STAINS AND STAINING. 291 



evaporate in contact with metallic zinc, until it attains the thick- 

 ness of a syrup ; the syrup is then saturated with potassium 

 iodide, and subsequently with iodine. Another formula on the 

 same lines directs the evaporation to be stopped at the consistence 

 of strong sulphuric acid, which is a long way short of a syrup. A 

 third formula starts from dry (fused) chloride of zinc, and adds 

 water, but in none of these formulae is there any hint that the 

 slightest difficulty is likely to be experienced in carrying out the 

 directions. 



The first difficulty likely to be encountered is with the zinc 

 chloride. When a neutral solution of chloride of zinc is evapo- 

 rated to a syrupy consistence it loses hydrochloric acid, becomes 

 basic, and precipitates oxychloride on being largely diluted with 

 water. When evaporated to a solid and fused, this dissociation is 

 very much greater ; if, in addition, the evaporation has been 

 carried out as directed over an excess of metaUic zinc, the product 

 contains such a quantity of oxide that the addition of water 

 results in a mud rather than a solution. The pharmacopoeia 

 process for making liquor zinci chloridi is on exactly the same 

 lines, with the result that on dilution one dispenser sends out a 

 muddy solution, another filters and sends out a clear one, a third 

 adds hydrochloric acid, and the whole results in a correspondence 

 in the Chemist and Druggist. 



The proper remedy is to neutralise the liquor zinci chloridi 

 with hydrochloric acid before making up to the final volume ; the 

 point taken being either when the solution ceases to precipitate 

 on being diluted with ten volumes of water, or when this diluted 

 solution just reddens methyl orange. 



Supposing that we start with a fairly neutral solution of zinc 

 chloride, and try to saturate it with potassium iodide, one of three 

 things will happen, according to the strength of the solution. If 

 very strong, sp. gr. 2 '2 (syrupy), it will scarcely dissolve any of the 

 salt, and although a quantity is taken up on heating, and the 

 solution is quite permanent, it has positively no staining power 

 whatever. If to four of the syrupy be added two of water, the 

 potassium iodide dissolves more freely, but in a day or two the 

 bottle becomes half filled with crystals (principally a double 

 chloride of zinc and potassium). If, however, to three of the 



