14 DUST AND' ITS DANGERS. 



germs which by chance have been upon it. 

 The cotton if properly packed in the tube will 

 catch and hold entangled in its meshes all the 

 dust particles no matter how small, and with 

 these all the bacteria which were in the air 

 which we force through the tube. If now we 

 carefully pull out the cotton plug with a pair 

 of perfectly clean forceps, and thoroughly rinse 

 it off in a small clean flat dish containing our 

 bacterial food which we call " the culture me- 

 dium," the germs will be distributed through 

 the medium, and we cover the dish and set it 

 aside in a warm place and let it stand until 

 each living germ has grown and multiplied till 

 it forms a visible colony. Now we count the col- 

 onies, and the number represents the number of 

 living germs which were present in the whole 

 volume of air which we forced through the cot- 

 ton plug. There are of course many details 

 and precautions against error which must be 

 observed, but this brief description will suffice 

 for our purposes here. 



It has been found in practice, however, that 

 it is better to use fine sand than cotton in the 

 tubes to catch the germs, since this is more 



