POSSIBLE EFFICIENCY OF HEAT-ENGINES. 831 



Phenylacetic acid, in a saturated solution (1 : 400), immediately. 



Indol, in a saturated solution (1 : 900), in twenty-four Lours. 



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 pro- 

 cess will take place, and the different substances will, as before, exhibit 

 their j>ower to delay the process in the following order : scatol, hydro- 

 cinnamic 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 decompo- 

 sition. 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 de- 

 stroy them. The idea is logically deducible from this theory that the 

 germs of disease finally produce their own destruction by the opera- 

 tion of their growth and development, and helps us to comprehend the 

 cyclical course which is characteristic of most infectious diseases. 



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POSSIBLE EFFICIENCY OF HEAT-ENGINES. 



Br Professor WILLIAM A. ANTHONY. 



THE 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 con- 

 clusion 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 X 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 



