CULTIVATIONS AT HIGHER TEMPERATURES. 85 



ones, and precautions must therefore be taken to prevent 

 this from occurring too quickly. If the cultivations 

 are made in test-tubes, the cotton-wool plug may either 

 be soaked thoroughly in paraffin, or may be covered with 

 an india-rubber cap. In either case the end is gained 

 in a very simple manner. The air in the test-tube be- 

 comes saturated with water, and after that takes no 

 more from the nutrient medium ; it is true that some 

 vapour condenses on the sides of the tubes and then 

 mixes with the condensation water, and that new 

 vapour is again formed, but this is unimportant, and 

 has no effect upon the medium. Only when the culti- 

 vations are made in media which have solidified in a 

 slanting direction, and when they remain a long time 

 in the incubator, does the upper portion become some- 

 what dry, even when it is entirely shut off from the 

 outer air. The upper layer, which is in contact with the 

 air, of course always dries up somewhat, and on this 

 account when bacteria, which are very sensitive in this 

 respect, are to be cultivated, it is a good plan to add 

 a little glycerine (4-6^) to tbe agar-agar ; this keeps 

 the surface moist without interfering with the growth 

 of the bacteria. On the contrary, it appears that a 

 little glycerine seems to increase the nutritive powers 

 of the medium, since many bacteria, even in puncture 

 cultures, thrive especially well when glycerine is mixed 

 with the gelatine or agar-agar. 



As a rule, it is not necessary to make plate cultiva- 



