84 



METHODS OF CULTIVATION OF BACTERIA. 



and e plastered over with sealing-wax, which is pricked, while still soft, with a 

 very fine needle. The gas flame, when only the peephole is supplying gas, 

 ought to be suiBciently large not to be blown out by small currents of air. If 

 the pressure of gas supplied to a regulator varies much in the 24 hours a press- 

 ure regulator ought to be interposed between the gas-tap and the instrument. 

 Several varieties of these can be obtained. In all cases g ought to be fixed to 

 b with a turn of wire. Greater accuracy in regulation can be obtained if some 

 liquid paraiiSn (" albolene ") is deposited upon the surface of the mercury to the 

 depth of 3 or 4 mm. 



The varieties of incubators are, as w^e have said, numerous. 

 The most complicated and expensive are made by German 



manufacturers. Many of 

 these are unsatisfactory. 

 They easily get out of order 

 and are difficult to repair. 

 We have found those of 

 Hearson, of London, ex- 

 tremely good, and in pro- 

 portion to their size much 

 cheaper than the German 

 articles. They are fitted 

 with an admirable regu- 

 lator. In America, manu- 

 facturers will be found who 

 can supply incubators the 

 equal of any of the foreign 

 makes on the market. It is 



Fig. 48. — Hearson 's incubator for use at 37° C. 



preferable in using an incubator to connect the regulator with the 

 gas supply and with the Bunsen by flexible metal tubing. It is 

 necessary to see that there is not too much evaporation from the 

 surface of cultures placed within incubators, otherwise they may 

 quickly dry up. It is thus advisable to raise the amount of water 

 vapour in the interior by having in the bottom of the incubator a flat 

 dish full of water, from which evaporation may take place. Tubes 

 which will require to be long in the incubator should have their 

 plugs covered either by india-rubber caps or by pieces of sheet rub- 

 ber tied over them. These caps should be previously sterilised in 

 i-iooo corrosive sublimate, and then dried. Before they are 

 placed on the tubes the cotton-wool plug ought to be well singed 

 in a flame. Or plugs may be impregnated with paraffin applied 

 at boiling heat, which thoroughly prevents the growth of moulds 



