THE ALUMNI JOURNAL 153 



pulp is also employed. The centrifuge is also used by one or two 

 manufacturers ; this, however, only removes the bulk of the precipi- 

 tate, and is usually employed for separating the large bulk of the 

 precipitate in the process of the manufacture. The liquid must be 

 passed through paper after this for complete clarification. 



IMPURITIES. 



The most likely impurity in peroxide is barium. This will not be 

 found, however, if sulphuric acid is present in a sufficient quantity, 

 as this will have removed any barium that may have gone into the 

 solution. Arsenic may be found in minute quantities. This usually 

 comes from using a cheap sulphuric acid. Hydrochloric acid, how- 

 ever, may be present, but hydrofluoric acid is seldom present in the 

 medicinal solution. 



PRESERVATIVES OE PEROXIDE. 



The most common preservatives for peroxide is acetanilid. For 

 many years this article was used by one or two manufacturers and 

 its employment jealously guarded as a secret. It was some time 

 before it became generally recognized by chemists, that the reason 

 some brands kept better than others was on account of the preserva- 

 tive used. Many of these chemists, in analyzing various brands, 

 overlooked acetanilid because this article is not recognized as a 

 preservative. Others made the mistake of examining too small a 

 quantity. By the concentration of four ounces or less a very minute 

 quantity of material is obtained to work upon. The first disinterested 

 chemist to discover acetanilid in a brand of peroxide concentrated 

 one gallon of the solution. The presence of acetanilid was deter- 

 mined rather by accident than by any prearranged system of ex- 

 amination. This man was perhaps more surprised to find acetanilid 

 present than was the first man who used it to discover that it was 

 a good preservative for peroxide. Only by repeated tests and the 

 checking of results, did he finally convince himself that he had found 

 acetanilid. When it became known that acetanilid was a preserva- 

 tive, it was an easy matter to extract it from a solution of peroxide 

 by shaking it out with ether and chloroform, evaporating off the 

 solvent at a low temperature. This procedure will remove about 95 

 per cent, of the amount present. The proportion of this article which 

 has been found to give the best results is about one-fifth of a grain per 

 fluid ounce. 



( To be continue a). 



