376 On the refractive and dispersive Powers of 
locity to that of the incident light is equal to the ratio from the 
sine of incidence to the sine of refraction. 
The same principle of active forces gives as the measurement 
of the total action of the body upon light, the increment of the 
square of the velocity of the ray; an increment which for this 
reason has been designated by the name of refractive power. 
This quantity ought evidently to depend on the nature of the 
body; but in one and the same substance it should remain in 
@ proportion to the density, for it is natural to suppose that an 
attraction is always exercised in proportion to the mass, what- 
ever in other respects may be the function of the distance ac- 
cording to which it varies. On this supposition, the refringent 
power, i. e. the ratio of the refractive power with the density, 
ought no longer to depend on any thing but the chemical con- 
stitution of the body, and remain constant when the density 
alone changes. 
This consequence of the theory of attraction has never been 
verified except in the gases. But if we reflect that their re- 
fractive power is extremely weak, and that consequently the in- 
crement of velocity which they impress on the light is very small, 
we shall be convinced, by the help of a very simple calculation, 
that the expression which the Newtonian theory gives for the re- 
fractive power is not the only one which, in the gases, remains 
in proportion to the density ; but that there exists an infinity of 
expressions different from the latter, all of which will satisfy the 
same condition. It results therefore, that, so far from the gases 
appearing to have a refracting power independent of their den-. 
sity, we have no right to conclude that solid and liquid bodies 
possess the same property. 
We are of opinion that the best method of deciding com- 
pletely this question, will be 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 otie of the bodies at least preserves a very strong action on 
the light. We have therefore made choice of liquids, which at 
the ordinary temperatures of the air furnish the most abundant 
vapours. We have measured the refractive power of each of 
these liquids and that of the vapours which come from them: by 
comparing those refractive powers with the known densities of 
the liquids and of the vapours, it was easy to see if in each of 
these bodies the refracting power, 7. e. the analytical expression 
2 
is was independent of the density. 
The result of our experiments proves rigorously the con- 
trary. hey all agree in giving for the vapours a refracting 
power 
