238 BACTERIOLOGY. 



Ion. If it were added to the former, the agar would dis- 

 solve much more slowly than in clear bouillon, and, more- 

 over, the coagulated albumin would materially interfere 

 with the subsequent filtration. As it is, only the impurities 

 in the agar are to be filtered off. The agar is neutral in 

 reaction, and hence, it is added to the already alkalized 

 bouillon. 



The filtered agar is filled, by means of a small funnel, 

 into plugged, sterilized test-tubes, and these are then ster- 

 ilized by steaming for 30 minutes, on each of three succes- 

 sive days. As many tubes as are necessary for immediate 

 use are inclined by placing on a thick glass rod, or narrow 

 strip of wood, or on the gas tubing. The small test-tubes 

 (12 X 150 mm.) are preferable for agar. The remaining 

 tubes are kept in the upright position, and are inclined 

 whenever it is necessary. Agar may be used, either, in the 

 inclined position, for streak cultures, or in the upright posi- 

 tion, for stab cultures. It may be mentioned incidentally 

 that inclined gelatin may often be employed to advantage. 



The agar solidifies in this inclined position. A large 

 _ surface can thus be obtained, which can be used for making 

 a streak culture, or even for the isolation of colonies. The 

 streak culture is made, as in the case of those on potato 

 (p. 186), by drawing the inoculated wire along the middle 

 of the inclined surface. In describing the use of the potato 

 tube (p. 186) it was shown that these could be used for ob- 

 taining isolated colonies. The same method can be applied 

 to the inclined agar. It enables one to obtain isolated 

 colonies in less than twenty-four hours, and that, without 

 the use of any special plating apparatus. A very small 

 amount of the material is taken up, on a platinum wire, 

 and spread over the entire inclined surface of an agar tube. 

 The same wire, without sterilization, is then rubbed over the 

 inclined surface of a second tube, and then in like manner 

 over a third and fourth. The tubes are then placed in an 

 incubator to develop. 



