ACTION OF COLD 25 



begin to develop vigorously, the hay infusion which contains 

 the spores being a most favourable medium for the growth 

 of the hay bacillus. As the cotton-wool plug prevents the 

 entrance of other organisms, a pure culture of the hay 

 bacillus is thus obtained. 



A similar method, using regulated temperatures, is em- 

 ployed to obtain pure cultures of the tetanus bacillus. 



(3) Action of Cold. — Micro-organisms are extremely resist- 

 ant with regard to cold. Frosch, in 1877, exposed various 

 cultures to a temperature of - 87° C, which he obtained by 

 means of liquid carbon dioxide, and found that most of the 

 organisms experimented upon multiplied on being placed 

 again under favourable conditions. Prudden has recently 

 made some extended experiments on the influence of freezing. 

 He found that while some organisms withstood the action 

 of cold for a long time, others failed to grow. The Bacillus 

 prodigiosus failed to grow after being frozen for fifty-one 

 days, as did also the Proteus vulgaris. The Staphylococcus 

 pyogenes aureus withstood freezing for sixty-five days, and 

 the typhoid bacillus for 103 days. Sub-cultures made 

 at intervals showed, however, a diminution in number 

 of the bacteria. A similar diminution in number would 

 perhaps have occurred in old cultures in which the material 

 for growth was exhausted, independent of freezing ; for the 

 bacteria, like the higher organisms, die in time as the result 

 of degenerative changes in the protoplasm of the cells, and 

 continued vitality in a culture depends on continued repro- 

 duction. 



Kepeated freezing and thawing was found by Prudden to 

 be more destructive to the typhoid bacillus than continuous 

 freezing. Cadeac and Malet kept portions of a tuberculous 

 lung in a frozen condition for four months, and found that 

 at the end of this time tuberculosis was produced in guinea- 

 pigs by injecting a small quantity of the material. 



