GELATIN AND POTATO MEDIA. 185 



on a clean piece of paper; the ends should then be trimmed 

 off at rig-ht angles, and finally the cylinders should be cut 

 diagonally in two. The potato wedges, thus obtained, are 

 now slipped, with the wide end downward, into sterilized 

 tubes (Fig. 34 a). The cut surface of the potato is inclined, 

 and hence can be readily inoculated by means of a platinum 

 wire. The potato tubes, thus prepared, can be submitted 

 to fractional sterilization in steam, being heated for about 

 20 minutes each day on three successive days. Or, they 

 may be given a single steam sterilization of ^-1 hour. 



The method, as given above, will yield potato cylin- 

 ders which are pure white, and will remain so. If a raw 

 potato is punched or cut, the cut -surf ace, as a result of ox- 

 idation, will soon take on a light pink tinge and, on subse- 

 quent heating, this turns to a dirty gray. This alteration 

 in the appearance can be avoided by at once submitting the 

 cut potato to the action of steam. If the potato has been 

 cooked before hand, it will not oxidize pn subsequent ex- 

 posure. 



Instead of the ordinary test-tube, Roux employed one 

 which was provided with a slight constriction near the end 

 (Fig. 34 b). The wedge of potato, in this case, does not rest 

 on the bottom of the tube, but on the constricted portion- 

 The space below this may be filled with water, or with any 

 desired liquid. The surface of the potato can, therefore, 

 be moistened, whenever it is desirable to do so. When a 

 five per cent, solution of glycerin is employed, the potato 

 becomes an excellent soil for the growth of the tubercle 

 bacillus. 



The Roux potato tube is consequently very useful. It can be 

 prepared by the student without the least difficulty. The clean, 

 sterile, wide test-tubes should be used. A narrow flame from the 

 blast-lamp should be directed horizontally, against the test-tube, at 

 about one inch from the bottom. The tube should be held vertically, 

 and rotated slowly, so that the point of the flame just strikes it. 

 The glass gradually softens and the constriction results. 



