138 



LABORATORY EXERCISES IN BACTERIOLOGY. 



At the lower end a small roll of fine wire gauze ; above this a layer of absorbent cotton ; 

 over the latter a layer of fine sand about three or four centimeters deep; and above 

 this, about the same thickness of a fine powder, as chalk, talcum, soapstone, pumice, 

 porcelain, or glass. 'Thus arranged, the apparatus is sterilized in the oven 'by pro- 

 longed baking and then a small amount of sterile water is poured into it to moisten 

 the different layers and cause the powder to become well packed. It is now attached 

 to the tap by a sterile rubber tube (bound tightly with copper wire), the water turned 





FIG. 41. COLLECTION FILTER, FOR USE IN CONCENTRATING BACTERIA FROM A LARGE 



AMOUNT OF WATER. 



A. Wire gauze. B. Cotton. C. Sand. D. Powder. Note recurved entrance tube at top 

 of filter to prevent force of water current from disturbing the powder in filter below. 



on in a gentle stream and a vessel of known capacity placed beneath by which the 

 amount of water which has passed through the filter may be measured. After a 

 suitable quantity has been filtered (ten or twenty gallons) the apparatus is detached 

 and the powder and sand shaken down into a measured quantity (five hundred cubic 

 centimeters) and well diffused in it. A known quantity of the latter, turbid with 

 grains of sand, powder, and bacteria, is then transferred to a tube of liquefied gelatine, 

 in which it is diffused by agitation and the medium thus inoculated is plated in one 

 of the usual manners. After growth (usually about three days) at room temperature 



