THE FILTRATION OF CULTURES. 



73 



pared must be as short as possible, otherwise the bacteria 

 will multiply, and again an erroneous idea of their number 

 be obtained. When samples have to be taken for transport 

 to the laboratory, these are best collected in four-ounce, 

 wide-mouthed, stoppered bottles, which are to be sterilised 

 by dry heat (the stopper must be removed during sterilisa- 

 tion, otherwise it will be tightly held by the neck of the 

 bottle). In using such a bottle it is best to immerse it in 

 the water, and then remove the stopper with forceps. 

 Plates must be prepared from such a sample as soon as 

 possible. 



Filtration of Cultures. For many purposes it is neces- 

 sary to filter all the organisms from fluids in which they may 

 have been growing. 

 This is especially 

 done in obtaining 

 the soluble toxic 

 products of bacteria. 

 The only filter cap- 

 able of keeping back 

 such minute bodies 

 as bacteria, is that 

 formed from a tube 

 of unglazed porce- 

 lain as introduced 

 by Chamberland. 

 There are several 

 filters, differing 

 slightly in detail, 

 all possessing this 

 common principle. 

 Sometimes the fluid 



FIG. 27. Geissler's vacuum pump arranged 

 ith manometer for filtering cultures. (The 

 tap and pump are intentionally drawn to a larger 



is forced through ta P and P um P are mtentlonal| y drawn to a lar s er 

 . scale than the manometer board to show details. ) 



the porcelain tube. 



In one case the filter consists practically of an ordinary spigot 

 screwed into the top of a porcelain tube. Through the 

 latter the fluid is forced and finds itself in a chamber 

 formed by a metal cylinder which surrounds the porcelain 



