478 ANNUAL EECOED OF SCIENCE AND INDUSTRY. 



off the solution into a very dilute solution of ammonia and 

 bromide of cadmium and returning it to the plate, when a 

 vigorous negative image will appear, formed of silver from 

 the reduced bromide. Treatment with dilute nitric acid 

 will convert this into a positive. Many modifications and 

 improvements have been projDosed, and are in course of trial. 

 Stillman, in England, and Newton, in this country, have done 

 much in this direction, while Mr. Carey Lea has shown that 

 the presence of iodide of silver is not at all incompatible 

 with a good working emulsion, and doubtless all compounds 

 that were found valuable in the old processes will be found 

 applicable in this. It is not unreasonable, also, to expect 

 that it may be made equal in sensitiveness to the w^et proc- 

 ess. An emulsion of gelatin and the bromide, as first sug- 

 gested by Dr. Maddox about the same time that the collo- 

 dion emulsion was first formed, is far more sensitive than the 

 latter. The removal of the soluble nitrates formed by wasli- 

 ing was originated by Mr. King in connection with this proc- 

 ess, lie placed the gelatin solution, charged with the prop- 

 er quantities of silver nitrate and potassium bromide, in a 

 dialyser, by which all the crystallizable salts, added in ex- 

 cess or formed in the solution, were removed. The dialyser 

 Was found to be entirely unnecessary by Mr. Kennett, who 

 simply poured the gelatin, mixed with the proper salts, into 

 a flat dish, and placed the film in pure water, after it had set 

 but not become dry, until all the crystallizable salts were ex- 

 tracted. The pellicle formed on drying this, cut into shreds, 

 forms the basis of a very sensitive emulsion, and can be trans- 

 ported to all parts of the world in opaque packets. It sim- 

 ply requires the addition of warm water to fit it for use. It 

 is, however, unsuitable for work in warm w^eather, and the 

 emulsion is liable to putrefaction, so that it must be used 

 soon after its preparation. In the multiplication of impres- 

 sions upon paper, photographically, from the negative, per- 

 haps no fact has been more annoying to the photographer 

 than the decided liability of the prints to fade and deterio- 

 rate. And yet the silver-printing process retains its place 

 almost universally. The carbon process affords equally beau- 

 tiful and undoubtedly permanent pictures, and is more flexi- 

 ble in its character; but it does not seem, in any of its mod- 

 ifications, to have reached the simplicity and certainty re- 



