IX EXPERIMENTS ON BIOGENESIS 99 



after these two conditions have been rigorously comj)lied 

 with, living organisms appear in the fluid, such organisms 

 must have originated abiogenetically. 



To kill any microbes contained in the fluid it is usually 

 quite sufficient to boil it thoroughly. As we have seen, 

 protoplasm enters into heat-rigor at a temperature consider- 

 ably below the boiling-point of water, so that, with an 

 exception which will be referred to presently, a few minutes' 

 boiling suffices to sterilize all ordinary infusions, i.e.^ to kill 

 any organisms they may contain. 



Then as to preventing the entrance of organisms or their 

 spores from without. This may be done in various ways. 

 One way is to take a flask with the neck drawn out into 

 a very slender tube, to boil the fluid in it for a sufficient 

 time, and then, while ebuUition is going on, to close the 

 end of the tube by melting the glass in the flame of a 

 Bunsen-burner or spirit-lamp, thus hermetically sealing the 

 flask. 



By this method not only organisms and their spores are 

 excluded from the flask but also air. But this is obviously 

 unnecessary : it is evident that air may be admitted to the 

 fluid with perfect impunity if only it can be filtered, that is, 

 passed through some substance which shall retain all solid 

 particles however small, and therefore of course bacteria, 

 monads, and their spores. 



A perfectly efficient filter for this purpose is furnished by 

 cotton-wool. A flask or test-tube is partly filled with the 

 infusion : the latter is boiled, and during ebullition cotton- 

 wool is pushed into the mouth of the vessel until a long and 

 firm plug is formed (Fig. 19). When the source of heat is 

 removed, and, by the cooling of the fluid, the steam which 

 filled the upper part of the tube condenses, air passes in to 

 supply its place, but as it does so it is filtered of even the 



H 2 



