certain Liquids, and of the Vapours which they form. 377 
power sensibly less than that of the liquids which have formed 
them. Thus, in order to cite but a single example, the refracting 
power of the liquid carburetted sulphur, with reference to that 
of the air, is a little more than three, whereas that of the same 
substance in the state of vapour, with the same reference, does 
not exceed two. 
If we nowcompare thisresult with thetheory, weshall be obliged, 
by admitting the Newtonian theory of refraction, to suppose, which 
is at least a singular conclusion, that the attraction of one and 
the same body for the light is not exerted in proportion to the 
density. Unfortunately, the number of the substances on which 
we may operate with precision in the state of vapour, is too 
small to entitle us to hope to deduce from the results of our ex- 
periments, any law relative to the variation which the change of 
density occasions to the affinity of bodies for light. The liquids 
which we tried were carburetted sulphur, sulphuric ether, and 
muriatic ether. 
In the absence of this direct method, it appeared to us that 
this law might be deduced from the comparison of the refracting 
power of the gases, and of that of the solid or liquid bodies 
which they form on being united. In fact, if in the combina- 
tions of gas which retain the gaseous state, the refracting power 
of the compound was, as has been hitherto supposed, equal to 
the sum of the refracting powers of its elements, it will thence 
result that the act of combination will not in any way modify the 
action of bodies on light. Hence we may conclude with proba- 
bility, that the refracting power of a solid or liquid compound 
differs from the sum of the refracting powers of its gaseous 
principles, only in the ratio of the augmentation which these 
last undergo by the effect of condensation. : 
Nevertheless, as the law relative to the refracting power of the 
compound gases had been founded but upon a small number of 
experiments, it was indispensable in the first place to ascertain 
its exactitude : now by the measurements which we have made of 
the refraction of a great number of gases, we have proved that 
this law has not always agreed with the results of observation. 
We see therefore that the refracting power of any body, far 
from being constant, as the Newtonian theory seemed to prove, 
on the most natural hypothesis which can be formed of attrac- 
tion, undergoes on the contrary variations, either by the effect 
of the change of density, or by the state of combination in which 
the body is found. In order to determine the influence of each ° 
of those causes in particular, it is necessary to measure with ex- 
actitude the refracting powers of a great number of substances, 
and those of the combinations to which they give rise. Although 
the labour which we have undertaken on this head already em- 
braces 
