EXAMINATION OF MILK. 19 



freshly precipitated basic zinc carbonate made as follows : 2 g. of zinc 

 sulphate are dissolved in about 100 cc. of water, heated, sodium car- 

 bonate solution gradually added until the reaction is markedly 

 alkaline, and the precipitate washed free from sulphates and sodium 

 carbonate with hot water, first by decantation and then on the filter) . 

 The well-pressed air-dried zinc lactate, (C 3 H 5 O 3 ) 2 Zn+3H 2 O, contains 

 3 molecules of water of crystallization (18.17 per cent.), which is driven 

 off at 100 to 110. The determination of the amount of water of 

 crystallization serves to distinguish it from the zinc sarcolactate, 

 which only contains 2 molecules of water (12.89 per cent.). Drying 

 over sulphuric acid instead of in the air is inadmissible, since a part 

 of the water of crystallization is lost. Indeed by drying for fourteen 

 days over sulphuric acid the whole of the water may be given off. 



