Fermentation and Kindred Phenomena. 83 



Spallanzani took up Needham's work, and by heating the her- 

 metically sealed infusions for a longer period arrived at the 

 opposite result — no infusoria appearing after the prolonged 

 heating. Needham, however, was prepared with an argument to 

 explain this result, his contention being that under the condi- 

 tions of Spallanzani's experiment the germinating power of the 

 infusion had been destroyed, and further that the air contained 

 in the closed vessel had been destroyed by the heat. The 

 latter part of this criticism acquired some force when it was 

 discovered that the gases contained in vessels of preserved pro- 

 visions contained no oxygen, and oxygen is, as we know, essential 

 to life. Swann, however, showed conclusively that if an infusion 

 previously boiled is placed in communication with air that has 

 been heated red hot, no putrefaction occurs. Ure and Helmholz 

 multiplied Swann's experiments with the same result, and 

 Schulz found that instead of calcining the air it is sufficient 

 before admitting it to the boiled infusions to allow it to pass 

 through energetic chemical substances, such as oil of vitriol, &c. 

 These experiments were really sufficient to decide the question 

 against the doctrine of spontaneous generation, but its supporters 

 were hard to defeat, and clung tenaciously to their belief. 

 Their objection at this stage of the controversy was ingenious, 

 if nothing else. By calcining the air, or by passing it through 

 energetic chemical substances, you destroy some principle in it 

 which is essential for the production of infusoria, they said. It 

 is all very well to say that you merely destroy the seeds or 

 germs, but you offer no proof of such a thing. 



This criticism had to be met, and it was met most ingeniously 

 by Schroeder and Dusch, the method which they employed 

 being simply a refinement of Redi's experiment with the gauze 

 cage round the meat. 



Instead of the gauze cage they used cotton wool, merely 

 allowing the air to filter through it before coming in contact 

 with the well boiled infusion. Under these conditions they 

 found that the latter remained perfectly sweet and fresh, showing 

 no trace of organisms when examined under the microscope, 

 nor the slightest symptom of putrefaction, except in the case of 



