i MILK 87 



By dissolving the copper in nitric acid, washing the tube and 

 asbestos with water and drying them, the filter can be used 

 again. As this method is based upon figures which have been 

 obtained empirically, it must always be carried out in exactly 

 the same way. 



(b) Cupric Oxide Method. 



This method of analysis is carried out exactly as the one 

 just described, save that the cuprous oxide is filtered on to a 

 small filter with aknown weightof ash. After thoroughly washing 

 with boiling water, the filter is dried in the oven, and the pre- 

 cipitate brought into a platinum crucible. The filter, to which 

 a little cuprous oxide is probably adhering, is burnt in the 

 usual way by the medium of a platinum wire, and the ash 

 added to the precipitate in the crucible. Two drops of nitric 

 acid are then poured on the mass and the crucible heated, at 

 first gently until the nitrate has decomposed, and then more 

 strongly until a red heat is obtained. When all the copper has 

 been converted into cupric oxide, which does not take very 

 long, the crucible is allowed to cool and then weighed. After 

 deducting the weight of the filter ash, the weight of cupric 

 oxide obtained is multiplied by 0'799, to convert it into 

 copper. An equivalent weight of milk sugar is obtained from 

 Table V. 



This method gives as good results as a reduction method, and 

 it is besides simpler and easier to carry out. 



2. Polarimetric Methods. 



The estimation of milk sugar by the polariscope had until 

 recently the reputation of being very unreliable. The cause of 

 this unfavourable opinion is to be sought in the failure to 

 understand the errors to which this method in its original form 

 is liable. Formerly it was believed, and the belief had only 

 very slight foundations, that there was present in the milk a 

 very small quantity of a dextrin-like substance, which 

 influenced the readings of the polariscope, and made them 

 uncertain. This view was upheld in comparatively recent 

 times by v. Raumer and Spath, 1 but since then A. Scheibe 2 



1 Zeitschrift fur angewandte Chemie, Vol. IX, 1896. 



2 Ref., p. 85. 



