1382 THE PROGRESS OF PHOTOGRAPHY. 
Strangely enough, this very fact interfered materially 
with their general use. Up to this time, photographers 
had habitually manipulated their plates under an amount 
of illumination which would prohibit the art at the 
present day. It took some years and much contention 
to convince the photographic world that rapid plates, of 
which Kennett’s were the forerunner, could only be ma- 
nipulated in a chamber from which all of the actinic 
rays of light were rigorously excluded. 
In 1874, M. J. 8. Stas, a celebrated Belgian chemist, 
published in the columns of a Belgian periodical a con- 
tribution to the chemistry of photography which gave: 
the clue to the secret of the present system of emulsion 
making. Stas, however, was not a photographer, and it 
does not appear that his researches attracted the atten- 
tion of any who were working in the photographic field. 
His statement was, and he clearly pointed out, that bro- 
mide of silver can exist in at least six well-defined physi- 
cal states, each having properties peculiar to itself. 
These are: 1st. White flakes. 2d. Yellow flakes. 
3d. Intense yellow powder. 4th. Pearly white powder. 
5th. Yellowish white powder. 6th. Pure intense yellow 
crystalized state. 
He said : ‘‘ The granular bromide, either dull or shin- 
ing, and the pearly white modification of it resulting 
from the action of boiling water on the first two, are the 
most sensitive substances to light with which I am ac- 
quainted.”’ 
Here was the clue, viz.: the action of heat. No one 
seems to have acted on this suggestion until four years 
later, when a Mr. Bennett, another amateur photog- 
rapher, of London, exhibited a number of gelatine nega- 
tives before the South London Photographic Society in 
1878. One of these was the interior of a room taken by 
ordinary gaslight after an hour’s exposure. Another 
represented a boat in motion—exposure one-twentieth of 
S82 
