4.JG Br. Allan Macfaihjcn [June 8, 



of procedure. The one efficient and trustworthy agent we possess is 

 heat. Heat and cold are the agents to be jointly employed in the 

 process, viz. a temperature sufficiently high to be fatal to organisms 

 producing a rapid decomposition of milk, as well as to those which 

 produce disease in man ; this to be followed by a rapid cooling to 

 preserve the fresh flavour and to prevent an increase of the bacteria 

 that still remain alive. The pasteurising process fulfils these 

 requirements. 



In conjunction with Dr. Hewlett, I had occasion to investigate in 

 how far the best pasteurising results might be obtained. We found 

 that GO to 68° C. applied for twenty minutes weeded out about 

 90 per cent, of the organisms present in the milk, leaving a 10 per 

 cent, residue of resistant forms. It was found advisable to fix the 

 pasteurising temperature at 68° C, in order to make certain of killing 

 any pathogenic organisms that may happen to be present. We 

 passed milk in a thin stream through a coil of metal piping, which 

 was heated on its outer surface by water. By regulating the length 

 of the coil, or the size of the tubing, or the rate of flow of the milk, 

 almost any desired temperature could be obtained. The temperature 

 we ultimately fixed at 70° C. The cooling was carried out in similar 

 coils placed in iced water. The thin stream of milk was quickly 

 heated and quickly cooled as it passed through the heated and cooled 

 tubing, and whilst it retained its natural flavour, the apparatus ac- 

 complished at 70° C. in thirty seconds a complete pasteurisation, 

 instead of in twenty minutes, i.e. about 90 per cent, of the bacteria 

 were killed, whilst the dipbtheria, typhoid, tubercle and pus or- 

 ganisms were destroyed in tlie same remarkably short period of 

 time, viz. thirty seconds. This will serve to illustrate how the 

 physical agent of heat may be employed, as w ell as the sensitiveness 

 of bacteria to heat when it is adequately employed. 



Bacteria are much more sensitive to high than to low tempera- 

 tures, and it is possible to proceed much further downwards than 

 upwards in the scale of temperature, without impairing their vitality. 

 Some will even multiply at zero, whilst others will remain alive 

 when frozen under ordinary conditions. 



I will conclude this discourse by briefly referring to experiments 

 recently made with most remarkable results upon the influence of 

 low temperatures on bacterial life. The experiments were conducted 

 at the suggestion of Sir James Crichton-Browne and Professor Dewar. 

 The necessary facilities were most kindly given at the Royal Institu- 

 tion, and the experiments were conducted under the personal super- 

 vision of Professor Dewar. The action of liquid air on bacteria was 

 first tested. A typical series of bacteria was emplojed for this 

 purpose, possessing varying degrees of resistance to external agents. 

 The bacteria were first simultaneously exposed to the temperature of 

 liquid air for twenty hours (about — 190° C). In no instance could 

 any impairment of the vitality of the organisms be detected as regards 

 their growth or functional activities. This was strikingly illustrated 



