and on the refractite Powers of different Gases. 137 
Newton discovered in the refracting power of the dia- 
mond what suggested to him that it was combustible; and 
M. de la Place announced, in a printed memoir, the creat 
refractive power of hydrogen. The refracting power of the 
other gases is intermediate between those of hydrogen and 
oxygen. That of the muriatic acid gas was tried, not- 
withstanding the great difficulties attending its manipula- 
tion, and it was found to surpass that of the atmospheric 
air. 
But it is the study of the refracting power of this latter 
fluid, and of its modifications on account of its density, that 
ought particularly to occupy philosophers, since the rays of 
light never reach us, except more or less turned from the 
right line by this influence. We might vary at pleasure the 
density of the common air inclosed im the apparatus; that 
is to say, produce a given difference between this medium 
and the external air, on leaving the one and the other iden- 
tical in their nature, and only changing the element of den- 
sity. When we have brought the density of the interior 
and exterior air to the term of equality, there will remain 
no more refrangent cause than the error in the parallelism 
of the two surfaces of each glass; and it is thus the absolute 
quantity of this correction was determined, which did not 
reach 17 seconds of a degree. This series of observations 
made upon the atmospheric air and upon the cther gases, 
taken at different densities, and always marking the state of 
the barometer, thermometer, and hygrometer, afforded the 
authors of the memoir a conviction of this important prin- 
ciple in the theory of refractions, namely, that from the most 
perfect vacuum to the ordinary degree of pressure of the at- 
mosphere, the refraction of the light in any given gas is al- 
ways proportional to its density, without this rule haying 
any occasion for the slightest modification. 
The authors propose to examine, by future experiments, 
the influence of a temperature more or less elevated, and of 
the condensation of the air beyond the ordinary atmospheri- 
eal limits upon the refractive power of this fluid. 
It did not appear to them that the state of the hygrometet 
had 
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