and on the refractive Powers of different Gases. 301 
fear, as well in the theory of astronomical refractions 4s in 
the reduction of observations. Thus the tables of refraction 
published by the Board of Longitude in France,.and which 
are founded upon this law, upon the coefficient of M. De- 
lambre, and upon the beautiful analysis of M. de Laplace, 
may be regarded as being as perfect as astronomers can ever 
require. 
Lastly, the coefficient of the barometer, as well as the 
exact relation of the specific gravities of the air and mercury, 
is a determination of some utility to astronomers, since upon 
that depends the height of the atmosphere (being supposed 
homogeneous), a height which is one of the elements of the 
theory of refractions. 
«¢ We are of opinion,” the authors conclude, ‘ that we 
may still deduce from our researches one other important 
truth; but it is requisite that we should recall the general 
eles found by naturalists and chemists upon the nature 
and constitution of the atmosphere. 
« Mr. Cavendish is the first who endeavoured to establish 
that the proportions of the two elements of the atmospheric 
air were constant, in spite of the distance of places and the 
difference of temperatures. The observations since made 
by M. de Mairy in Spain, M. Berthollet in Egypt and in 
France, Mr. Dayy in England, and by Beddoes on the air 
brought from the coast of Guinea, have confirmed this grand 
result. But one of the finest experiments made on this sub- 
ject is that of Gay-Lussac, who, having been elevated alone 
in a balloon to the height of 6900 metres, the greatest ever 
attained by any person, brought some atmospheric air from 
“these regions. This air, being analysed on his return, com- 
paratively with that on the surface of the earth, gave the 
same principles in the same proportions: this proves that 
the chemical constitution of the atmosphere at these great 
heights is the same as at the surface of the earth. This re- 
sult has been since confirmed by the experiments made by 
Messrs. Humboldt and Gay-Lussac on eudiometry. The 
air of the surface of the earth, analysed at different days, at 
various hours and temperatures, presented no change in its 
composition : it always contained 0°21 of oxygen in volume, 
0'787 
