INCUBATION OF CULTURES. 27 
ture may melt the gelatin, while the latter will only 
delay the growth of the colonies. It will probably be 
necessary to find one such place in the hot weather 
(¢.g., the cellar) and another one in the winter (¢.g., a 
cupboard not far from the hot-water pipes). 
It is necessary that cultures which are being incuba- 
ted should be kept in the dark, as light is inimical to the 
development of nearly all bacteria. 
Incubation at the body temperature presents more 
difficulty. An incubator is almost essential when much 
work has to be done, and of these Hearson’s is by far 
the best. The smallest size costs about £6 1os. and 
is a thoroughly reliable and durable affair. Cheaper 
instruments are also procurable; the Edinburgh in- 
cubator (Alex. Frazer, 22, Teviot Place, Edinburgh) 
costs about £4 10s. complete. Foreign incubators can 
be obtained at an even lower price, but are not very 
durable. 
Much can be done without the use of so expensive an 
apparatus if the practitioner can find a room in which 
the temperature keeps approximately constant through- 
out the twenty-four hours. A tin biscuit box (or any 
other metal box) is covered with cotton-wool on the top 
and sides, the bottom being left bare, and mounted on a 
tripod stand.. It is heated by means of an ordinary 
night-light (two may be necessary if the weather is cold) 
shielded from draughts by means of a wide lamp- 
chimney or a tin cylinder made out of an ordinary 
canister. The temperature is observed by means of a 
thermometer projecting through a hole in the lid, 
and the night-light raised or lowered until the 
temperature reaches the desired figure. The whole 
apparatus should be placed on a metal tray containing 
a small quantity of water and put in the middle of the 
