ADDRESS. Ixxxi 



depends upon the closeness of the compression of the wool. Now, Schrocder 

 and Dusch found, that, in the case of all the putrefiable materials which they 

 used (except milk and yolk of egg), an infusion boiled, and then allowed to 

 come into contact with no air but such as had been filtered through cotton- 

 wool, neither putrefied nor fermented, nor developed living forms. It 

 is hard to imagine what the fine sieve formed by the cotton-wool could 

 have stopped except miniite solid particles. Still the evidence was incom- 

 plete until it had been positively shown, first, that ordinary air does contain 

 such particles ; and, secondly, that filtration through cotton-wool arrests 

 these particles and allows only physically pure air to pass. This demon- 

 stration has been furnished within the last year by the remarkable experi- 

 ments of Professor Tj'ndall. It has been a common objection of Abiogenists 

 that, if the doctrine of Biogeny is true, the air must be thick with germs ; and 

 they regard this as the height of absurdity. Eut Nature occasionally is ex- 

 ceedingly unreasonable, and Professor Tyndall has proved that this particular 

 absurdity may nevertheless be a reality. He has demonstrated that ordinary 

 air is no better than a sort of stirabout of excessively minute sohd particles ; 

 that these particles are almost wholly destructible by heat; and that they are 

 strained off, and the air rendered optically p\ire, by being passed through 

 cotton-wool. 



Eut it remains yet in the order of logic, though not of history, to show 

 that, among these solid destructible particles, there really do exist germs 

 capable of giving rise to the development of living forms in suitable menstrua. 

 This piece of work was done by M. Pasteur in those beautiful researches 

 which will ever render his name famous ; and which, in spite of all attacks 

 upon them, appear to mo now, as they did seven years ago*, to be 

 models of accurate experimentation and logical reasoning. He strained air 

 through cotton-wool, and found, as Schrocder and Dusch had done, that it 

 contained nothing competent to give rise to the development of life in fluids 

 highly fitted for that purpose. Eut the important further links in the chain 

 of evidence added by Pasteur are three. In the first place, he subjected to 

 microscopic examination the cotton-wool which had served as strainer, 

 and found that sundry bodies, clearly recognizable as germs, were among 

 the solid particles strained off. Secondly, he proved that these germs 

 Avcre competent to give rise to living forms by simply sowing them in a solution 

 fitted for their development. And, thirdly, he showed, that the incapacity of 

 air straiued through cotton-wool to give rise to life, was not due to any occult 

 change eflPected in constituents of the air by the wool, by proving that the 

 cotton-wool might be dispensed with altogether, and perfectly free access left 

 between the exterior air and that in the experimental flask. If the neck of 

 the flask is drawn out into a tube and bent downwards ; and if, after the con- 

 tained fluid has been carefully boiled, the tube is heated sufficiently to destroy 

 any germs which may be present in the air which enters as the fluid cools, 

 the apparatus may be left to itself for any time, and no Hfe will appear in 

 the fluid. The reason is plain. Although there is free communication 

 between the atmosphere laden with germs and the germless air in the flask, 

 contact between the two takes place only in the tube ; and as the germs 

 cannot fall upwards, and there are no currents, they never reach the interior 

 of the flask. But if the tube be broken short off where it proceeds from the 

 flask, and free access be thus given to germs falling vertically out of the air, 



* " Lectures to Working Men on the Causes of the Phenomena of Organic Nature," 

 1863. ° 



