RECENT DISCOVERIES IN PHOTOGRAPHY. 
399 
shreds, reduced to powder, or packed away in any other suitable 
form, ready for being dissolved when wanted. 
It is necessary, in order to obtain the best results, that 
organic matter be added to the emulsion. The kind recom- 
mended by Mr. Bolton is composed of forty grains of tannin 
dissolved in an ounce of an alcoholic saturated solution of soap, 
twenty minims of this being added to each ounce of emulsion. 
The development of the image is best effected in the follow- 
ing manner : — First wet the surface of the plate by pouiing 
over it methylated alcohol, followed by a rinsing with water. 
Next apply a four-grain solution of pyrogallic acid, which in 
the course of about a minute generally brings out a very feeble 
picture ; but at the end of this time, whether a picture be 
visible or not, pour off the developing solution into a vessel 
containing a few drops of greatly diluted ammonia (one drachm 
of ammonia to thirteen drachms of water), together with an 
equal proportion of a ten-grain solution of bromide of cadmium, 
and apply again to the plate. This will immediately bring out 
the image in great vigour, the silver of which the picture is 
formed being obtained at the expense of the bromide, which is 
reduced. 
At this stage the negative image may be converted into a 
positive. To effect this it is only necessary to apply diluted 
nitric acid, which dissolves metallic silver, but leaves the bro- 
mide of that metal unaltered. Now, as the opaque portions 
are composed of reduced silver, such parts are consequently 
denuded of bromide; hence the solvent action of the acid 
renders the glass more or less transparent in the exact ratio of 
its previous opacity. This principle is now being successfully 
applied in the production of transparencies and enlargements 
by a single operation. 
But simultaneous with, or rather previous to the successful 
working out of the interesting photographic process just 
described, Dr. B. L. Maddox had conceived the idea of emulsi- 
fying gelatine, instead of collodion, with bromide of silver. 
Having worked out his idea to a practical issue with that intel- 
ligent assiduity so characteristic of this gentleman, he had to 
withdraw from this pursuit, which, however, was taken up by 
others. It was soon found that when the gelatine was well 
charged with bromide of silver it was more sensitive than col- 
lodion ; but the crystallisable nitrates resulting from the decom- 
position by which was formed the bromide of silver precluded 
the possibility of fully utilising this quality, for the film was 
unable to retain these nitrates without undergoing disintegra- 
tion. Mr. King, to whom allusion has been made, effected the 
removal of the soluble salts by the well-known principle of 
dialysis. The gelatine having been liquefied, bromide of potas- 
