief that the atomic theory, beautiful and consistent as it appears, is only a temporary expedient for representing the facts of chemistry to the mind. Although in the present state of the science it gives absolutely essential aid both to investigation and study, I have the conviction that it is a temporary scaffolding around the imperfect building, which will be removed as soon as its usefulness is passed. I have been called a blind partisan of the atomic theory, but, after this disclaimer, you will understand me when, during the remainder of this course of lectures, I shall endeavor to present its principles as forcibly as I can.

LECTURE VI.

ELEMENTARY SUBSTANCES AND COMBINING PROPORTIONS.

IN my last lecture I stated that in a chemical compound the qualities of the constituents are wholly merged in those of the product, and that this circumstance distinguishes a true compound from a mechanical mixture in which the qualities of each ingredient are to a greater or less extent preserved. This distinction is one of very great importance in chemistry, and I will begin my lecture this evening by asking your attention to a simple experiment, which will recall the principal points of our discussion at the last lecture and at the same time illustrate still other aspects of this important subject.

I have prepared a mixture of finely-divided iron (iron reduced by hydrogen) and flowers of sulphur. The two powders have been rubbed together in a mortar until the mass appears perfectly homogeneous and it is impossible with the unaided eye to distinguish the grains of either substance, and yet nothing is easier than to show that both are here wholly unchanged.

For this purpose I will, in the first place, pour upon a portion of the powder some of this colorless liquid called sulphide of carbon, which dissolves sulphur with great eagerness. After shaking the two together we