122 Influence of Light on Development of Bacteria. 



their destruction ; while fourteen hours of exposure at 47° 0. 

 (116.3° F.), three to four hours at 50°-52° C. (122°-125.6° F.), 

 and one hour at 60° C. (140° F.) sufficed to produce the same 

 effect. Under a hot Australian sun there is no difficulty 

 about getting a temperature of 140° F. or over, 125° F. being 

 quite common, and so the destruction of bacteria by insol- 

 ation is easily accounted for. Whether a high enough 

 temperature for that purpose is readily attained in England 

 may be doubtful, and the fact that Professor Tyndall never 

 succeeded in sterilizing his solutions, meets its explanation 

 in this way. It is possible that, in June or July, when 

 Messrs. Downes and Blunt carried on most of their investi- 

 gations, a heat of 125° F. may be occasionally reached for 

 three or four hours continuously, and this would suffice. 

 An anomaly, to them apparently unaccountable, viz., that 

 solution in very small test tubes was more easily sterilized 

 than when contained in larger ones, may be explained by 

 the circumstance that a small body of fluid would more 

 speedily and certainly be raised to the required temperature 

 than a larger one. The fact that Professor Tyndall, in his 

 experiments, used flasks, which I presume were of consider- 

 able size, would on the same principle account for his failure 

 to get complete destruction of germs — the attainment of tem- 

 porary torpidity, by a temperature slightly exceeding 104° F., 

 being comparatively easy. 



While, therefore, it might be going beyond my compe- 

 tence to deny to direct sunlight any influence inimical 

 to the development of bacteria, I have no hesitation in 

 expressing the opinion that such inimical influence of light 

 'per se is not established, either by my own experiments, or 

 by those which I have ventured to criticise, and to interpret 

 in a different sense from their authors. I can explain their 

 error only by supposing that it had not occurred to them as 

 possible, that bacteria might be paralysed, or even killed, 

 by continuous exposure to ordinary summer heat. An 

 expression, contained in one of Messrs. Downes and Blunt's 

 Memoirs, already quoted, to the effect "that temperature 

 may retard or counteract the preservative quality of the 

 solar rays," seems to show clearly that it was actually their 

 opinion, that any elevation of temperature, to which their 

 solutions were exposed, could act only by hastening the 

 development to such an extent as to overcome the destructive 

 power of light as light. Professor Tyndall says, " On many 

 occasions the temperature of the exposed flasks was far more 



