nomena.

But, neither the conditions of the burning nor the chemical change itself, although so beautifully illustrated here, are nearly so prominent facts as the manifestation of light and heat, which attends, the process; and these brilliant phenomena wholly engrossed the attention of the world until comparatively recently, and indeed they still point out what is really the most important circumstance connected with this class of pheThe union of combustible bodies with oxygen is attended with the development of an immense amount of energy, which takes the form of light or heat, as the case may be. Moreover, it is also true that the amount of energy thus developed depends solely on the amount of combustible burnt, and not at all on the circumstance that the burning is rapid or slow. Thus, in the case before us, the amount of heat developed by the burning of an ounce of phosphorus is a perfectly definite quantity, and would not be increased if the combustion were made vastly more intense. So it is with other combustibles. The table before you gives the amount of energy developed by the burning of one pound of several of the more common combusCalorific Power from One Pound of Each Combustible.

tibles, estimated, in the first place, in our common units of heat, and, in the second place, in foot-pounds. But, although the amount of energy is thus constant, de

PHOSPHORUS BURNT IN OXYGEN GAS.

187

pending solely on the amount of the combustible burnt, the brilliancy of the effect may differ immensely. A striking illustration of this fact I can readily show you.

For this purpose I will now repeat the last experiment, with only this difference, that, instead of burning the phosphorus in air, I will burn the same amount as before in a globe filled with pure oxygen. We shall, of course, expect a more violent action, because, there being here no nitrogen-molecules, there are five times as many molecules of oxygen in the same space. Hence, there are five times as many molecules of oxygen in contact with the phosphorus at once, and five will combine with the phosphorus in the same time that one did before. But, with this exception, all the other conditions of the two experiments are identical. We have the same combustible, and the same amount of it burnt. We have, therefore, the same amount of energy developed, and yet how different the effect! Phosphorus burns brightly even in air, but here we have vastly greater brilliancy, and the intensity of the light is blinding.

What is the cause of the difference? One obvious explanation will occur to all: The energy in this last experiment has been concentrated. Although only the same amount of heat is produced in the two cases, yet, in the last, it is liberated in one fifth of the time, and the effect is proportionally more intense. The intensity of the effect is shown simply in two circumstances: first, a higher temperature; and, secondly, a more brilliant light. Of these, the first is fully accounted for in the explanation just suggested; for, if five times as much heat is liberated in a given time, it must necessarily raise the temperature of surrounding bodies to a much higher degree. I need not go beyond your famil

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 IX.

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

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