158 Proceedings of Philosophical Societies. (Fes. 
whose refracting power had been determined by the preceding ex- 
periments. The dispersive force of the liquids was easily examined ; 
but this was not the case with the vapours. The refraction which 
they occasion in a prism being very small, the dispersion, which is 
only a very small part of the refraction, is scarcely sensible. Ac- 
cordingly, notwithstanding the importance of such a determination 
in gases and vapours, philosophers seem to have despaired of de-~ 
ducing them from observation. ‘The object which the authors had 
in view required a direct measurement, and they have accomplished 
it by a method which they promise to describe in detail ; and they 
announce that experiments made on the same vapour under different 
circumstances agree sufficiently with each other to show that their 
determinations approach pretty near the truth. 
They have ascertained that the dispersive power really diminishes 
with the density; but that the dispersive power diminishes at a 
greater rate than the refracting power; so that if we call 2 the ratio 
of the sine of incidence to the sine of refraction, and ¢ the density 
of the body, the refractive power (=) is not only variable for the 
same class of rays, but the law according to which this change takes 
place is different for the different coloured rays. 
In carburet of sulphur, already chosen as an example, the ratio 
of the dispersive power to the refracting power is 0°14 ina liquid 
state, while it is reduced to less than 0°08 in a state of vapour. 
Thus while the variation of the refractive power may be stilt 
explained by admitting that the attraction of the same body for 
light varies according to a different law than that of the direct ratio 
of the densities, we see that, to explain the variation observed in the 
dispersive power, it would be necessary to suppose besides that the 
action of a body on the differently coloured rays follows, in the 
changes of density, a different law for each of these rays. 
These different suppositions doubtless diminish both the simplicity 
and probability of the Newtonian theory. But before coming to 
any decision, the authors repeat that it is necessary to examine with 
a great deal of care the changes which the refracting forces of bodies 
undergo, either by variations of density, or by the effect of combi- 
nation. It is no less indispensable to join to these determinations 
those relative to the dispersive forces, which philosophers hitherto 
have not examined, and which may by means of numerous precau~ 
tions be deduced from direct experiments. 
The work which the authors propose to publish on this subject is 
far advanced. They thought, however, that it might be useful to 
make known at present the results which they have drawn from 
their experiments on liquids and vapours. 
Mecanique Analytique, by J. L. Lagrange; new edition, re- 
vised and corrected by the author. Vol. If. Paris. Mad. V. 
Courcier. 
The editors, in a very short advertisement, give an account of 
the causes of delay in the publication of this second volume. M. 
