82 PUBLIC HEALTH CHEMISTRY 



months. In the milk trade a much more dilute solution 

 is used, namely I of formalin in 80 of water (2 oz. per 

 gallon) equal to 0-5 per cent of CH 2 O. Rideal states that 

 one-fourth of a pint of such a solution in 17 to 18 gallons 

 of milk (= i in 126 to 144) keeps the milk fresh for at least 

 three days, and does not give it any taste or smell. This 

 dilution is roughly I part of CH 2 O in 100,000. 



3. Salicylic Acid. Now rarely used in this country, is 

 still largely employed on the Continent. It is best detected 

 by Pellet's method : 100 c.c. of milk sample are diluted 

 with as much distilled water, and heated to 60 C. ; I c.c. of 

 acetic acid, and some mercuric nitrate are then added. 

 The resulting curd is filtered off and rejected. The filtrate 

 is repeatedly extracted with ether, the various extractions 

 are mixed, a portion is evaporated, the residue dissolved in 

 distilled water, and the solution tested with ferric chloride 

 solution. A violet coloration not discharged by acetic 

 acid, is positive. When present in considerable amount, 

 the direct addition of ferric chloride gives a pale brown 

 colour, and in the filtered whey a violet may be detected. 



4. Benzoates. Curdle the milk with acetic acid, extract 

 the whey with chloroform, neutralize carefully after dilu- 

 tion. Add ferric chloride, when benzoates give a buff- 

 coloured precipitate insoluble in acetic acid. 



5. Sulphites. 



(1) Add dilute phosphoric acid and heat gently ; observe 

 odour (SO 2 ). 



(2) Take 20 c.c. in a tube, add Zn and HC1. Place paper 

 soaked in lead acetate solution over mouth of tube and heat 

 latter gently. Darkening indicates presence of sulphites, 

 but a negative result is more reliable. 



Hydrogen Peroxide. First suggested by Budde : milk 

 said to be " Buddeized." To 10 c.c. of milk sample, add 

 i c.c. of i per cent, ortol solution (freshly made), when in 

 presence of hydroxyl a dull crimson colour is produced 

 unless the milk has been heated above 72 C., when the 

 addition of a little fresh milk is required. Paraphenylene- 

 diamine similarly gives a blue coloration. Schardinger's 

 reagent is decolorized by normal milk, but not by treated 

 milk. In the presence of organic matter hydroxyl splits 



