26 BACTERIOLOGICAL DIAGNOSIS. 
and remains constant for long periods whatever be the 
external temperature. It is hardly necessary for the 
practitioner to purchase one of these. Careful search 
in the house will usually reveal some cupboard or 
Fic. 10.—Incubator. 
corner in which the temperature will remain sufficiently 
near 20° C. for a sufficiently long period; it is more 
important that it should not rise above 22° C. than that 
it should not fall below 18° C., as the former tempera: 
