3. The warm air, carbonic acid gas and water vapor passing away from the lungs in respiration carry with them a large amount of heat. This corresponds to the loss of heat in the locomotive through the smoke passing out the smokestack, and in both cases the loss is greater when work is being done and less during inaction. The refuse products of the body (as the ashes of the locomotive) also carry away heat. This is the third portion of heat and is a large one.
Work is done in the locomotive by the expanding steam in the cylinders of the engine. The steam is cooled as it expands. Hence heat disappears when work is done; that is, is converted into mechanical energy, and a steam engine is hence called a heat engine; an engine for converting heat into work, according to the law of the conservation of energy. As the pistons are pushed to and fro by the tremendous pressure of the expanding steam, the reciprocating motion is communicated to the great drivers of the engine by strong arms of steel. But how is work done in the body? That is a question of prime importance and of surpassing interest. When muscle contracts and force is exerted, as when the body is lifted or an oar is pulled, muscular tissue (or material stored in muscular tissue) is oxidized; that is, burned, and heat is produced; yet not as much heat appears as would have appeared on the combustion of the same amount of body material if no work had been done. Apparently, then, heat has been converted into work. But we cannot trace the process with the same clearness as in the cylinder of a steam engine. Whether the potential energy of the body material is directly converted into work, or whether combustion first produces heat and a part of this heat is then converted into work, we do not know. In other words, we do not know whether the animal body as a machine for doing mechanical work is a heat engine or some other kind of engine. This is a fundamental question, as well as a very difficult one, and to a student of thermodynamics and physiology it prompts all sorts of speculation.
When one tries to picture to himself how the potential energy of food or body tissue can be directly converted into mechanical work, he is apt to turn to the other alternative and imagine that in some way the body is a heat engine. For we know that heat results from the oxidation of tissue, and we also know how heat can be converted into mechanical work. But we are at once confronted with a difficulty. One of the fundamental laws of thermodynamics requires that when heat is converted into work there shall be a difference of temperature between the source of heat and the place to which the heated material employed passes after doing the work. In other words, in a heat engine, whatever the mechanism, there must be a fall of temperature, which is greater as the relative amount of work, or efficiency, is greater. In the human body the efficiency perhaps surpasses that of the best steam engines; hence there should be a fall of temperature comparable with that between the boiler and condenser of a steam engine. This may be 100 degrees or more, and we do not know of any such difference of temperature in the body. Indeed, we know, on the contrary, that the temperature of the body is remarkably uniform, as already stated. It is possible, however, that there are molecular differences of large amount. In other words, if we could make an ultra-microscopic survey of temperature in a muscle during contraction, there might be found places of high temperature where combustion was occurring, and all the requirements of a heat engine of molecular dimensions fulfilled. But this is a matter of speculation. The process may yet be found to be electrical, or something else quite different from that of a steam engine.
We thus find between the animal body and a locomotive engine a striking parallel. In many particulars the chemical and physical processes going on in the latter are found also in the former. In both, the fundamental law of the conservation of energy is strictly observed. Nevertheless, the animal body considered simply as a machine is far more complex in its structure and operation than the engine, and far more of mystery envelops its working. Much remains for the chemist and physicist and physiologist to reveal, and no more fascinating field of research exists.
CHAPTERS ON THE STARS.
By Professor SIMON NEWCOMB, U. S. N.
THE SPECTRA OF THE STARS.
The principles on which spectrum analysis rests can be stated so concisely that I shall set them forth for the special use of such readers as may not be entirely familiar with the subject. Every one knows that when the rays of the sun pass through a triangular prism of glass or other transparent substance they are unequally refracted, and thus separated into rays of different colors. These colors are not distinct, but each runs into the other by insensible gradations, from deep red through orange, yellow, green and blue to a faint violet.
This result is due to the fact that the light of the sun is composed of rays of an infinite number of wave-lengths, or, as we might express it, of an infinite number of shades of color, since to every wave-length corresponds a definite shade. Such a spreading out of elementary colors in the form of a visible sheet is called a spectrum. By the spectrum of an incandescent object is meant the spectrum formed by the light emitted by the object when passed through a refracting prism, or otherwise separated into its elementary colors. The interest and value which attach to the study of spectra arise from the fact that different bodies give different kinds of spectra, according to their constitution, their temperature and the substances of which they are composed. In this manner it is possible, by a study of the spectrum of a body, to reach certain inferences respecting its constitution.
In order that such a study should lead to a definite conclusion, we must recall that to each special shade of color corresponds a definite position in the spectrum. That is to say, there is a special kind of light having a certain wave-length and therefore a certain shade which will be refracted through a certain fixed angle, and will therefore fall into a definite position in the spectrum. This position is, for every possible kind of light, expressed by a number indicating its wave-length.