234 THE EVOLUTION OF LIFE 
if they were merely to be heated to 100° C., or in 
a solution of calcium-chloride for temperatures rang- 
ing from 115-130 C. If temperatures above this 
were had recourse to, I have employed a colza 
oil bath, and then very great care is required to 
keep the bath at a definite temperature. Unless 
precautions are taken to slow the process of heating 
when the desired temperature is nearly attained, it 
will almost certainly run up four or five degrees higher 
than was intended. The calcium bath heats more 
slowly, and, of course, can be prepared so as to boil 
at any given temperature up to 130 C.; so that, for 
short periods, the heating of the tubes in a bath of 
this kind can be kept at any definite point that 
may be desired. [ have employed a deep can, 
narrowing above, the thermometer being fixed so 
that its bulb may be situated in the middle strata of 
the fluid. Not more than five or six tubes ought to 
be heated at the same time, as if one should burst 
the commotion caused may lead to the bursting of 
others. ‘The shape of the can is then a protection. 
With careful sealing, however, and ordinary glass no 
accident of this kind should occur, though with the 
thinner tubes of uviol glass, presently to be men- 
tioned, it has happened on two or three oc- 
casions. 
When the tubes have been exposed to the given 
temperature for the time desired they should 
be quickly taken out of the bath with a large 
wooden forceps and allowed to cool. Then, after 
being cleaned and labelled, they are either placed 
in an incubator, at a comparatively high tempera- 

