M, C. Lea on a Theory of Photo-chemistry. 73 
dent and appreciable in the case of iodid of silver, that we may 
ence obtain views subsequently to receive a wide extension. 
Now when pure iodid of silver is exposed to light, it changes 
slightly in color, and has acquired a new property, that of at- 
tracting to itself a metallic precipitate in the act of forming, or 
a metallic vapor already formed. Some have seen in this action 
of light upon the iodid, a distinct reduction to sub-iodid. But 
of this, proof is altogether wanting. An extension of the ex- 
posure many million fold does not produce a reduction appre- 
ciable by the most delicate reagents, and I have been enabled to 
prove that such iodid perfectly recovers its esata in the 
ark, at is, a film of such iodid, exposed for many hours to 
a er sun does not further darken beyond the change pro- 
duced by the first instants of diffuse light. And if then put 
aside in the dark for a brief time it re-acquires the capacity by 
exposure for a second, to receive an image. 
ut if, during this exposure, or subsequent to it, a body capa- 
ble of combining with iodine be brought into contact with it, a 
true chemical decomposition takes place. The silver loses half 
of its iodine, is reduced to sub-iodid, and a vapor, or a precipl- 
se goed favorable circumstances, is attracted to the parts so 
upon 
What then is the nature of this change, this impression, 
received in a second, and then slowly passing spontaneously 
ped ? Evidently a physical, not a chemical change, ough 
l g the way to a chemical change. But what is that physi- 
When light falls upon a compound body, its molecules are 
if ied to a ce 
if a certain note be sounded near it with sufficient force, will 
bes , the excursion of its atoms exceeding the limits 1m- 
would resist this and much greater strain. Just so with light. 
bodies will be decomposed, others will resist. So when 
ure chlorid of silver is exposed to the light, it presently assumes 
f of its chlorine. 
to ee i 
ight, no chemical change takes place. But the impression 
light is for a time oraleiaed Now the analogy which exists 
between this effect and phosphorescence has not been before 
ceived. The “ physical” impression of light rs a persistence o 
is tcal’’) lel see pees 
visible or luminous rays, in phosphorescence. e vibrations ex- 
~~ by light are in ‘both cases not given out instantaneously 
J : 
