286 ILLINOIS STATE ACADEMY OF SCIENCE 



mass of AS2S3 owing- to the relatively greater active sur- 

 face. 



In the above discussion of the mechanism of the re- 

 action it is assumed that the HoS-pentathionic acid re- 

 action proceeds only in the intermicellular liquid, and 

 not in the adsorbed layer enveloping the AS2S3 particles, 

 since it is taken for granted that AsoSa and S-micelles, 

 respectively, retain the HoS and pentathionic acid ad- 

 sorbed as stabilizing electrolyte, ont of the sphere of 

 reaction with each other. The photochemical formation 

 of colloidal snlfnr and pentathionic acid takes place 

 only in the adsorbed layer. 



It is noteworthy to remark that the photochemical re- 

 action is not reversible, samples of the sol which had 

 been exposed to light suffering only a very slight de- 

 crease in conductance on being kept in the dark for per- 

 iods as long as several days. This slight decrease in 

 conductance is explained as being due to the fact that 

 the slow HoS-pentathionic acid reaction, which in itself 

 tends to lower the conductance by formation of H2O and 

 S from the active electrolyte, pentathionic acid, contin- 

 ues for some time after the action of light ceases, the 

 resultant effect being a slight diminution of the concen- 

 tration of the intermicellular pentathionic acid. 



There may be some question as to the nature of the 

 electrolyte formed in the photochemical process. The 

 fact that Freundlich and Scliolz have demonstrated the 

 existence of pentathionic acid in colloidal sulfur is not 

 to be taken as a priori evidence of the existence of the 

 same stal)ilizing acid for the colloidal sulfur formed in 

 AsoSa hydrosol. Whether the acid formed is penta- 

 thionic, tetrathionic or another of the thionic acids known 

 to exist in Wackenroder's solution, or a mixture of two 

 or more members of the family, is conjectural. It would 

 seem more plausible, possibly, to assume the formation 

 of tetrathionic acid, since this is undoubtedly the first 

 product formed in the preparation of Wackenroder's 

 solution. 



