134 Report of the Department of Bactekiologv of the 



In placing uninsulated wire so close to paraffined wood, there 

 might seem to be some danger of fire. This danger, however, is 

 more apparent than real. The wire used offers so much resistance 

 that even though the thermostat should fail to break the circuit 

 the temperature would not become extremely high; and if the im- 

 probable should happen and a sufficiently high temperature should 

 be reached, the renewal of oxygen through the cracks of the doors 

 would not be sufficient to support combustion. Even though it 

 were, there would be nothing to burn but the inside woodwork, for 

 cork-board is incombustible. As a further protection, however, 

 each length of wire has been broken at one point and soldered to- 

 gether again in such a way that if the solder melts the two ends of 

 wire will fall apart and break the circuit. Using these precautions, 

 there has been no indication of the slightest danger from fire during 

 a whole year of use. Compared to the danger of fire from a gas 

 incubator the fire risk here is wholly negligible. 



Each chamber is connected with the main circuit by means of 

 what is called deck cable. This double-wired cable passes through 

 a close fitting hole in the back of each chamber, where it is held in 

 place by means of cement (the same as used for the cork-board), 

 which was smeared upon the cable before drawing it into its final 

 position. Outside the incubators, the only electrical appliance is 

 the fuse-box FF. 



All external resistance is avoided; because with a voltage of 110 

 or 120 sufficient resistance is furnished by the heating wires to pre- 

 vent the passage of too powerful a current. The advantage of 

 this arrangement is obvious, for it allows practically all the energy 

 to be utilized within the chamber. The avoidance of external 

 resistance and the good heat insulation insure a low cost for main- 

 tenance. 



COOLING SYSTEM. 



There are three principal methods of cooling ordinarily used in 

 low temperature incubators. The first is a circulatory system of 

 either pure water or brine; the second is by means of a continually 

 flowing stream of cold water; while the third method is employed 

 in the Panum type of incubator where cooling is effected by direct 

 conduction from an ice-chamber through thin, uninsulated walls. 

 The first of these three, as it often requires an extra thermostat, 

 is quite complex; and the last method is very inaccurate; while 

 the second, provided a stream of sufficiently cold water can be 

 obtained, is both simple and accurate. Because of the difficulty in 

 obtaining such a stream during hot weather, this method is slightly 

 modified in the present incubator by using as a cooling medium the 

 drip-water from the ice-chamber. 



The 18° chamber alone is cooled. The cooling apparatus con- 

 sists of a one-inch brass pipe through which flows the drip-water 



