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.