402 THE POPULAR SCIENCE MONTHLY. 



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 ammo- 

 nia, and adds sufficient gelatine when the emulsion is ripened. Dr. 

 Eder's method was to add the full amount of gelatine with the am- 

 monia. Mr. Cowan's method gives greater rapidity and greater cer- 

 tainty. 



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 act- 

 ing 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 sen- 

 sitiveness of a gelatine plate, I should show you, not some of those 

 marvelous instantaneous photographs, but a photograph by Mr. Hen- 

 derson, 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. 



