No. 4.] GRADING OF MILK. 163 



in addition to other precautions concerning the health of his 

 cows. 



Modern control over milk has as its chief basis the bacterial 

 testing of milk. There has been some unnecessary mystery- 

 concerning the bacterial testing of milk, especially in the minds 

 of milk producers. As a matter of fact, such tests are rather 

 simple. The various steps can be easily mastered by any 

 bright high school boy or girl. Milk samples must first be 

 taken in properly sterilized sample bottles. These are usually 

 of small size, so that a large number may be conveniently 

 packed in a sampling case and surrounded with cracked ice. To 

 make bacterial testing of practical value to the milk industry 

 it is necessary that the work shall be done on a large scale, 

 and numerous samples tested in a short period of time. This 

 can be done only by instituting the most efficient and simple 

 methods for laboratory work. The milk samples and glassware 

 for testing should be arranged on long tables, high enough to 

 permit the work to be done in a standing position. The opera- 

 tor first shakes the milk sample thoroughly to obtain a perfect 

 mixture. A portion is then drawn up in a small glass tube, 

 called a pipette, to an exact mark measuring 1 cubic centi- 

 meter, or about one-third of a teaspoonful. This is then in- 

 jected into a bottle holding 99 cubic centimeters of sterilized water, 

 which gives a dilution of 1 per cent. The mixture of milk and 

 water is then thoroughly shaken. From the bottle of mixed 

 water and milk a sample is then drawn in a sterile glass pipette to 

 an exact mark amounting to 1 cubic centimeter, and discharged 

 into a small glass dish, called a petri dish, which has been 

 previously sterilized. Upon this is then poured a jelly paste of 

 agar, which is a Japanese seaweed mixed with a beef-tea solu- 

 tion of proper strength. This jelly must be carefully mixed by 

 shaking the dish with the dilution. In a few moments it will 

 harden in the bottom of the glass dish, thereby fixing fast in the 

 jelly all the bacteria which may be in the milk. The dish is 

 then placed in a hot closet, called an incubator, at the same 

 temperature used for the incubation of hens' eggs, where it re- 

 mains for a period of two days. When first placed in the hot 

 closet the appearance of this glass plate is perfectly transpar- 

 ent. If, however, bacteria are present in the milk, at the end 



