Page:Popular Science Monthly Volume 22.djvu/416

402 low temperature in a liquid condition for many days. Colonel Wortley afterward claimed that he could get the same sensitiveness by heating up to 150° Fahr. for a short time; and then Mr. Mansfield got it in a few minutes by boiling. Another method was then introduced by Dr. Monkhoven for the production of very sensitive gelatine emulsions by adding ammonia with the nitrate of silver. The ammonia process found many admirers, among them Dr. Eder, whose method of adding a large quantity of ammonia has given very sensitive pictures, and very vigorous ones when the sensitiveness is not too great. A process introduced by Mr. Cowan is even superior to that of Dr. Eder. He emulsifies his bromide in a very small quantity of gelatine with ammonia, and adds sufficient gelatine when the emulsion is ripened. Dr. Eder's method was to add the full amount of gelatine with the ammonia. Mr. Cowan's method gives greater rapidity and greater certainty.

What is the reason of the sensitiveness of the gelatine emulsion? Pictures can be taken with it in a tenth of the time necessary for a wet plate, and perhaps a thousandth of that necessary for an ordinary dry plate. The first reason is, that the emulsion has a blue form. Another reason is, that you can use a more powerful developer. If you separate bromide of silver which has been emulsified in gelatine, and place it in collodion, the extreme rapidity will be gone, for the simple reason that you can not use as strong a developer as you can with a gelatine emulsion; in fact, the property that gelatine possesses of acting as a physical restrainer comes into play: each little particle or aggregation of particles of the salt is surrounded by gelatine, which prevents the developer acting rapidly on them. Again, the fact that by boiling, or by the ammonia process, you get a coarser deposit of bromide of silver, also points to increased sensitiveness. Furthermore, if you boil or heat bromide, or any haloid salt of silver, with an organic substance, it has a tendency to separate into a metallic state; in fact, the bromide of silver is then in a state of very tottering equilibrium; the bromine is ready to be given off at the very slightest disturbance of the molecule, much more so than before it is boiled. I think that the fact that you so often get fogged emulsion when you overboil is proof of this statement. If you were to ask me to illustrate the sensitiveness of a gelatine plate, I should show you, not some of those marvelous instantaneous photographs, but a photograph by Mr. Henderson, by moonlight, and another of some under-ground cellars at Reigate, by Mr. William Brooks, taken by lamp-light. If anything can show what gelatine plates can do, it is the fact that candle-light and moonlight can be utilized for impressing the surface with an image. Dr. Vogel has recently introduced an emulsion made with acetic acid, gelatine, pyroxyline, and bromide of silver, which is very clean and very fairly rapid. Plates are more readily coated with it than with gelatine emulsion, but less so than with collodion emulsion.