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 

 (e.g., the cellar) and another one in the winter (e.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 los. 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 i os. 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 



