68 , A MANUAL OF BACTERIOLOGY 



changed. At least one incubator is required, and it is 

 convenient to have two or three. If there be only one 

 the regulator should be set for a temperature of 37 C. ; 

 if more, another should be kept at about 20 C. The 

 incubator at 37 C. is termed the warm or blood-heat, 

 and that at 20 C. the cool or room temperature one. A 

 warm room or cupboard will serve most of the purposes 

 of the cool incubator. A third incubator set for 42 C. 

 is useful for water examination, and a fourth at 25 C. for 

 fermentation work. 



A substitute for the large and expensive incubator can 

 readily be devised. An ordinary chemical hot- water oven 

 may be employed, or simply a smaller tin set in a some- 

 what larger one, the interspace being filled with water ; 

 and, with a little scheming, regulators can be dispensed 

 with by making use of a small gas or lamp flame, varying 

 its size and distance from the bottom until the right 

 temperature has been attained. Gas is a great con- 

 venience, but if not available, regulating oil lamps can be 

 obtained to take its place. Electricity has also been 

 adapted for heating incubators. 



Gelatin will remain solid only at temperatures below 

 24 C., and cannot therefore be placed in the blood-heat 

 incubator without becoming for practical purposes a 

 fluid medium. Agar, however and this is one of its 

 most valuable properties does not liquefy below a 

 temperature of 97 r 99 C., though when once liquefied 

 it does not set again until the temperature has fallen 

 to about 45 C. Gelatin is therefore usually reserved 

 for use at low temperatures, while agar, blood-serum, 

 potato, and the fluid media can be used indifferently 

 either at low or at high temperatures. Agar is often 

 a better cultivating medium than gelatin, even at low 

 temperatures, probably because it is so much moister. 

 The growths in fluid media are usually of the nature of a 



