HISTORICAL INTRODUCTION. 5 



Schulze demonstrated that in spite of free access to air, which hadnot 

 been heated, the infusions remained free from germs. 



Schwann, in 1837, arrived at similar results. He found that 

 putrescible substances remained sterile if exposed to an abundant 

 supply of air which was heated by being passed through a melted mix- 

 ture of metals. This convinced him that the cause of the decompo- 

 sition which would otherwise have occurred must exLst in the air. 



The objection remained that in the experiments of Schulze and 

 Schwann, the air which was admitted to the flasks had undergone 

 either a chemical or a thermal change, and therefore the theory of 

 Needham was not yet entirely disposed of. 



In 1854 the final blow was dealt by Schroder and Van Dusch. 

 These investigators demonstrated that decomposition could be obviated 

 without resorting either to thermal or chemical treatment of the 

 air, as simple filtration of the air through cotton-wool was shown ta 

 be efficacious in excluding germs. Finally, Hoffman in 1860, and 

 independently, Chevreuil and Pasteur in 1861, showed that even 

 cotton-wool could be dispensed with, as a sterile solution would 

 remain sterile when the neck of the vessel was bent into an 

 S -shaped curve. Micro-organisms in the air entering the flask 

 were deposited by gravitation in the bend of the tube. 



The advocates of spontaneous generation were ready with fresh 

 objections. They now urged that the medium lost its power of 

 undergoing decompositioii by being boUed. This objection was at 

 once set aside by the fact that when unfiltered air was admitted to 

 the infusion, decomposition set in. Additional evidence was brought 

 against spontaneous generation by the experiments of Pasteur, 

 Burdon Sanderson, Lister, and others, in which it was shown that 

 blood, urine, and milk would remain without decomposition, when all 

 precautions were adopted to avoid contamination in filling the 

 sterilised flasks. 



Even at this stage of this great scientific controversy fresh 

 difficulties arose, for it was found that in certain solutions which had 

 been boiled and hermetically sealed in flasks micro-organisms made 

 their appearance. In 1872 Charlton Bastian published a research 

 which was to prove that spontaneous generation actually took 

 place. Decoctions of turnip and cheese which had been filtered, 

 neutralised, and boiled for ten minutes, and hermetically sealed 

 during the boiling, were found after a time to contain micro 

 organisms. These results, however, were before long explained by 

 the fact that in milk, infusions of hay, and certain other decoctions, 

 the spores of bacilli are present, which are much more resistant 



