FOR THE ADVANCEMENT OF SCIENCE, 1857. 499 



through air, and the other through water ; and thus the last con- 

 clusion of the emission-theory has been set aside. 



It would occupy too much of your time were I to touch, even 

 lightly, upon the subject of the chemical action of light, and the 

 many beautiful and important discoveries of the Art to which it 

 has given rise. I may, however, mention, as one of the latest of 

 the marvels of photography, that M. Poitevin has succeeded in pro- 

 ducing plates in relief , for the purposes of engraving, by the action 

 of light alone. The process depends upon the change in the 

 affinity for water, produced by the action of light upon a thin 

 plate of gelatine, which is impregnated with bichromate of potash. 



In the whole range of experimental science there is no fact 

 more familiar, or longer known, than the development of Heat by 

 friction. The most ignorant savage is acquainted with it, and it was 

 probably known to the first generation of mankind. Yet, familiar 

 as it is, the science of which it is the germ dates back but a very 

 few years. 



It was known from the time of Black, that heat disappeared in 

 producing certain changes of state in bodies, and reappeared when 

 the order of those changes was reversed ; and that the amount of 

 heat, thus converted, had a given relation to the effect produced. 

 In one of these changes namely, evaporation a definite mecha- 

 nical force is developed, which is again absorbed when the vapour 

 is restored by pressure to the liquid state. It was, therefore, not 

 unnatural to conjecture, that in all cases in which heat is deve- 

 loped by mechanical action, or vice versa, a definite relation would 

 be found to subsist between the amount of the action, and that of 

 the heat developed or absorbed. 



This conjecture was put to the test of experiment by Mayer 

 and Joule, in 1842, and was verified by the result. It was found 

 that heat and mechanical power were mutually convertible; and that 

 the relation between them was definite, 772 foot-pounds of motive 

 power being equivalent to a unit of heat that is, to the amount of 

 heat requisite to raise a pound of water through one degree of 

 Fahrenheit. The science of Thermo-dynamics, based upon this 

 fact, and upon a few other obvious facts, or self-evident principles, 

 has grown up in the hands of Clausius, Thomson, and Kankine, 

 into large proportions, and is each day making fresh conquests 

 from the region of the unknown. 



2x2 



