66 BIOLOGY AND TECHNIQUE 



These observers were able to show that the spores of anthrax 

 destroyed by boiling water at 100 C. in from one to twelve minutes, 

 whereas dry hot air was efficient only after three hours' exposure to 

 140 C. Extensive confirmation of these differences has been brought 

 by many workers. An explanation of the phenomena observed 

 is probably to be found in the changes in the coagulability of 

 proteids brought about in them by the abstraction of water. 

 Lewith, 1 working with various proteids, found that these sub- 

 stances are coagulated by heat at lower temperatures when they 

 contain abundant quantities of water, than when water has been 

 abstracted from them On the basis of actual experiment with egg 

 albumin he obtained the following results, 2 which illustrate the point 

 in question: 



Egg albumin in dilute aqueous solution, coagulated at 56 C. 

 " . " with 25 per cent water, " " 74-80 C. 



" " " 18 " " " " " 80-90 C. 



" 6 " " " " " 145 C. 



Absolutely anhydrous albumin, according to Haas, 3 may be heated 

 to 170 C. without coagulation. It is thus clear that bacteria exposed 

 to hot air may be considerably dehydrated before the temperature rises 

 sufficiently to cause death by coagulation, complete dehydration neces- 

 sitating their destruction possibly by actual burning. 



Bacteria exposed to moist air or steam, on the other hand, may ab- 

 sorb water and become proportionately more coagulable. 



The same principle, as Lewith points out, probably explains the great 

 resistance to heat observed in the case of the highly concentrated pro- 

 toplasm of spores. 



Apart from the actually greater efficiency of moist heat when com- 

 pared with dry heat of an equal temperature, an advantage of great 

 practical significance possessed by moist heat lies in its greater powers of 

 penetration. An experiment carried out by Koch and his associates 

 illustrates this point clearly. Small packages of garden soil were sur- 

 rounded by varying thicknesses of linen with thermometers so placed 

 that the temperature under a definite number of layers could be deter- 



1 Lewith, Arch. f. exp. Path. u. Pharm., xxvi, 1890. 



2 Lewith, loc. cit., p. 351. 



s Haas, Prag. med. Woch., 34-36, 1876. 



