12 DUST AND ITS DANGERS. 



This food medium usually contains some 

 form of gelatine. The gelatinized material is 

 usually melted when the planting is being done, 

 and when it cools the bacteria are held firmly 

 in the position in which they lodged when they 

 were put in. 



The bacteria placed under these conditions 

 multiply with such great rapidity that usually 

 in a short time the progeny of a single living 

 germ will have accumulated to such a degree 

 right in the spot where the germ lodged 

 that the mass of them, which we call a " col- 

 ony " will be readily visible to the naked eye, 

 or under a low power of the microscope (see 

 Fig. i). Now since we can readily see the 

 mass of bacteria which has grown where only 

 a single germ had lodged we have only to 

 count the colonies to know how many living 

 bacteria were present in the volume of air 

 or fluid which we have tested. 



We can now, futhermore, subject the little 

 colonies which form our bacterial crop to a va- 

 riety of examinations and tests, and make out 

 what kinds there are, and further learn their 

 effects upon man or animals. 



