BACTERIOLOGICAL STUDY OF WATER. 595 



Note. — Make cover-slips from your own mouth ; 

 make plates on both gelatin and agar-agar, at the same 

 time. Compare the number of bacteria 

 found by microscopic examination of 

 the cover-slips with the number of 

 colonies that develop on the plates. 



For the collection of samples from 

 the deeper portions of streams, lakes, 

 etc., a number of convenient devices 

 have been made. A very satisfactory 

 apparatus has been made for me by 

 Messrs. Charles Lentz& Sons, of Phila- 

 delphia. It consists of a metal frame- 

 work, in which is encased a bottle 

 provided with a ground-glass stopper. 

 To the stopper a spring clamp is at- 

 tached, and this in turn is operated by 

 a string, so that when the weighted 

 apparatus is allowed to sink into the 

 stream the stopper may be removed 

 from the bottle at any depth by simply 

 pulling upon the string. When the 

 bottle is filled with water the stopper 

 is allowed to spring back into position 

 by releasing the string. The whole 

 apparatus (depicted in Fig. 103) is pro- 

 vided with a weight that insures its 

 sinking, and a heavy cord by which it may be lowered 

 and raised. It should be sterilized before using. After 

 collecting the sample the bottle should be wiped dry 

 with a sterilized towel. Before removing the stopper 

 the mouth of the bottle should be rinsed with alcohol 



Bottle for collecting 

 water. 



