156 Proceedings of Philosophical Sovieties. (Fes. 
is not the only one which in the gases remains proportional to the 
density ; and that there exists an infinity of expressions which 
would all satisfy the same condition. Therefore, though the gases 
appear to have a refractive power independent of thei Jensity, we 
have no reason to conclude that solid and liquid bodies possess this 
property. 
The authors thought that the best way of deciding this question 
completely was to compare the refractive power of different liquids 
with that of the vapours which these liquids form. In this case the 
change of density is very considerable, and one of the bodies at 
least preserves a strong action on light. They made choice of the 
liquids which furnish the most abundant vapour at the ordinary 
temperature of the atmosphere. They measured the refracting 
power of each of these liquids, and that of the vapours derived 
from them. By comparing these powers with the known densities 
of the liquids and vapours, it was easy to see whether in each of 
these bodies the refracting power was independent of the density. 
The result of their experiments proves the contrary. They all 
agree to give for vapours a refracting power sensibly less than that 
of the liquids which have formed them. Thus the refracting power 
of liquid carburet of sulphur, referred to that of air, is a little 
greater than three; while that of the same substance in a state of 
vapour, referred likewise to air, does not exceed two. 
If we compare this result with theory, we find ourselves obliged, 
according to the Newtonian hypothesis, to suppose, what is cer- 
tainly a singular supposition, that the attraction of the same body 
for light does not act in proportion to its density. Unfortunately, 
the number of bodies on which we can operate with precision in 
the state of vapour is too small to enable us to conclude from these 
experiments any law relative to the variation which the change of 
density makes the affinity of the body for light undergo. The 
liquids tried by the authors were carburet of sulphur, sulphuric 
ether, and muriatic ether. 
In want of this direct method, the authors were of opinion that 
this law might be deduced from a comparison of the refractive 
power of gases, and that of the solid or liquid bodies which they 
form by uniting. If in the gaseous combinations which preserve 
the gaseous state, the refracting power of the compound were, us 
has been hitherto believed, equal to the sum of the refracting 
powers, it would follow that the act of combination will not in the 
least modify the action of the body on light, from which we may 
conclude with probability that the refracting power of a solid or 
liquid compound does not differ from the sum of the refracting 
powers of its gaseous principles, but in proportion to the increase 
which these last receive by condensation, 
However, as the law relative to the refracting force of the com- 
pound gases had been. established only on a small number of expe- 
riments, it was necessary, in the first place, to be certain of the 
accuracy of that law. But the measures which the authors have 
