190 BIOLOGY AND TECHNIQUE 



for the growth of gonococcus. Cohen and Markle 13 applied this to 

 meningococcus and found again that partial anaerobiosis favored 

 growth. It was later found by Wherry and Erwin, 14 as well as by 

 Cohen and Fleming 15 that the growth of a number of different 

 bacteria was favored when the air was partially replaced by carbon 

 dioxide. Gates 16 confirmed this, as did Kohman. 17 We have tried 

 this out in our own laboratory and, like other observers, have found 

 that, in the case of meningococcus and also with the influenza 

 bacillus, growth is definitely stimulated by replacing about 10 per 

 cent of the air with carbon dioxide'. Both Kohman, Gates and others 

 believe that the principle underlying this is due to the effect of the 

 carbon dioxide on the reaction of the medium. As the organisms 

 grow they produce acid which displaces equivalent amounts of dis- 

 solved carbon dioxide, in consequence of which the acidity is hot 

 materially increased. 



THE INCUBATION OF CULTURES 



After bacteria have been transferred to suitable culture media, 

 it is necessary to expose them to a temperature favorable to their 

 development. In the case of many saprophytes, the ordinary room 

 temperature is sufficiently near the optimum to obviate the use of 

 any special apparatus for maintaining a suitable temperature; in 

 the case of most pathogenic bacteria, however, the body temperature 

 of man, 37.5 C., is either a necessary requirement for their growth, 

 or at any rate favors speedy and characteristic development. . 



For the purpose of obtaining a uniform temperature of any 

 required degree, the apparatus in general use is the so-called incu- 

 bator or thermostat. This may be adjusted for gelatin cultivation 

 at 20 to 22 C., or for agar, broth, or other media at 37.5 C. 



Incubators, while varying in detail, are all constructed upon the 

 same principles. They consist of double-walled copper chambers, 

 which are fitted with a set of double doors, the outer being made 

 of asbestos-covered metal, the inner of glass. The space between 

 the two walls is filled with water, which, being a poor heat con- 



13 Cohen and Markle, Abst. of Bacter., 2, 1918, 10. 



14 Wherry and Erwin, Jour, of Infec. Dis., 22, 1918, 194. 



15 Cohen and Fleming, Jour, of Infec. Dis., 23, 337, 1918, 

 19 Gates, Jour, of Exper. Med., 29, 325, 1919. 



" Kohman, Jour, of Bacter., 4, 1919. 



