EXPERIMENTAL CONDITIONS 37 

organic matter derived from pre-existing living 
things. 
The fact that the conditions for life-origination are 
so much more favourable in the open fields of nature 
than they are in our experimental vessels, would 
make it impossible, therefore, to deny its present 
occurrence, even if all experiments under strict con- 
ditions had been attended by negative results. No 
experiment can be regarded as trustworthy unless 
the vessel and its contents have been subjected to a 
degree of heat deemed adequate to kill all pre-existing 
living things that may have been contained therein— 
that is, unless the vessel and its contents have been 
reduced, as near as possible to the condition of 
our earth before living matter had originated on 
its surface. 
After the initial purification by heat, we have like- 
wise to make sure that the fluid is kept safe from 
external contamination—that is, from contamination 
by germs contained in the air. 
Thus the disadvantages with which we have to 
contend are two—the degrading effect of the initial 
purifying heat process, and the partial exclusion of 
actinic rays by the glass vessel in which the fluid is 
contained — rays which might be potential, or at 
least helpful, in bringing about the combinations in 
question. 
The fallacies to be guarded against are also two. 
We must be sure that the heat employed is adequate 
to kill all pre-existing living things within the 
experimental vessels, short of rendering the medium 
