BACTERIOLOGICAL TECHNIQUE 



29 



been sterilised by passing through the flame, is dipped into the 

 solution to be examined and then inserted into the gelatine 

 to about half the depth and then withdrawn, the plug of 

 cotton wool again singed and then replaced. Such a culturs 

 is known as a stab culture, and is chiefly useful when inocu- 

 lating from a pure cultivation. If the culture is a mixed 

 one, the gelatine is melted before removing the cotton-wool 

 plug, by allowing the tube to stand for a few mmutes in a 

 beaker of water which has been heated to a temperature 

 some ten or twenty degrees above the melting-point of the 

 gelatine. After inoculation and r:iixing the culture with 

 the melted gelatine, the latter is poured into a sterile 

 Petri dish. 



The gelatine is allowed to set in the Petri dish, which is 

 then placed in a moist chamber. 

 The latter is a similar glass 

 vessel of a much larger size, in 

 which some moist blotting-paper 

 or a small Petri dish of water 

 has been placed. 



In order to accelerate the 

 growth of organisms on the 

 gelatine in the Petri dish it 

 may be necessary to place the 

 latter in an incubator. 



The incubator consists essen- 

 tially of a water-] acketed cham- 

 ber heated by a gas flame, the 

 size of which, and consequently 

 the temperature produced, can 

 be very exactly regulated by a 

 thermostat. A very satisfactory 

 form of incubator is the Hearson 



incubator shown in Fig. 7, though less expensive arrange- 

 ments are obtainable. A set of instructions for adjusting the 

 temperature of the Hearson incubator is issued with the 



Fig. 7. — Heakson Inoubatoe 

 wrrH Theemo - Rbqulatok. 

 {Messrs. Flatters & Ganiett, 

 Ltd.). 



