178 BACTERIOLOGY. 



gelatin melts at about 25°. In warm weather, as in sum- 

 mer, gelatin plates cannot be developed at the prevailing 

 room temperature. In order to keep the gelatin solid, it is 

 customary in that case to set the plates in an ice-chest. 

 Inasmuch as the temperature in the ice-chest is about 10°, 

 the growth of the colonies will be materially retarded, and 

 this at times may lead to annoying delays. 



The nutrient gelatin, as commonly prepared, contains 

 10 per cent, of gelatin. Unless otherwise specified, this is 

 assumed to be the composition of the medium when "gela- 

 tin " is mentioned. Occasionally a nutrient medium con- 

 taining 15 per cent, of gelatin is employed. This melts at a 

 higher temperature than does the ordinary gelatin, and 

 hence, is specially made use of during warm weather. It 

 should be borne in mind, however, that the cultural charac- 

 teristics of an organism may vary considerably with the 

 hardness of the medium in which it is grown. Atypical cul- 

 tures will not infrequently be obtained when an organism is 

 grown on hard gelatin. 



The apparatus shown in Fig. 33 is employed in this laboratory, 

 to cultivate bacteria at a constant low temperature, regardless of that 

 which prevails in the room. It will be found as useful for this 

 purpose, as the incubator is for higher temperatures. It has the 

 merit of being simple and inexpensive. The apparatus is made of 

 galvanized iron or of copper. The outer box is 22 cm. high, 68 cm. 

 long and 35 cm. wide, while the inner compartment is 18 x 53 x 27 cm. 

 The narrow tube (1.2 cm. diameter) to the left stops short, on the 

 inside of the box. It is connected directly with the water-supply pipe, 

 and not by means of a narrow faucet. Next to this inflow is the wide 

 outflow tube (2 cm. diameter), which passes along the bottom of the 

 box to the opposite end, where it is turned upward. A short rubber 

 tube is slipped over the end of the upright tube, and, by moving it up 

 or down, the height of the water can be regulated at will. The bent 

 wide tube on the side is a safety over-flow. The inner box is held in 

 place and kept from floating, by means of two stout rods. The water 

 flows into the large box, under and around the inner compartment, 

 and then leaves by the outflow and overflow tubes. 



The temperature in the inner compartment will depend upon the 

 rate of ilow of the water, and on the temperature of the water. With 



