146 
POPULAR SCIENCE REVIEW. 
A brief glance at the cause and nature of this decay or 
fading of photographs may not here be improper, seeing that it 
was the means of leading to* important results, to a descrip- 
tion of one of which we have devoted this article. 
The blacks of photographic prints on ordinary unsized 
paper consist of silver. To aid in the proper fixing of a 
photograph, or destroying its further sensitiveness to light, 
hyposulphite of soda in solution is employed. The action of 
this salt on the silver in the pores of the paper is of an ex- 
tremely complex nature, and long washing is requisite to 
secure its removal. If not thoroughly removed, an action 
continues to be exerted which ultimately results in the de- 
struction of the picture, the blacks of which are converted 
into a sulphide of silver. But the sulphurous gases with 
which the atmosphere is impregnated, joined to the complex 
effects produced by the albumen (with which photographic 
paper is usually prepared) acting on the silver in a manner not 
yet clearly understood, exert a destructive iufluence on photo- 
graphs. The introduction of gold-toning has mitigated this 
evil to a considerable extent, but an inspection of some recent 
pictorial productions of photographers of reputation suffices to 
show that it still exists, notwithstanding the known care taken 
by them to obviate it. 
It was this knowledge of the liability of silver prints to 
fade that induced Mr. Talbot, upwards of fourteen years ago, 
to search through the arcana of science for a more stable 
substance than silver of which to form the photographic 
image, his search being accelerated, as he informed the 
writer, by the fact that even the paste by which the pictures in 
his Pencil of Nature (the first illustrated photographic work 
ever published) were attached to the mounting board had set 
up a process of decomposition. 
The most stable substance which presented itself to him 
was carbon; but, eminently unaffected by light as it was, the 
question of how to utilise it in the production of a photograph 
was one that occupied much time and involved much labour 
in answering. The ink used by the engraver, he considered, 
was permanent ; and if means existed by which a photograph 
could be automatically engraved on a metal plate, then would 
the product of this plate be permanent when printed with 
a carbonaceous ink. Hence resulted a discovery of infinitely 
more importance than he himself could possibly have foreseen, 
from which have proceeded numerous ramifications, one of the 
latest and possibly most important of these being Woodbury’s 
method of photo-relief printing, to the elucidation of the prin- 
ciples and practice of which we now address ourselves. 
In his endeavour to obtain an engraved plate by means of 
