SUNDRY APPLICATIONS OF CASEIN. 159 



is obtained by the use of a large number of metallic salts, 

 such as alkali chlorides, alkali sulphates, alkali nitrates, a 

 number of salts of the heavy metals, etc. ; and this peculiarity 

 seemed to the writer to offer a prospect of success in using 

 casein as a medium for photographic pellicles. 



The success foreshadowed was eventually realised in the 

 following manner. When casein is stirred up with water 

 and dissolved in a weak solution of acid, e.g., citric acid, by 

 gentle heat, it forms a clear, gelatinous mass, which sets to a 

 jelly on cooling. Physically, it behaves exactly like gelatine, 

 and there is not the least difficulty in coating paper uni- 

 formly with the warmed solution. The coating sets im- 

 mediately and forms a glossy stratum, which, however, is 

 still slightly soluble in water. To make it insoluble in water 

 and weak acids, it is treated by floating or immersing the 

 paper in a solution of common salt or ammonium chloride, 

 etc., and this treatment does not deprive the stratum of its 

 permeability to aqueous solutions. At the same time, the 

 use of a chloride in this operation imparts to the stratum 

 the chloride necessary for the subsequent formation of silver 

 chloride, without eliminating the citric acid. It is easy, by 

 suitably modifying the concentration of the chloride solu- 

 tion, to incorporate any desired quantity of chloride in the 

 stratum ; and since, as already mentioned, the original 

 quantity of citric acid is left unaffected, or only washed 

 out to a very slight extent by the chloride solution, it is 

 possible in this way to adjust the relative proportions of 

 chloride and citric acid to any desired extent. The paper 

 prepared in this way will, of course, keep for any length of 

 time when dry. Sensitising is effected, the same way as for 

 albuminised paper, in a neutral or faintly acid silver bath ; 

 but the processes involved in the operation are evidently of 

 a very different character to those occurring in the case of 

 albuminised paper, or such as has been prepared with an 



