32 AIDS TO BACTERIOLOGY 



The Incubator. The pathogenic bacteria grow best at 

 the temperature of the body of the host, and for their 

 culture an incubator with a temperature of 37 C. is 

 employed. Many of the saprophy tic forms will not develop 

 at so high a temperature, and they are cultivated either in 

 a warm room or in a ' cool ' incubator, at about 22 C. 

 The incubator consists essentially of a double-walled 

 chamber, the space between the walls being filled with 

 water warmed by a gas-burner. The outer wall is covered 

 with some non-conducting material. In the Hearson 

 incubator a regular temperature is secured by an Excelsior 

 gas-valve, in which the pressure of ether and other vapours 

 is employed in a flexible envelope; this, acting upon a 

 lever, controls the gas-supply. Page and Reichert thermo- 

 stats are also used, where a fall in temperature occasions 

 the contraction of mercury and allows more gas to pass, 

 while if too hot, mercurial expansion ensues, and by par- 

 tially cutting off the gas, diminishes the flame. In case 

 the main gas opening should become closed by the expan- 

 sion of the mercury, a by-pass allows the maintenance of 

 a pilot light. 



The ' cool ' incubator is similar in principle, but is sur- 

 mounted by a vessel containing ice. The regulation of 

 temperature within the chamber is effected by a small 

 stream of water, which runs continuously through the 

 apparatus in one of three directions, the choice being 

 automatically determined by a thermostatic capsule. A 

 third incubator, giving a temperature of 42 C., is useful 

 for the culture of typhoid and colon bacilli in water, etc., 

 examinations. 



Centrifuges. For the removal of fine particles from 

 suspension, centrifugal force is employed. The Gerber 

 machine, as used in milk analysis, when fitted with centri- 

 fuge tubes, answers well, and special machines are made 

 for the purpose. For small quantities of material, haema- 

 tocrites are used. 



Inoculating Needles. For the transference and spread- 

 ing, etc., of material, pieces of platinum wire fused into 

 glass, or fixed into aluminium rods, are used. For ordinary 

 purposes O4 millimetre (27-28 B.W.G.) wire is suitable, but 

 the greater stiffness of a 0*7 millimetre wire is sometimes 

 necessary. Needles, both straight and with loops (of vary- 

 ing sizes up to 4 millimetres in diameter or bigger) are used. 



