THE CULTIVATION OF MICRO-ORGANISMS 121 



prefer rubber stoppers, which may be boiled and stored in bi- 

 chloride of mercury solution. Cut the cotton plug even with the- 

 edge of the tube; singe it in the flame; push it into the tube about 

 i cm., and insert the rubber stopper (Fig. 44). 



Low-temperature Incubator. An incubator regulated for so- 

 called "room temperature" is very desirable for the cultivation 

 of bacteria upon gelatin and for the bacteriological analysis 

 of water. In our climate the temperature of the rooms of the 

 laboratory often reaches a point at which gelatin melts, and 

 for this reason in a low-temperature incubator provision has to 

 be made for cooling when the room temperature is too high as 

 well as for heating when it is too low. 



A form of incubator devised by Rogers 1 for this purpose 

 consists of a refrigerator or of a specially constructed chamber 

 heated by electricity and controlled by an electric thermoregu- 

 lator. Below is given a description of an incubator constructed 

 according to Rogers' plans. This incubator has been in use 

 for some time and has given entire satisfaction since the pre- 

 cautions noted below were followed. There would appear no 

 reason why this incubator should not be employed for high 

 temperatures as well as for low, but so far it has been run at 22 C. 

 The temperature has kept very constant. The incubator con- 

 sists of a refrigerator, 30 inches high, 24 inches wide, 18 inches 

 from front to back, all outside measurements. Instead of the 

 ordinary drip pipe, there is a coil of i-inch galvanized iron pipe 

 run down the back of the cooling chamber attached water-tight 

 to the ice tank. From the bottom of the cooling chamber the 

 coil runs up perpendicularly nearly to the bottom of the ice 

 compartment, and then runs horizontally through the wall of the 

 refrigerator. A bracket on the outside supports a drip-pan. 

 A thermometer encased in a fenestrated metal jacket is inserted 

 about half way up on one side. A lump of ice, about 50 pounds, 

 placed in the ice compartment serves to keep the tem- 



1 L. A. Rogers. On electrically controlled low temperature incubators. Cen- 

 tralblatt fur Bakteriologie, etc., Bd. XV, Abt. II, pp. 236-239, Sept. 23, 1905. 



