Page:Popular Science Monthly Volume 17.djvu/851

Rh  Phenylacetic acid, in a saturated solution (1:400), immediately. Indol, in a saturated solution (1:900), in twenty-four hours. Kresol, in the proportion of 5:1,000 of the mixture, in twenty-four hours. Carbolic acid, in the proportion of 20:1,000 of the mixture, immediately.

Two points strike us in this review: first, the difference in the amount of poison required to produce the aseptic and the antiseptic effect; again, it is curious that carbolic acid, the favorite antiseptic, appears to be the weakest on the list. It is at the same time one of the most soluble, while scatol, the most difficult of solution, is the strongest.

If we add the substances we have been examining to a saccharine solution exposed to fermentation, a slackening of the fermenting process will take place, and the different substances will, as before, exhibit their power to delay the process in the following order: scatol, hydrocinnamic acid, indol, phenylacetic acid, kresol, carbolic acid.

These facts seem to justify us in looking for specific disinfectants and prophylactics among the aromatic products of chemical decomposition. They also give a strong air of plausibility to the theory that the bacteria produce, through the chemical changes of which they are the direct cause, the most effective substances that can be used to destroy them. The idea is logically deducible from this theory that the germs of disease finally produce their own destruction by the operation of their growth and development, and helps us to comprehend the cyclical course which is characteristic of most infectious diseases.



HE theory of thermodynamics, which asserts the equivalence of heat and mechanical work, has now been generally accepted by men of science for thirty years. The equivalence of heat and work is accepted as an established fact by engineers and mechanics, and the mechanical equivalent of heat, as determined by Joule, is made the basis of computations regarding the energy of fuel, etc., by practical men, without a question as to its correctness. But there is one conclusion to which the theory leads, of great practical importance as regards the theory of steam-engines, which does not seem to have been as generally accepted, and yet it is just as firmly established as the fundamental principle of the equivalence of heat and work. A pound of carbon by its combustion in oxygen yields 14,400 heat-units, equal to 14,400 $$\times$$ 772 $$=$$ 11,116,800 foot-pounds of energy. One horse-power should, therefore, be developed by the combustion of about one sixth of a pound of carbon per hour, while our best steam-engines require 