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. Fully 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 ’o1, d’Arsonval (Bibliog., XX XIII) and 
Fig. 69.* 
Smith & Swingle, the Effect of Freezing on Bacteria, Proc. Sixth Ann. Meeting 
Soc. Am. Bacteriologists, December 27, 1904; Science, N. S., Vol. X XI, 1905, pp. 
481-483. For opposing views see ’02, Macfadyen, Bibliog., XX XIII. 
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. ¢., over 50 per cent. In this latter case the agar plates were incubated 
7 days at 30° C, before the count was made. 
