PHOTOGRAPHY AND SOME OP ITS APPLICATIONS. 623 
salts. Among the thousands of combinations which chemists 
have effected in their laboratories_, there are some which^ when 
exposed^ as perfectly clear solutions^ to the action of lights 
undergo decomposition^ the base being set free in the insoluble 
form as a black or coloured deposit. Of these compounds^ 
the salts of silver (especially the nitrate) are the best known. 
When solution of nitrate of silver is exposed^ in contact with 
organic matter^ to the action of the sun^s rays_, the base (oxide 
of silver) separates from its acid as an insoluble black sub- 
stance. The common marking ink of the housekeeper is 
an apt illustration of this fact ; it is a solution of nitrate of silver^ 
usually sold in small black bottles_, which prevent the action of 
the sun^s rays upon the salt. When this material is placed upon 
linen^ and exposed to the lights it undergoes the usual change/ 
and leaves a blacky permanent stain upon the fabric. This is 
the simplest illustration of photography that can be given. 
But let us select another. If we take a piece of paper which 
has been wetted with solution of nitrate of silver (lunar 
caustic)^ and allowed to dry^ and having fastened closely to it 
a piece of fern^ expose it to the action of sun-light_, we shall 
find that all those portions of the paper which have been 
unshaded by the fern become blacky while the covered portions 
retain their ordinary colour_, thus leaving us an outlinear 
representation of a fern_, in white_, upon a black ground. If^ 
however_, having removed the fern_, we now again expose the 
paper_, our picture disappears^ and we have left only a black- 
ened sheet. What is the rationale of this ? When the paper 
was first exposed^ the nitrate of silver (with which it had been 
saturated) was decomposed by the sun-light in all those 
places where it was unprotected, leaving an area of undecom- 
posed nitrate, corresponding to that of the frond/-’ and 
when the sheet was a second time exposed in an unprotected 
state, this white space also was blackened. So far, the reader 
will say, the photographic process is a very inefficient one. 
Not so, however. It is evident that if, after the first expo- 
sure, we brought the paper to a darkened room, and then, by 
means of certain chemical solutions, washed out all traces of 
the undecomposed nitrate of silver from the clear unstained 
portion of the paper, our picture would then be an enduring 
one. Now, this operation, which is termed can actually 
be performed. A solution of hyposulphite of soda will dis- 
solve away all the undecomposed nitrate, and thus render the 
representation unalterably fixed. The term fixing-’-’ is fairly 
applied, as far as the general result of the process is con- 
cerned j but the operation is fundamentally one of removing 
the unaltered silver salt, and leaving that which has been 
already /icTed by the actinic (ray-acting) influence of the sun. 
2 T 2 
