LABORATORY WORK 305 



teat of a cow into a sterile test tube. Now milk the cow about one- 

 half dry and draw 6 more jets in a second sterile tube. Remove 

 to laboratory and plate at once in plain gelatin, diluting 10 times. 

 Compare the numbers and kinds of colonies in the two sets of plates. 



26. Bacteria on the Hands. — Wash the hands thoroughly 

 in 200 c.c. of sterile water. Place 1 c.c. of this water in a tube 

 of melted agar. Count the colonies and calculate the number of 

 bacteria removed from the hands by washing. 



2J. Estimation of Dirt in Milk* — Procure some milk from 

 a clean and carefully kept dairy and also some from a dairy 

 where the milking is done in a careless, slovenly fashion. Deter- 

 mine the amount of dirt in the milk in each case, as follows : 



a. First method. Place a quart or a liter of each sample of 

 milk in a tall glass cylinder, and allow it to stand for several hours. 

 A deposit of dirt will collect at the bottom, which may easily be 

 seen by looking through the glass. With a siphon carefully re- 

 move the milk to within % inch of the bottom, but do not disturb 

 the sediment. Fill up with clean water and allow to settle again.. 

 Once more, after settling, siphon off the liquid, and fill the glass; 

 with water. Repeat this operation several times, until the water be- 

 comes fairly clear. Then, after allowing the material to settle again, 

 siphon off a considerable portion of the liquid, and after thoroughly 

 mixing the sediment with the remainder of the water in the cylinder, 

 pour it through carefully weighed filter paper; wash the filtrate by 

 means of water through the filter paper. Remove the filter paper 

 and dry, until it comes to a constant weight. Weigh. The differ- 



, ence in weight between this and the original weight of the filter 

 paper will give the weighed amount of dirt collected. 



b. Second method. Place a definite quantity of milk, usually 

 10 c.c, in one of the glass tubes of a centrifugal machine. Set the 



* This experiment is impossible without a chemist's balance and apparatus for drying, 

 such as used in quantitative chemistry and analysis. 



