4 Knecht, An Interesting Reaction of Copper Salts. 



tested as described. Other substances such as tinned 

 vegetables may be similarly examined. 



The reaction does not lend itself to the estimation of 

 traces of copper by colorimetry, since comparison by trans- 

 mitted light is useless for small quantities. Even if a means 

 of comparing the precipitates by reflected light (which 

 would involve devising and fitting up of special apparatus) 

 could be devised, it is doubtful whether such a method 

 would take the place of the recognised and more convenient 

 ferrocyanide test. 



In conclusion, I may be permitted to draw attention 

 to the remarkable behaviour of an acid solution of titanous 

 chloride towards mercuric chloride. In consequence of 

 the powerful reducing action of the former, one would 

 expect instant precipitation of metallic mercury, but 

 although such precipitation does take place on boiling, 

 no change is noticed in cold solutions beyond the forma- 

 tion of a very slight quantity of a crystalline precipitate, 

 the composition of which I have not ascertained. Metallic 

 mercury vigorously shaken with a solution of titanic 

 chloride, yields a small amount of the titanous salt and 

 the reaction is therefore a reversible one, as in the case of 

 copper, but this fact nevertheless does not explain the 

 non-precipitation of mercuric chloride in the cold. 



