METHODS OF EXAMINING MILK 259 



ring is formed. This test will detect 1 part of formalde- 

 hyde in 200,000 parts of milk. 



Leach's Test. The reagent used in this test is a solu- 

 tion of hydrochloric acid and ferric chloride consisting of 

 998 c.c. of hydrochloric acid of a specific gravity of 1.2, 

 and 2 c.c. of a 10 per cent, aqueous solution of ferric 

 chloride. Place about 10 c.c. of milk and an equal quan- 

 tity of this solution in a white cup or casserole and slowly 

 heat over a flame to boiling, giving the fluid a rotary 

 motion. If formaldehyde is present a blue or violet color 

 appears. 



Salicylic Acid. Salicylic acid is not often used to pre- 

 serve milk because it is not very soluble in that fluid; 

 0.04 per cent, will prevent milk from souring for 36 

 hours. The presence of salicylic acid may be detected 

 by the following test: 



Remonfs Test. Place 20 c.c. of milk in a measuring 

 cylinder or large test tube, add 2 to 3 drops of sulphuric 

 acid and 20 c.c. of ether. Mix by shaking and stand aside 

 until the ether separates and forms a layer at the top. 

 The fat of the milk and any salicylic acid present is 

 dissolved by the ether. The ether solution is drawn off 

 with a pipette, placed in a small crucible and evaporated 

 by floating the crucible in hot water. The residue is 

 rubbed up with a small quantity of 40 per cent, alcohol 

 and filtered and a few drops of ferric chloride solution 

 are added to the filtrate. A violet color appears if salicy- 

 lic acid is present. 



Benzole Acid. Like salicylic acid, benzoic acid is also 

 not very soluble, and consequently is not used very often 

 to preserve milk. The various tests for this substance 

 are all more or less complicated. 



Peter's test, as described by Leff mann, is as follows : 

 " The material is made slightly acid and extracted with 



