Font SY 207 
XXVI. Upon the Affinities of Bodies for Light; and parti- 
cularly upon the refractive Powers of different Gases*. 
Aprer some preliminary reflections upon the utility of 
studying the radical properties (if we may be allowed the 
expression). of different substances, taking them in the siate 
of gas, on account’of their greater simplicity in that form, 
and because they then belong to three sciences—pbysics, 
chemistry, and astronomy; the authors of the above me- 
moir proceed to detail the object of their experiments, and 
their method of investigation. 
We know that light, on passing from one transparent 
medium into another of a different density or chemical na- 
ture, experiences a deviation known in physics and astro- 
nomy by the name of refraction. Newton proved that this 
change of direction was owing to an attraction which bodies 
exercise upon the element of hght, and which acting only 
at very small distances completely resembles the chemical 
affinities. The principal idea of the authors of the memoir 
was to employ the physical fact of the deviation of light in 
different gases, in order to study its connection with their 
chemical nature ; they conceived the idea of applying this 
subtle element to the aériform substances, nearly as the che- 
mists apply their reagents to liquids ; an ingenious idea, and 
which became very fertile in their hands. | 
The first and most interesting of the aériform substances 
is, without doubt, that which envelops the terrestrial globe | 
by the name of atsnosphere, and preserves beat and life at its 
surface. It was also the first aériform substance examined 
by the authors. Two general processes present themselves 
in studying the refractions of the common air: the one (astro- 
nomical) consists in comparing the positions of the stars af- 
fected by refraction with their true positions, that is to sav, 
such as found by calculation: the difference is the effect of 
refraction, The other process is physical, and completely 
analogous to that in use for determining the refracting power 
* Extracted from a memoir read at the National Institute by Messrs. Biot 
and Arrago,. 
K4 of 
