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34 THE ADVANCE OF PHOTOGRAPHY
is different from that described on p. 33. They are not developed by precipitating silver which adheres to the exposed spots, but by a process of reduction acting directly on the exposed portions of the bromide of silver and reducing it to metal. This was effected by washing with a very powerful reducing agent, such as pyrogallic acid mixed with ammonia (known as the alkaline or chemical developer), or by potassio-ferrous oxalate.
This emulsion process with bromide of silver was an important step ; still, the sensitiveness of the plates thus obtained was not high enough for them to compete successfully with the wet plates. This was not achieved till a new vehicle was adopted for the application of the bromide of silver — namely, gelatine. By dissolving gelatine and bromide of potassium in warm water, and adding a solution of silver, bromide of silver is formed, which remains suspended in the mucilaginous fluid. This, when cold, becomes rigid, and can then be cut up and washed with cold water to remove the superfluous salts ; warmth again causes it to dissolve, and it is applied to the glass plates in a horizontal position. The film soon sets, but it takes much longer to dry than a collodion film — twenty-four hours, or more.
This process would scarcely have excited any attention as compared with the use of collodion, but that it was discovered that the sensitiveness of these plates under certain conditions was as great as that of wet plates, and that this sensitiveness of the gelatine emulsion could be greatly increased simply by long warming. In fact, plates thus prepared were more than ten times as sensitive as collodion plates. This, which was discovered by Bennett in 1878, soon made gelatine plates extremely popular.
The plates were first of all manufactured on a large scale in England, and afterwards on the Continent.
This chapter has been devoted almost exclusively to the historical development of photography. Many of the