202 M. C. Lea on the action of Light upon Iodid of Silver. 
Chemists are very familiar with the fact that in cases of slow 
precipitation, the precipitate tends to follow the path which the 
sed to stir the solution, has travelled over. The anal- 
ogy which this and the development of a latent image present 
cannot be missed, and they are brought closer still by the pres- 
sure images just described. In all cases a molecular alteration 
takes place, and the molecules thus altered seem to have their 
attraction for exterior objects increased, and for each other and 
tbe surrounding homologous particles diminished. 
This has been beautifully illustrated by some experiments 
lately published. A steel burnisher being drawn firmly over @ 
plate of glass, yet so as not to scratch it or abrade the surface, 
the glass on examination by polarized light showed its structure 
altered by a line of color where the burnisher passed. This 
molecular effect slowly diminishes, and in a few days the parti- 
eles return to their wonted state. 
The foregoing experiment beautifully illustrates the changes 
of molecular condition whic ies are capable of undergoing. 
I have just remarked, that by drawing a blunt glass rod over the 
inside of a glass vessel, the parts so treated attract to themselves 
of a latent photographic image, and the experiment just cited, 
reveals by polarized light a change of structure in those paths 
over which the body has been drawn. In the course of hours 
or days, this molecular change gradually passes away, and the 
_ body recovers its original condition, just as ave shown that 
when as in the case of pure iodid of silver, isolated, no reduction 
takes place, the effect of light gradually passes off, leaving the 
iodid free to receive a new impression. 
It seems almost superfluous to say that what is here stated of 
_ the development of pressure images is not given by way of ex- 
planation, but as illustration and confirmation of the general 
view expressed. 
Before concluding, a few words seem required as to what ex- 
= is to be given to the ordinary process of Negative 
graphy. 
In the foregoing I have reasoned on the action of light upon 
pure iodid of silver isolated. In the camera, light is made to 
act upon lodid of silver in a very different condition. It is in 
contact with nitrate of silver and with organic matter, and here 
t. 
of silver is presen 
' 
: 
" 
4 
