160 CASEIN. 



alkaline solution of casein. It does not seem probable that 

 insoluble, free casein and silver nitrate react with formation 

 of silver caseate and nitric acid; and both the behaviour of 

 the finished paper and the results of wet tests speak against 

 the occurrence of this reaction. The reaction is more prob- 

 ably confined to the silver nitrate, the chloride and the citric 

 acid alone, silver chloride and citrate being formed in the 

 casein stratum, which itself remains chemically unaltered. 

 The whole behaviour of the stratum favours this assumption, 

 which is very difficult to prove chemically, both the casein 

 and silver caseate being insoluble ; for whilst the stratum 

 obtained by means of casein, alkali and silver nitrate is 

 soluble when the print comes to be fixed in hypo, etc., that 

 obtained in the manner just described remains perfectly 

 intact and tough. The typical purple-brown colour of silver 

 caseate could not be detected anywhere, the print having 

 the same bluish or reddish purple tone as the gelatino- 

 chloride or collodio- chloride papers. At the same time, the 

 character of the prints more nearly resembled that of 

 gelatine papers than albuminised papers. Alkali caseates 

 and the water-soluble egg albumen of albuminised paper, on 

 the other hand, react on silver nitrate to form silver al- 

 buminate and silver caseate respectively. (An analogous 

 reaction may be mentioned in the behaviour of citric acid 

 and silver nitrate, inasmuch as alkali citrate and silver 

 nitrate furnish silver citrate in almost quantitative amount, 

 whereas none is formed in the case of the free citric acid 

 and silver nitrate.) 



In this manner the utilisation of casein as a medium for 

 photographic pellicles may be claimed to have been accom- 

 plished, inasmuch as a stratum of free casein is obtained 

 which encloses the image-forming substances, silver chloride 

 and silver citrate. The use of this method should certainly 

 not be restricted to the preparation of chloride printing- out 



