ENERGY DEVELOPED.

101

told you, 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

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

THE ATOMIC THEORY.

103

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 V.

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

COMBINATION OF SULPHUR AND IRON.

105

find left on the bottom of our glass beaker a quantity of a black powder, which, as the magnet shows at once, is iron. In the second place I will stir up another portion of the mixture with alcohol, using this liquid to hold the powder in suspension so that I can pick out the grains of iron with a magnet. Using this bar-magnet as a stirring-rod, I can thus readily wash out the sulphur from the iron which adheres to the magnet, and we recognize at once the yellow color as the particles of sulphur settle to the bottom of the jar.

Having shown you now that both iron and sulphur are here present, with their qualities wholly unaltered, I will next take a third portion of the powder, and, having made with it a small conical heap, apply a lighted match to the apex of the cone. A glow at once spreads through the whole mass, which is an evidence to me that a chemical change has taken place, and in that change the sulphur and iron have disappeared. The mass has somewhat caked together, but we can easily pulverize it again, and our product is then a black powder not differing very greatly in external appearance from the original material. But from this black powder the sulphide of carbon can dissolve no sulphur, and the magnet can remove no iron.

The qualities both of the iron and the sulphur have disappeared, and those of a new substance we call sulphide of iron have taken their place, and the only evidence we have that the material of the sulphur and the material of the iron are still here is the weight of the sulphide of iron, which is exactly equal to that of the sulphur and iron combined. So long as the sulphide of iron remains sulphide of iron, no scrutiny can detect in it either sulphur or iron, and we must have recourse to other chemical processes in order to repro