Om the Affinities of Bodies for Light, ee, 293 
*Thé authors of the memoir, having ascertained by their 
experiments the powerful action of hydrogen upon fight, 
naturally infer from this, that it is the presence ofthis prin- 
‘ciple in water, in gums, oils, and other inflammable sub- 
stances, which gives them that great refractive power which 
Newton has so well observed. This refracting influence of 
hydrogen is eminently conspicuous in ammonia, which ts 
composed of hydrogen and azote; the refractive power of 
this gas being double that of the air, and surpassing that of 
water. 
The great velocity, and the extreme tenuity of the mole- 
cules of licht, give it a particular advantage in this mode of 
research, where it is employed as a re-agent, the greater or 
less degree of condensation of the constituent principles of a 
‘body having little influence upon its refractive power in 
comparison with that produced by the affinity of these same 
principles with light, excepting certain extraordinary cases 
in which the condensations are very considerable. In every 
case, by multiplying the refractive power of each principle 
by the ponderous quantity which enters into the combina- 
tion, the sum of these products gives the refractive power 
of the compound. 
In a simple mixture, without intimate combination among 
the component parts, the refractive power observed in the 
compound is exactly equal to that which the calculation 
gives according to the proportion of the constituent princi- 
ples; thus the refraction of the common air is exactly equal 
to-what ought to be produced by a mixture of 0-21 of oxy- 
gen in volunfe with 0°787 of azote and 0-003 of carbonic 
acid. By calculating the refraction according to the rela- 
tive quantity of thesé principles, we obtain it as exactly as 
by direct observation. : 
This law holds in combinations in which cbbribiienrions 1s 
not very strong: for instance, in ammoniacal gas, m which 
the constituent principles, the azote and the hydrogen, are 
reduced to one-half of their total volume by the effect of 
condénsation, the refraction observed is exactly what agrees 
with a mixture of 0°S05 of azote (in weight), and of O° 195 
T-3 of 
