NUTRIENT JELL Y IN TEST- TUBES. 7 9 



charged, should be once more submitted to ten minutes steaming on 

 each of two successive days, by which is avoided the danger of con- 

 tamination by the atmosphere, during the manipulative process. 



The stock of test-tubes thus prepared is kept under observation for 

 seven or eight days before it is used as a cultivating medium. If any 

 organisms have gained access to the jelly in the process of prepara- 

 tion, they will become apparent during this period by altering its 

 appearance. In this way can be excluded any danger of the develop- 

 ment of foreign elements alongside the special cultures established in 

 the test-tubes, which would render useless any results obtained. The 

 method of inoculating the nutrient jelly is as follows : — A short piece 

 of platinum wire is mounted on a solid glass rod (see Fig. 21), and is 

 sterilised by heating to a white heat in the flame of a Bunsen burner. 

 It is then allowed to cool, without touching any free surface, whence 

 it might derive contaminating matter. After it is loaded with a 

 minute trace of the seed material, the organisms contained in which 

 it is desired to cultivate, it is introduced into the sterile tube. The 

 test-tube is held inverted in the left hand (§ 42, p. 68), and the plug of 

 cotton wool is twisted once or twice in the mouth of the test-tube, 

 to break down any adhesions between it and the neck of the tube 

 which might prevent its rapid removal. 1 The plug is then removed 

 from the mouth of the test-tube, and held between two of the un- 

 occupied fingers of the left hand (see Fig. 34), care being taken that 

 no part of it which passes within the test-tube comes in contact with 

 any source of infection other than the air itself; at the same time 

 this portion of the plug is directed downwards in order to avoid the 

 germs gravitating through the atmosphere. The platinum wire is now 

 plunged twice or thrice into the gelatine mass, and gently removed, 

 so that two or three linear tracts are produced in which the particles 

 of the inoculated material are deposited in a widely dispersed con- 

 dition ; and along the linear tracts thus produced, appearances may be 

 subsequently observed, which in many cases are characteristic of the 

 special kind of seed material which has been sown. The platinum 



1 If the plugs are dusty, it is well to burn the outer surface of the plug 

 by passing it rapidly through a flame before removing it from the test-tube. 

 The wadding burns very rapidly, and must be extinguished at once. 



