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: 
Remont’s 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. 
Benzoic 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 Leffmann, is as follows: 
“ The material is made slightly acid and extracted with 
