GELATIN AND POTATO MEDIA. 175 



before. With a sterilized, cooled platinum wire, make three 

 successive transfers of gelatin from tube 2 to tube 3. Re- 

 turn the cotton plugs to their tubes, sterilize the wire, and 

 set the tubes aside in the tumbler. 



Each of the three gelatin tubes has now, been inocu- 

 lated. Tube 1 usually has a very large number of bacteria, 

 while tube 2 has less and tube 3 should have but a small 

 number, so that, subsequently, when colonies develop 

 these should be separated from one another by an appre- 

 ciable distance. This successive dilution, it will be ob- 

 served, corresponds to that performed when making potato 

 cultures. It is necessary to take a very minute amount of 

 material for the inoculation of tube 1 in order to to obtain 

 good dilutions. Moreover, working as the class does with 

 pure cultures, it will be sufficient to make transfers of only 

 one or two loopsful of gelatin, instead of three. 



In transferring gelatin from one tube to another, care 

 must be taken to prevent the wire from coming into contact 

 with the neck, or wall of the tube. Unnecessary contact 

 of the end of the glass rod with gelatin should be avoided, 

 and, if gelatin does cover the end it should be burned off 

 completely. 



The ice-plating apparatus should now be prepared for use. 

 Broken ice is placed in the glass dish, which is then filled with water, 

 so that when the plate is placed in position it rests directly on the 

 ice water. No air-space should be present. The apparatus should be 

 levelled and is then ready for use (Fig. 31). Instead of the ordinary 

 plating apparatus, one similar to that shown in Fig. 32 can be em- 

 ployed. It is made of galvanized iron or of copper, and is 15 cm. 

 (6 in.) high. The inflow tube {1 cm.) extends along the bottom to the 

 farther end of the box. The wide outflow tube (2 cm.) does not extend 

 inward. Tap-water is allowed to flow into the apparatus. It runs the 

 entire length of the box and thus reduces the temperature of the 

 surface plate to that of the water (about 15°). The box is 68.5 cm. 

 (27 in.) long, 28 cm. (11 in.) wide, and 7.5 cm. (3 in.) deep. 



Remove a sterilized glass plate from the iron box, by 

 grasping the edges with two fingers; place it upon the 



