72 M. C. Lea on a Theory of Photo-chemistry. 
who showed that a very large number of bodies continued to — 
emit light for an appreciable time after the direct influence of 
light ceased to operate upon them. Although the time might 
be but a very small fraction of a second, still it was rende 
ssa evident to the sense, and the exact period could be 
en we consider the enormous rapidity with 
shah the phenomena of light take place, even the fraction of 
a second is a long time, and it would be exceedingly rash to at- 
tempt to limit such phenome ena to our powers of observation. 
Just as with heat there exists in all probability an absolute — | 
zero at which heat-vibrations cease, so probably there is a light _ 
zero at which the body ceases to vibrate luminous Most 
bodies (to our oat ng reach this zero immediately when 
carried into darkness. Phosphorescent bodies, form, however, 
a secking exception 
Let us suppose that a body be surrounded by other bodies 
equally illuminated, and that temporarily an additional quantity 
of light falls upon it. On the cessation of this illumination, 
the body will recover its condition of equilibrium with surround- 
ing bodies, by losing its excess of light in the following manner: 
1. By reflection. 2. y transmission. 8. By conversion into 
heat. 4. By chemical action. 5. By radiation 
That is, the body, if it be transparent, or have reflecting sur- 
faces, will part with a certain quantity of its light in those ways. 
If it — susceptible of chemical decomposition, a certain portion 
of light will be consumed in effecting that decomposition. And 
what farther loss is necessary to take place in order to reach an 
equilibrium, must take place by conversion into heat and by ra 
we have already seen, this radiation may be oa 
instantaneous, as in the case of most bodies, or it may re 
minutes, hours, or even days, as in the case of phoephoreoat 
bodies. This fact is of the utmost importance in the atte 
I here make to explain the phenomena of photo-chemistry. 
In their influence upon combustion and decomposition, the 
nomena of light and heat exhibit a striking parallelism. 
h tends in some cases, to promote combination, but in the 
vast majority of cases, to dissociate elements already combined. 
Such PE BE s is the action of light in the cases which I pro- 
pose to cousi | 
I have shown elsewhere, that, contrary to long-established _ 
opinion, perfectly pure iodid of silver, isolated from all other 
substances, is aanailicee| to light, and this fact, now I believe uni 
versally admitted, must form the corner-stone of Somme e 
try. For iodid of silver is pongo oe the only substance fitted to 
give us a clear view into the action of light upon matter in 
general, by which — this action is so much more evi | 
