AMINATION OF MILK 55 



the dust came. In the cow stable many operations are carried 

 on that tend to raise a large amount of dust, which in many 

 cases comes from materials that are very rich in bacterial life. 

 The dried manure which is scattered over the floor, and in time 

 pulverized, is an important source. The dust raised whenever 

 cattle are brushed is largely made up of manure particles, 

 epidermal scales, and scurf from the skin. The operation of 

 feeding dry feed (hay, corn, fodder, etc.) is a source of dust, 

 for the way in which* these materials are harvested tends to 

 contaminate them with soil from the fields, and the surface 

 layers of the soil are especially rich in bacterial life. The 

 dust raised during the various barn operations is compara- 

 tively heavy, and for the most part soon settles, carrying its 

 load of germ life. 



The relative extent and importance of this source of con- 

 tamination under varying conditions may be determined as 

 follows: Tubes of lactose gelatin are melted, poured into 

 sterile Petri dishes, and allowed to become completely solidi- 

 fied. A number of these plates are exposed in the stable 

 when dust is present in minimum quantities, i.e. at a time 

 when no operation that produces dust has been carried on 

 for two or three hours. The remaining plates should be ex- 

 posed shortly after some dust-producing operation has been 

 completed. This exposure should be made by placing the 

 plates on a level surface, removing the cover,/ which is placed 

 right side up by the side of the plate, so as not to add a further 

 contamination when it is replaced. The exposures should be 

 made in different parts of the stable and the plates so located 

 as to be fully exposed to the falling dust, and yet be protected 

 from dirt being thrown on to them. A definite period of ex- 

 posure should be made, although this time may vary within 

 reasonable limits (thirty to one hundred and twenty seconds). 



