232 Cincinnati Society oj Natural History. 



walls of the containing vessel, sodium sulphate and finally 

 fine sand were substituted. It was found that the sulphate 

 exerted an inhibitory effect upon the colonies and was, there- 

 fore, undesirable. The sand worked admirably, but leads to 

 some trouble in counting the colonies. Still, where the col- 

 onies are well developed this difficulty is very slight. The 

 same aspirator was used as before for drawing air through the 

 apparatus. 



The filter was made from ordinary glass tubing of approxi- 

 mately ^-inch bore. This was drawn out at one end so as to 

 lessen the bore to 1-10-inch or so. In the neck thus formed 

 a small, loose, cotton plug was fitted. A layer of carefully 

 sifted sand (40-mesh) was placed upon this cotton, and another 

 cotton plug closed up the mouth of the tube or filter. The 

 whole is thus sterilized by dry heat, preferably in a glass box 

 from which the filter can be removed when wanted. When 

 used the filter is fastened in a clamp and attached to the aspi- 

 rator. It is desirable, also, that the clamps and other close- 

 lying parts be sterilized; this can be effected in a number of 

 ways: e. g., by washing with sublimate solution. When the 

 aspirator is started the cotton plug is removed. The bacteria 

 enter with the air drawn through the filter, but are held back 

 by the sand. The filter material can be added to any desir- 

 able medium. 



This method worked quite satisfactorily, and its simplicity 

 and cheapness would seem to recommend it for all ordinary 

 work. If care is taken to insure good suction and a steady 

 current, perhaps the results are as accurate as those obtained 

 with the most elaborate and expensive apparatus. 



The number of experiments or determinations made are 

 very limited, but perhaps of sufficient interest to warrant 

 their insertion here. It is thought, however, that a table 

 summarizing the data would be sufficient. Such a table 

 follows: 



22 



