42 THE EVOLUTION OF LIFE 
not concern ourselves at all minutely with the 
number of the organisms existing either in air or in 
water. Nor need we trouble ourselves to enquire 
how many different kinds of organisms are to be 
found in the air. In these experiments we find, as 
a rule, within our experimental vessels only different 
varieties of Bacteria, and of Torule, or other Fungus 
germs, such as are shown in Plate [.! 
We know perfectly well that these organisms are 
apt to be found in variable quantity in the air, or in 
the water that may be contained in our experimental 
vessels. So that an initial purifying heat, to which 
the experimental vessels and their contents are 
submitted, is had recourse to solely with the view of 
killing all such pre-existing forms of life. Though 
we try to exclude them as far as possible, it is 
practically immaterial, when we are operating with 
hermetically sealed vessels, whether a few germs 
may be contained therein or not. We have to 
strive to kill all that may be there ; so that, before 
proceeding with such experiments, the most careful 
consideration must be given to all available evi- 
dence bearing upon the question of the destructive 
influence of heat upon different kinds of living 
matter, and especially upon the above-mentioned 
micro-organisms and their germs. 
1 In this Plate, Fig. 1, A, shows some very small Bacteria developing 
from minute particles ; while in B, larger and stained Bacilli, from a 
hay infusion, are represented. 
