THERMAL RELATIONS. 83 



growth for the time being. Such statements have been based on certain qualitative 

 tests and do not tell the whole truth. In the writer's experiments with liquid air 

 great differences have been detected, the reduction by exposure for one-half hour 

 varying from 15 per cent, or less, to 90 per cent, or more, according to the species 

 tested. Full}' 50 per cent of many sorts, grown in bouillon, are destroyed by a single 

 short exposure (see figs. 68 and 69). Query : Is intense cold any more harmful to 

 bacteria than simple freezing? Are young or old cultures most susceptible? 

 Are they killed by the rupture of the cell-wall due to the formation of ice- crystals, or 

 simply by the abstraction of water? Why do some resist several freezings? Can 

 endospores be killed in this way? Consult 'or, d'Arsonval (Bibliog., XXXIII) and 



Fig. 69 * 



Smith & Swingle, the Effect of Freezing on Bacteria, Proc. Sixth Ann. Meeting 

 Soc. Am. Bacteriologists, December 27, ^04; Science, N. S., Vol. XXI, 1905, pp. 

 48^483. For opposing views see '02, Macfadyen, Bibliog., XXXIII. 



Live steam acts upon the growing bacteria very quickly. All bacteria not in 

 spore form, or in some other way protected from the direct action of the heat by 

 what surrounds them, are promptly destroyed by steam heat at 100 C., an exposure 

 of a minute or two being ample, except, possibly, in case of some of the thermo- 



*Fic. 69. Same as fig. 68, but made after exposure for twenty hours to liquid air. Number 

 of colonies reduced two-thirds. Exposed in test-tubes of Jena-glass for one-half hour, the reduc- 

 tion was nearly as great, i. e., over 50 per cent. In this latter case the agar plates were incubated 

 7 days at 30 C, before the count was made. 



