PHOSPHORUS BURNT IN OXYGEN GAS.

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iar experience to establish this principle, although temperature is a complex effect, depending, not only on the amount of heat liberated, but also on the nature of the material to be heated, and on conditions which determine the rapidity with which the heat is dissipated. But the matter of the light is not so obvious. Why should more rapid burning be attended with more brilliant light? It is so in the present case; but is it always so? We can best answer this question by a few experiments, which will teach us what are the conditions under which energy takes the form of light; but these experiments we must reserve until the next lect

ure.

LECTURE X.

THE THEORY OF COMBUSTION.

As our last hour closed, we were studying the phenomena of combustion. I had already illustrated the fact that, so far as the chemical change was concerned, these processes were examples of simple synthesis, consisting in the union of the combustible atoms with the oxygen atoms of the air, and that the sole circumstance which distinguished these processes from other synthetical reactions was the amount of energy developed. There were three points to which I directed your attention in connection with this subject: 1. The condition of molecular activity, measured by the temperature or point of ignition, which the process requires. 2. The chemical change itself, always very simple. 3. The amount of energy developed, and the form of its manifestation. This last point is the phase of these phenomena which absorbs the attention of beholders, and the one which we have chiefly to study. I stated in the last lecture that the amount of energy developed depended solely on the nature and amount of the combustible burnt, but I also showed that both the intensity and the mode of manifestation of this energy varied very greatly with the circumstances of the experiment. The intensity of the action we traced at

HYDROGEN GAS BURNT IN AIR.

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once to the rapidity of the combustion, but the conditions which determine whether the energy developed shall take the form of heat or light we have still to investigate, and no combustible is so well adapted as hydrogen gas to teach us what we seek to know.

Here, then, we have a burning jet of hydrogen. It is not best for me to describe, in this connection, either the process or the apparatus by which this elementary substance is made, and a constant supply maintained at the burner, as I wish now to ask your attention exclusively to the phenomena attending the burning of the gas; and let me point out to you, in the first place, that hydrogen burns with a very well-marked flame. The flame is so slightly luminous that I am afraid it cannot be seen at the end of the hall, but I can make it visible by puffing into it a little charcoal-powder.

Now, all gases burn with a flame, and flame is simply a mass of gas burning on its exterior surface. As the gas issues from the orifice of the burner, the current pushes aside the air, and a mass of gas rises from the jet. If the gas is lighted—that is, raised to the point of ignition-this mass begins to combine with the oxygen atoms of the air at the surface of contact, and the size of the flame depends on the rapidity with which the gas is consumed as compared with the rapidity with which it is supplied. By regulating the supply with a cock, as every one knows, I can enlarge or diminish the size at will.

The conical form of a quiet flame results from the circumstance that the gas, as it rises, is consumed, and thus the burning mass, which may have a considerable diameter near the orifice of the jet, rapidly shrinks to a point as it burns in ascending.

But we must not spend too much time with these

details, lest we should lose sight of the chemical philosophy, which it is the main object of this course to illustrate. The chemical change here is even more simple than in the experiment with phosphorus, and consists solely in a direct union of the hydrogen atoms of the gas with the oxygen atoms of the air. Indeed, in another connection, we studied the reaction at an early stage in this course of lectures; when, in order to illustrate the characteristic feature of chemical combination, we exploded a mixture of hydrogen and oxygen gases. The reaction obtained under those conditions was identical with that here. We had not then learned to express the chemical change with symbols; but now I may venture to write the reaction on the black-board: + 0=0 = 2H2O. Oxygen Gas. Steam.

2H-H Hydrogen Gas.

It would be very easy to show you that, as the symbols indicate, from two volumes of hydrogen, and one of oxygen, two volumes of steam are formed; but the experiment requires a great deal of time, and the result could not readily be made visible to this audience. I must content myself with proving that water is really produced by the hydrogen flame.

The apparatus we use looks complicated, but is, in fact, very simple (Fig. 29). By means of an aspirator the products of combustion are sucked through a long glass tube, which is kept cool by a current of water in a jacket outside. The flame burns under the open and flaring mouth of the tube, and the liquid, which condenses, drops into a bottle at the other end.

You must not expect that any considerable amount of water can be produced in this way. In the union of the two gases to liquid water, a condensation of 1,800 times takes place, so that, in order to obtain a

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quart of liquid water, we must burn 1,200 quarts of hydrogen gas, and take from the air 600 quarts of pure oxygen; and this, on the scale of our experiment, would be a very slow process. We have here obtained barely an ounce of liquid, although the jet has been burning for more than an hour. In order to show that the product is really water, I will apply the same test I used in a former experiment. We will pour the liquid into a shallow dish, and drop upon it a bit of potassium. . . . The hydrogen-flame, which at once bursts forth, gives the evidence we seek.

FIG. 29.-The Synthesis of Water.

Such, then, being the nature of the chemical process before us, let me pass on to that feature of this flame which is at once the most conspicuous and the most important phase of the phenomenon, namely, the development of energy. Here, again, we have become acquainted with the important facts bearing on this question. In a previous lecture I told you that, in the burning of a pound of hydrogen, sufficient energy was developed to raise a weight of 47,888,400 pounds to the height of one foot, and these figures are included,