THE CYANIDE OF ZINC AND MERCURY 327 



used in weak solution (i part to 4,000) along with the gentian violet in the water 

 in which the cyanide salt was diffused. Bichloride of mercury interferes, to 

 a certain extent, with the precipitation of the gentian violet, and, leaving some 

 of the dye in solution, causes tinting of the gauze independently of the presence 

 of the cyanide salt, and at the same time it impairs the efficacy of the dye in 

 fixing the salt to the fabric. Yet the use of the bichloride of mercury is a matter 

 of great importance, for reasons which I have given elsewhere,^ and it became 

 necessary to look for some other dye on which the bichloride might not exert 

 this prejudicial influence. I have found that there are several colouring matters 

 which answer the purpose fairly well. Thus both carmine and prussian blue attach 

 the cyanide salt to a cotton fabric perfectly so long as it is moist, but when it is 

 thoroughly dry they are not very good as regards the question of dusting. The 

 dye which I have found to compty best with all the requisite conditions is logwood, 

 or rather the essential ingredient of logwood — haematoxylin, which is a definite 

 crystalline substance, and not unduly expensive. 



The manner in which I have found it best to use this substance is the following. 

 It is incomparably better to apply it to the freshly precipitated and wet cvanide 

 than to mix it with the salt after its particles have been aggregated in the process 

 of drying. It may be well to mention here the manner in which the cvanide 

 is prepared. Cyanide of potassium, cyanide of mercury, and sulphate of zinc 

 are mixed together in solution in quantities proportioned to the atomic weights 

 of 2KCy, HgCy._„ and ZnS04 + 7H^O; the cyanide of potassium and cvanide 

 of mercury being dissolved together in 1^ oz. of water for ever}^ 100 grs. of 

 potassium cyanide, and added to the sulphate of zinc dissolved in three times 

 that amount of water. The precipitate is collected on a strainer, and when 

 well drained is washed with two successive portions of water, equal in quantity 

 to that used for the solutions — viz. 6 oz. for every 100 grs. of potassium cyanide ; 

 at least this amount of washing being essential in order to free the precipitate 

 sufficiently from the highly irritating soluble salts which are associated with 

 it in its formation. The precipitate having been thus washed and drained, but 

 not dried, it is thoroughly diffused with pestle and mortar in distilled water 

 (6 oz. for every 100 grs. of potassium cyanide), containing in solution i part of 

 haematoxylin for every 100 parts of the cyanide salt, the amount of which is 

 known from the circumstance that the dry product of cyanide salt is almost 

 exactly equal in weight to the potassium cvanide employed. Haematoxylin 

 is readily soluble in a small c|uantity of hot water and remains in solution when 

 added to a large quantity of cold water. The cvanide salt, while it precipitates 

 the haematoxylin, changes its colour to a pale-bluish tint. This is advanta- 



' Vide Lancet, loc. cit. (p. 309 of tliis volume). 



