METHODS OF SAMPLING MILK 29 



When this is used as a milk preservative, it is a wise 

 precaution to add a little coloring matter to the milk 

 in order to warn every one of its abnormal character. 

 Corrosive sublimate, mixed with coloring matter, is 

 put up in convenient tablet form and has found exten- 

 sive use in preserving composite samples. All things 

 considered, it is probably the most satisfactory of the 

 preservatives commonly employed. 



Formalin is a liquid containing about 40 per cent, 

 of the chemical compound known as formaldehyde. It 

 is *an effective antiseptic and has the advantage of 

 being in liquid form. One cubic centimeter of forma- 

 lin should keep a pint or quart sample of milk two 

 weeks or more. Formalin possesses the disadvantage 

 of so hardening the milk-casein that it is not as readily 

 dissolved by sulphuric acid (see p. 63) as is the casein 

 of untreated milk. An excessive use of corrosive sub- 

 limate may produce a similar hardening of casein. 



Bichromate of potash, also called potassium bichro- 

 mate, is extensively used in preserving samples of 

 milk for testing. It is best to use it in powdered form. 

 It has the following advantages : ( i ) It is compara- 

 tively inexpensive. (2) It colors milk yellow and thus 

 shows its presence. (3) It is not a very violent poison, 

 though not entirely harmless. (4) It is efficient in 

 keeping milk for one or two weeks. However, it has 

 some disadvantages as a preservative of composite 

 samples of milk: (i) If too much bichromate is used, 

 the solution of the casein in sulphuric acid is some- 

 what difficult and the final results of testing may not 

 be clear. (2) In hot weather, it is often difficult to 

 keep samples without using an excessive amount of 



