MUENCK&S FILTER. 77 



matter. If the latter is allowed to accumulate the pores 

 become filled up. 



The success of filtration must be tested by inoculating 

 tubes of media from the nitrate, and observing if growth 

 takes place, as there may be minute perforations in the 

 candles sufficiently large to allow bacteria to pass through. 

 Filtered fluids keep for a long time if the openings of the 

 glass vessels in which they are placed are kept thoroughly 

 closed. Sometimes the fluids may be evaporated to dry- 

 ness in vacuo over sulphuric acid and kept in an air-tight 

 bottle in a dry state. 



Instead of being filtered off, the bacteria may be killed 

 by various antiseptics, chiefly volatile oils, such as oil of 

 mustard (Roux). These oils have no injurious effect on 

 the chemical substances in the fluid, and they may be 

 subsequently removed by evaporation. It is not practic- 

 able to kill the bacteria by heat when their soluble pro- 

 ducts are to be studied, as many of the latter are destroyed 

 by a lower temperature than is required to kill the bacteria 

 themselves. 



The Storing and Incubation of Cultures. Gelatine 

 cultures must be grown at a temperature below their 

 melting point, i.e. for 10 per cent gelatine, below 22 C. 

 They are usually kept in ordinary rooms, which vary, of 

 course, in temperature at different times, but which have 

 usually a range of from about 12 C. to 18 C. Agar and 

 serum media are usually employed to grow bacteria at a 

 higher temperature, corresponding to that at which the 

 organisms grow best, usually 37 C. in the case of pathogenic 

 organisms. For the purpose of maintaining a uniform 

 temperature incubators are used. These vary much in the 

 details of their structure, but all consist of a chamber with 

 double walls between which some fluid (water or glycerine 

 and water) is placed, which, when raised to a certain tem- 

 perature, ensures a fairly constant distribution of the heat 

 round the chamber. The latter is also furnished with 

 double doors, the inner being usually of glass. Heat is 

 supplied from a burner fixed below. These burners vary 



