iv.] PREPARATION OF CULTURE-MEDIA. 37 



or two weeks, they remain free of any growth and are to 

 be considered sterile. 



Test-tubes containing solid nourishing material are 

 generally kept sufficiently inclined during solidification of the 

 material (see a former chapter) to allow the material to spread 

 into a layer of large area ; although useful, it is not essential. 



When test-tubes with sterile fluid blood-serum are to be 

 subjected to the process of solidification (see page 23), it is 

 advisable to keep the tubes in a slanting position, so as to 

 allow the serum to spread out into a layer which is suffi- 

 ciently transparent even after solidification. For this 

 purpose I place these test-tubes into a double-walled tin 

 trough, about five inches broad, twenty inches long ; between 

 the two walls and also inside the trough is water of the tem- 

 perature of 62 to 65 C. The test-tubes are placed side by 

 side, so that they rest with the bottom part on the floor, 

 with the top part on the opposite ledge of the trough. The 

 difference of level is such, that it allows the serum to spread 

 out in the test-tubes into a large area. By means of burners 

 the temperature of the water is then gradually raised to 

 68 to 70 C. 



In order to protect the solid materials, contained in test- 

 tubes, from drying up, it is advantageous to keep them in 

 large wide-mouthed bottles with well-fitting (by vaseline, 

 grease, or otherwise) glass stoppers. This is particularly 

 necessary in the case of test-tubes, containing Agar-Agar 

 mixture or blood-serum, and which are to be kept in the 

 incubator for many days or weeks at a temperature of 35 to 

 3 C. 



