lyiii REPORT— 1857. 
refractive index should also have a maximum value at the same temperature, 
This has been disproved by M. Jamin, by observing the interference of two 
rays, one of which has passed through air, and the other through water; 
and thus the last conclusion of the emission-theory has been set aside. 
It would oceupy 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. Peitevin 
has succeeded in producing 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, 
ar longer known, than the development of Heat by friction, The most 
ignorant savage is acquainted with it,—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 mechanical force is developed, which is again absorhed 
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 developed by 
mechanical action, or vice versd, 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 1849, 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 wnit 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 Rankine, into large pro- 
portions, and is each day making fresh conquests from the region of the 
unknown, 
Thus far the science of Heat is made to rest wholly upon the faets of ex- 
periment, and is independent of any hypothesis respecting the molecular 
constitution of bodies, The dynamical theory of heat, however, has mate- 
rially aided in establishing true physical conceptions of the nature of heat, 
The old hypothesis of caloric, as a separate substance, was indeed rendered i 
improbable by the experiments of Rumford and Davy, and by the reasonings — 
