RESISTANCE TO HEAT AND COLD. 13 



mous multitudes prove that their rate of multiplication is ade- 

 quate to meet the demands made upon them, and is not seri- 

 ously impeded by the low temperature of the waters, never 

 much above the freezing-point, in which they habitually live. 



The maximum limit of heat which living matter can resist 

 is no less variable than its minimum limit. Kuhne found 

 that marine Amoebae were killed when the temperature 

 reached 35° C. (95° Fahr.), while this was not the case with 

 fresh-water Amoebae, which survived a heat of 5°, or even 10°, 

 C. higher. Actinophrys Eichhomii was not killed until the 

 temperature rose to 44° or 45° 0. Didymium serpida is killed 

 at 35° C. ; while another Myxomycete, ^Ethalium septicum, 

 succumbs only at 40° C. 



Colin (" Untersuchungen uber Bacterien," Beitrage zur 

 Blologie der Pflanzen, Heft 2, 1872) has given the results of 

 a series of experiments conducted with the view of ascertain- 

 ing the temperature at which Bacteria are destroyed when 

 living in a fluid of definite chemical composition, and free 

 from all such complications as must arise from the inequalities 

 of physical condition when solid particles other than the Bac- 

 teria coexist with them. The fluid employed contained 0.1 

 gramme potassium phosphate, 0.1 gr. crystallized magnesium 

 sulphate, 0.1 gr. tribasic calcium phosphate, and 0.2 gr. am- 

 monium tartrate, dissolved in 20 cubic centimetres of distilled 

 water. If to a certain quantity of this " normal fluid " a small 

 proportion of water containing Bacteria was added, the mul- 

 tiplication of the Bacteria went on with rapidity, whether the 

 mouth of the containing flask was open or hermetically closed. 

 Hermetically-sealed flasks, containing portions of the normal 

 fluid infected with Bacteria, were submerged in water heated 

 to various temperatures, the flask being carefully shaken, with- 

 out being raised out of the water, during its submergence. 



The result was, that in those flasks which were thus sub- 

 jected, for an hour, to a heat of 60°-62° C. (140°-143° Fahr.), 

 the Bacteria underwent no development, and the fluid re- 

 mained perfectly clear. On the other hand, in similar experi- 

 ments in which the flasks were heated only to 40° or 50° C. 

 (104°-122° Fahr.), the fluid became turbid, in consequence of 

 the multiplication of the Bacteria, in the course of from two 

 to three days. 



I am in the habit of demonstrating annually, that Pasteur's 

 solution and hay-infusion, after five minutes' boiling in a flask 

 properly stopped with cotton-wool, remain perfectly clear of 

 living organisms, however long they may be kept. The same 



