CHAPTER III. 

 INCUBATORS. 



Introduction. 



Section I. Devices for automatically regulating the temperature of incubators, p. 59. 



Section II. Incubators heated by coal gas, p. 61. 



Section III. Incubators heated by electricity, p. 65. 



Section IV. Incubators heated by petrol, gasoline, or petroleum oil, p. 66. 



AN incubator is a piece of apparatus designed to maintain cultures of 

 organisms constantly at any temperature which may be best suited to their 

 growth. 



The shape of the incubator, provided it be adapted to the size of the tubes, 

 flasks, etc., which it will have to receive, is generally speaking of little conse- 

 quence, though on the whole the rectangular form is the most convenient 

 because the space can be most completely utilized. 



Any metal box which had a door and could be heated by a convenient source of 

 heat could in an emergency be made to serve as an incubator. A rectangular tin or 

 copper box for instance, raised on a suitable stand, and heated by a [Bunsen burner 

 or] small oil lamp, placed below it at such a distance as to keep the temperature 

 within the box at the level required, would do quite well. But with such a rough 

 and ready piece of apparatus the difficulty would be to keep the temperature con- 

 stant, for quite apart from the question of the control of the heat supply the 

 temperature would be influenced by the temperature of the outside air. And in 

 practice these difficulties are so considerable that it has been found necessary to 

 design special forms of apparatus in which the temperature can be kept more fully 

 under control. These of course are more complicated and more expensive than 

 the simple arrangement just referred to, but are nevertheless indispensable if 

 satisfactory results are to be obtained. 



There are two essential points for which provision must be made in the 

 construction of an incubator. 



Firstly, the instrument must be protected as far as possible from variations 

 in the atmospheric temperature, and from loss of heat by radiation and con- 

 vection. 



Secondly, it must have some form of automatically acting regulator, which will 

 readily respond to variations of temperature. 



The former condition is satisfied by surrounding the outer surface with 

 some non-conductor of heat, e.g. wood or felt or a water jacket ; or, since 

 polished metal surfaces radiate heat very feebly, a veneer of brightly polished 

 copper will serve the purpose equally well. 



To get the most satisfactory results, the temperature throughout the 

 incubator must be as uniform as possible. If the incubator were an hermeti- 



