84 Fermentation and Kindred Phenomena. 



milk and eggs. It is, they argued, difficult to imagine that the 

 wool can have removed anything from the air except solid 

 particles, and these must be the germs of the infusoria. 



It only remained to demonstrate, first, that these germs are 

 actually present in air, and secondly, that they are retained by 

 the cotton wool. Independently Tyndall and Pasteur devoted 

 themselves to this branch of the subject, and arrived at positive 

 results by two totally different methods. 



In Pasteur's beautiful researches, which are remarkable for 

 their simplicity, elegance, and aptness, ordinary air was filtered 

 through cotton wool, and as thus purified was found to have 

 lost its power of inducing putrefaction in organic liquids. 

 Pasteur then submitted the minute residue which was left to 

 microscopic examination. In it he had no difficulty in recog- 

 nising the spores of minute organisms ; and to complete the 

 proof that these spores are actually the seeds of putrefactive 

 organisms he brought them into a previously boiled infusion, 

 and found that in the course of a few hours the liquid was in 

 active putrefaction. Tyndall's experiments were based upon 

 totally different considerations. Every one knows that when 

 a ray of sunlight enters a dark room its path is clearly visible. 

 The ray looks like a faint luminous cloud, and if the cloud is 

 examined narrowly myriads of particles are seen to be floating 

 in it. Now, Tyndall found that by allowing air to remain 

 perfectly quiet and undisturbed for a day or two these particles 

 by their natural gravity subside, and a ray of light when now 

 passed through the air no longer shows any visible track. He 

 proved by a simple experiment that air before subsidence 

 causes organic infusions to putrefy, whereas after subsidence 

 the infusion may be exposed for any length of time to it 

 without undergoing the slightest putrefactive change. 



We may, therefore, consider it as definitely proved that 

 putrefaction is caused by minute organisms, the spores of which 

 are present in air, and that it is not due to any spontaneous 

 change occurring in the putrescible matter, nor to any specific 

 action of the air as such. The minute organisms are produced 

 from spores or eggs, and the doctrine of spontaneous generation 

 we may consider as finally refuted, 



