2 BULLETIN 951, IJ. S. DEPAHTMENT OF AGRICULTURE. 



cultures made from the diseased tissues are usually incubated at about 

 the same temperature as that mentioned for the tubercle bacillus. 

 There are, however, certain thermophilic organisms that may be en- 

 countered in bacteriological investigations of diseased tissues, and in 

 order to be able to investigate these organisms smaller compartments 

 are provided. These are located jnside the larger incubating com- 

 partment and are furnished with separate heating devices. The sys- 

 tem used for controlling the temperature is identical with that of the 

 larger room, the controlling device being adjusted to maintain the 

 temperature at a higher jjoint. 



CONSTRUCTION OF TEMPERATURE-CONTROL COMPARTMENT. 



In the control of temperatures the proper construction of the com- 

 partment is of equal importance with that of the controlling ap- 

 paratus. The walls, floors, and ceiling should be so constructed as 

 to minimize the transfer of heat from the inside to the outside of the 

 room, thus economizing on the quantity of heat required to maintain 

 the desired room temperature. Not only will the proper construction 

 of the room reduce the quantity of heat necessary to maintain the 

 desired temperature, but should the supply of heat be discontinued 

 the insulated walls of the room will assist in holding the temperature 

 near the desired point for a considerable time. Figure 1 shows the 

 rate at which the temperature falls when the source of heat is entirely 

 cut off. The curve shows that for a period of approximately one week 

 the drop in temperature was only about 6° C. The temperature sur- 

 rounding the incubator room — that is, the temperature in the labora- 

 tory during this time^ — averaged about 21.1° C. It is obvious, there- 

 fore, that in a well-installed compartment cultures may continue to 

 grow several days after the supply of heat has been discontinued. 



The dimensions and construction of incubating compartments used 

 in laboratories of the bureau at Washington are illustrated in figure 

 2. The finished inside dimensions of the rooms are 7 feet wide by 7 

 feet 6^ inches deep by 8 feet 5 inches high. The capacity of each 

 room, in terms of standard 300 c. c. flasks, is about 2,600. In all 

 there are 7 rooms of these dimensions. 



CONSTRUCTION OF ROOM. 



Floors. — On the laboratory floor one course of 1-inch cork board 

 was laid down in hot asphalt. All transverse joints were broken and 

 all joints were slushed with hot asphalt and made tight by butting 

 the sheets of cork board closely together. The top surface of the cork 

 board was flooded with hot asphalt about one-eighth inch thick, and 

 directly on top of this a wearing concrete floor was laid, the thick- 

 ness of the floor being 2 inches in the center of the room and 3 inches 



