THERMAL RELATIONS. 



77 



air in the bottom of the chest may be kept fairly constant for some days or weeks, but 

 witli marked external fluctuations of temperature tnistworth)' results can be obtained 

 only by constantly watching the box. 

 What one needs for this work is a good- 

 sized room kept at o° C, or a little below, 

 in which thermostats ma}- be installed at 

 temperatures a little above freezing, c. g.^ 

 + 2°, +5°, +7°, etc. It would then be 

 very eas)' to determine the minimum 

 temperature at which any organism will 

 grow — as easy as it is now to determine the 



maximum. Different levels in the same room may afford constairt 

 and useful differences in temperature. 



The thermal death-point, which is a purely arbitrary standard, 

 depending on the age and kind of culture, its volume, and the length 

 of exposure, as well as the temperature, is when properly determined 

 not least valuable. The writer, following that one of Dr. Sternberg's 

 methods which is easiest to carry out, uses lo cc. portions of moder- 

 ately alkaline (+io or +15) peptonized beef-brothf in test-tubes of 

 uniform diameter (16 to 17 nnn.), inoculates from recent bouillon- 

 cultures with care not to touch the sides of the tube above the fluid, 

 thrusts the tubes deep into the hot water, and exposes for ten minutes. 

 All who make this test are urged to ttse standard alkaline beef- 

 bouillon (for all organisms growing well in this medium) and to 

 limit the exposure to exactly ten minutes, so that easy comparisons 

 may be made. The five minutes exposure which has been recom- 

 mended by some authors is rather too short, since it only a little 

 more than suffices to warm the fluid up to the required temperature. 

 Inoculatiorr while the tubes are in the bath and after the fluid has 

 been brought to the required temperature is inconvenient and has no 

 special advantage. Fig. 64.* 



*FiG. 64. — Roux's thermo-rcgulator, made by Maison Wiesncgg (P. Lequeux), Paris. The parts 

 requiring description are as follows: A, bar composed of two metals (which expand and contract un- 

 equally) attached at bottom and free at the top, which moves with increased heat in the direction of 

 the arrow; B, arm on which the upper part of the apparatus moves freely when K is turned; C, 

 stiff spring; D, long rod which controls the gas-inflow, and the spring movement of which is in 

 the direction of the arrow except when controlled by the counter movement of A, due to lessened 

 heat ; E, gas-inflow ; F, gas-chamber, of glass ; G, gas-outflow, to the burner ; H, ruljlier stopper ; 

 I, cylinder screwing into L, and provided with capped upright tulic filled with vaseline to prevent 

 gas from escaping in the direction of D. The button shown in the gas-chamber at the left is part 

 of D, and the gas enters the chamber between it and the left end of L, the size of the opening, and 

 consequently the amount of gas, varying with the slightest movement of A. Different temperatures 

 are obtained by turning the button K. The constant gas-flow is provided for by a small opening on 

 the lower side of L at its extreme left, in the gas-chamber. yVbout two-fifths actual size. 



fThe thermal death-point in acid media is considerably higher — at least that of several organ- 

 isms which have been tested in the author's laboratory. 



