TiCHBORNE — On the Dissociation of 2Mecules in Solution. 231 



potassium. Solutions of the yellow or neutral clironiate of potassium, 

 and also tlie red, or acid, chromate of potassium, were made of such a 

 streno-th that 10,000 parts contained an equivalent of each salt. In 

 other words, the yellow solution of chi'omate of potassium contained 

 194-5 parts of salt in the 10,000 parts, and the red cliromate contained 

 147"5 parts in the 10,000, but each solution contained the same amount 

 of the colour-producing molecule, or the chi'omic acid, only in different 

 conditions of satui'ation. If we take 1000 parts of the yellow solution 

 of chromate of potassium, we find that, even at the ordinary tempera- 

 ture, lo'o C, partial dissociation has occurred. Presuming that we 

 have started with perfectly neutral crystals, we shall find that, on the 

 addition of a volumetric solution of soda, the colom- becomes lighter 

 until a quantity has been added which represents -059 of red chromic 

 salt • therefore' we are justified in coming to the conclusion that at a 

 temperatru'e of 15-5, sufficient of the acid is dissociated from the neu- 

 tral salt to form that quantity of the acid salt — that in fact there is 

 even a slight or partial dissociation in the act of solution. If this 

 neutral solution is then brought to the boiling point, and we use 

 some considerable bulk for the experiment, we shall find that a marked 

 decomposition has taken place in the salt, as evidenced in the change 

 of coloiu". In a few carefully conducted experiments it was found 

 that it gave a decomposition which averaged about ^Vth of the yellow 

 salt present; e.g., 1000 parts of the solution of the yellow salt, men- 

 tioned above, were brought rapidly to the boil in a flask furnished with 

 a long neck, so as to condense the steam which flowed back into the 

 flask. Another 1000 parts of the yellow solution was then placed into 

 another flask, and at a temporatiu'e of 15-5 C. A standard solution of 

 sulphuric acid was added, degree by degree, until the exact colour 

 was obtaiaed, which agreed with the boiling solution of chromate. The 

 standard solution contained one-half an equivalent in 10,000 parts of 

 sulphuric acid, and fifty-one degrees had been used, which corresponded 

 with 0-75 parts of the red salt formed, and which also represented 

 •51 parts of chromic anliydiide dissociated at a temperature of 100° C. 

 from 19-45 parts of the salt. 



If the neutral solution of yellow chromate be now inclosed in a 

 sealed tube (such a one as described in my previous paper*), and sub- 

 mitted to a high temperature, say 250° C, it would appear that about 

 75 per cent, of the yellow chromate will be converted into the red 

 salt.f The experiments in connexion with chromates may throw some 

 considerable light upon one of those mysteries of the photographic art 

 which up to the present time has not been properly or satisfactoiily 

 explained. One, if not the most important, method of taking sun- 

 pictures is based upon the action of light upon a film of gelatine, contain- 

 ing a little red chromate of potassium. Such a mixture is highly 



* 1. c, p. 2.50. 



t Chromic acid is said to fonn fouj salts with potassium ; KjOCrOj, Kx02CrOs 

 KjOSCrO,, and Ki04Cr03. 



