146 BACTERIOLOGY. 



dishes are about 8 cm. in diameter and about 1.5 to 2 

 cm. in height, the sides being vertical. They may 

 readily be sterilized either by hot air or steam. They 

 are very useful for this work, as they do away with the 

 necessity for the cooling-stage and levelling-tripod, 

 though in warm weather the cooling-stage may be used 

 to hasten the solidification of gelatin. A cooling-stage 

 of very convenient design for use with these dishes 

 consists of a closed, flat metal box, either of copper or 

 block tin, and round or square in shape, so arranged 

 that it can be filled with cold water, or that cold water 

 can constantly be passed through it by means of a 

 rubber tube connected with a spigot. The inlet for the 



FIG. 26. 



Petri double dish, now generally used instead of plates. 



water should be just above the bottom of the box, and 

 the outlet just beneath the top and slightly turned 

 upward and then downward, so as to insure filling the 

 space with water. The box should be sufficiently strong 

 to resist the pressure of the water. A convenient size- 

 is from 20-25 cm. in diameter and about 1.5 to 2 cm. 

 high. It is simple in construction, and can be made by 

 any copper-spinner. An idea of its construction is 

 given in Fig. 27. 



When gelatin or agar-agar is to be cooled, it 

 is only necessary to place the dishes containing il 



