152 THE EVOLUTION OF LIFE 
The first modification was a super-heating of the 
potash-tubes. To this I had no objection, and as it 
will be fully referred to in my discussion with M. 
Pasteur, nothing need be said on that subject now. 
The second modification was this : liquor potassz 
was added in quantity sufficient to ‘exactly 
neutralise’ the urine employed; this was then 
boiled over the flame for five minutes before the 
neck of the flask was sealed. The flasks were 
“then set aside in a warm place (70° to 80° F.) 
for a fortnight,” and Dr Roberts says, the liquor 
potasse was not added ‘till the lapse of time had 
satisfied me that it had been rendered permanently 
barren.” Professor Tyndall says that in these 
respects “the procedure described by Dr Roberts 
was accurately pursued” in his own experiments. 
And he says at the close of his brief communication, 
“The experiments have already extended to one 
hundred and five instances, not one of which shows 
the least countenance to the doctrine of spontaneous 
generation.” This I can readily believe, as the 
methods adopted by these experimenters certainly 
entailed the addition of liquor potassz considerably 
in excess of the proper amount, and the method 
adopted by them was also wrong in other ways. 
In the first place there was no necessity to depart 
from the ordinary method of the “control” experi- 
ment—especially in dealing with such a fluid as 
boiled acid urine which Dr Roberts and Professor 
Tyndall, as well as other experimenters, had declared 
to be invariably sterilised by a brief elevation to 
100° C..{gee p: 432). 

