256 METHODS OF ISOLATION 



Bacillus typhosus, whilst the second will be an almost 

 pure culture of the Bacillus aquatilis. 



6. Differential Sterilisation. 



(a) Non-sporing Bacteria. Similarly, advantage may 

 be taken of the varying thermal death-points of bac- 

 teria. From a mixture of two organisms whose ther- 

 mal death-points differ by, say, 4 C. e. g., Bacillus 

 pyocyaneus, thermal death-point 55 C., and Bacillus 

 mesentericus vulgatus, thermal death-point 60 C. a 

 pure cultivation of the latter may be obtained by heat- 

 ing the mixture in a water-bath to 58 C. and keeping 

 it at that point for ten minutes. The mixture is then 

 planted on to fresh media and incubated, when the re- 

 sulting growth will be found to consist entirely of the 

 B. mesentericus. 



(b) Sparing Bacteria. This method finds its chief 

 practical application in the differentiation of a spore- 

 bearing organism from one which does not form spores. 

 In this case the mixture is heated in a water-bath at 

 80 C. for fifteen to twenty minutes. At the end of 

 this time the non-sporing bacteria are dead, and cul- 

 tivations made from the mixture will yield a growth 

 resulting from the germination of the spores only. 



Differential sterilisation at 80 C. is most conveni- 

 ently carried out in a water-bath of special construction, 

 designed by Balfour Stewart (Fig. 140) . It consists of a 

 double-walled copper vessel mounted on legs, and pro- 

 vided with a tubulure communicating with the space 

 between the walls. This space is nearly filled with 

 benzole (boiling-point 80 C. ; pure benzole, free from 

 thiophene must be employed for the purpose, otherwise 

 the boiling-point gradually and perceptibly rises in the 

 course of time), and to the tubulure is fitted a long 

 glass tube, some 2 metres long and about 0.75 cm. 

 diameter, serving as a condensing tube (a tube half 

 this length if provided with a condensing bulb at 



