EXAMINATION OF MILK. 7 



which contains a narrow strip of filter-paper moistened with 

 a solution of basic lead acetate; blackening due to the forma- 

 tion of lead sulphide (Siegfried). 2. Grind together 0.1 to 

 0.2 g. of the substance with thirty times its weight of oxidiz- 

 ing mixture (3 parts of potassium nitrate and 1 part of sodium 

 carbonate), and heat the mixture slowjy in a crucible or small 

 dish until it is completely fused and all the carbon has been 

 burned. The sulphur is oxidized to sulphuric acid, which 

 forms alkali sulphate. After cooling dissolve the fused mass 

 by warming with some water, filter in case the solution is not 

 perfectly clear, acidify with hydrochloric acid, and add barium 

 chloride solution. If the substance contained sulphur, a pre- 

 cipitate of barium sulphate will separate at once, or after some 

 time. Since the test depends upon the conversion of sulphur 

 into sulphuric acid, the substance and the reagents must of 

 course contain no sulphates. 



This method is not adapted to detect traces of sulphur: in this 

 case we must evaporate the solution of the fused mass to dryness 

 several times with hydrochloric acid in order to drive out the nitric 

 acid. If we then add water to the residue, we sometimes find the 

 solution is cloudy, due to the presence of silicic acid. It must then 

 be filtered again, since clearness of the solution is absolutely essential. 

 The sulphur may also be detected by heating to red heat with sodium 

 (or potassium), sodium sulphide being formed. In many cases it is 

 sufficient tc heat with sodium carbonate. See in this connection the 

 " Detection of Sodium Sulphide" in the chapter on "Bile," section 

 "Taurin." 



2. Calcium Phosphate (F). 



The calcium phosphate is washed with water, then dis- 

 solved by pouring on the filter 20 cc. of dilute hydrochloric 

 acid (1 part hydrochloric acid, 2 parts water). The filtrate 

 is frequently somewhat cloudy, "but it may be rendered clear 

 by allowing it to stand and by filtering several times, or also 

 by repeatedly pouring it back upon the filter. The greater 



