the various refracting bodies presents a very extensive scale, 
{ 292 | 
4 
XLVI. Upon the Affinities of Bodies for Light; and pare 
ticularly upon the refractive Powers of different ite ial 
[Concluded from p- 158.] 5 
Mv A> authors ¢ of the memoir now Relate us commence the 
second part of their work by a very ingenious and just ob- 
servation. ‘ The action of bodies upon light,” they say, 
“¢ is not exercised in a sensible manner except at very short, 
distances; the intensity of this action is necessarily con- 
nected with the nature of the particles of the bodies, and 
with their arrangement; that is to say, with their most in- 
timate properties : so that the philosopher who observes the 
refractive powers of substances in order to compare them 
with each other, acts exactly like the chemist who presents, 
one and the same base successively to all the acids, or one 
and the same acid to all the alkalis, in order to determine 
their respective powers and their degrees of saturation. In 
our experiments the substance we present to all the bodies 
is light, and we compute the action which they exercise 
upon it by their refractive power; that is to say, by the in- 
crease of effect which the action of their particles tends to A 
impress on them. 
‘© There is here a particular advantage, which is not to, be 
met with in the same degree in any chemical experiment; : 
it is the almost inconceivable intensity of the action of i 
bodies upon light, an intensity which sometimes goes the ; 
length of impressing upon it, in an infinitely small instant ¥ 
of time, a velocity double of that which it has in space ; i 
and which at least always modifies it in a sensible manner, i 
even in bodies of the weakest refractive power.” 
The diversity of the velocities impressed upon light by 
upon intermediate points of which all these may find a place 
at great intervals, and which thus presents a method, of 
distinguishing them by this particular character, and even. 
of pursuing them, in some measure, in various combina- 
tions. 
*® From Billioth. Brit. vol. xxxii. 
+ Messrs. Biot and Arrago. 
Fhe @ 
