13 



particles. After waiting two or three days he examined it again, 

 when the electric ray showed the air to be quite pure, all the 

 suspended particles having subsided and been retained by the 

 glycerine. He then boiled all the infusions, and found that 

 they remained perlectly pure after a lengthened period. Ditllinger 

 repeated these experiments under more trying conditions, and 

 was able to confirm his results. 



From these experiments we are able, I think, to conclude — 

 First, that organic infusions have no inherent tendency to decom- 

 pose. Secondly, that the decomposition is not set up by the 

 contact of pure air, that is air free from floating particles; and 

 Thirdly, that decomposition is associated with the presence of 

 organisms which derive their origin from the atmosphere, or, at 

 any rate, irom without. 



In the stand before me are three test tubes. Two of them 

 are closed with plugs of cotton wool, and contain sterilized infu- 

 sions. They have been kept for fourteen days at a temperature 

 varying between 6b'^ and 90^^ F.; but notwithstanding that the 

 air can pass through the plugs, and has actually done so, owing 

 to the variations of temperature, the liquids remain as clear 

 and bright as at first. The third tube contains a vegetable 

 infusion, not sterilized, and although it has been exposed to the 

 same conditions for only three days it is already turbid and full of 

 organisms. 



Our next step is to inquire into the nature of these minute 

 particles. First of all Tyndall, by passing air through red hot 

 platinum tubes, showed that they were largely organic. In order 

 to collect them for microscopical examination, various devices 

 have been adopted. Pasteur passed large volumes of air through 

 cotton wool or gun cotton, and then dissolved it. Pouchet used 

 what he called an aeroscope, which was a contrivance by which 

 he caught the particles on a glass slip covered with glycerine. 

 Cohn passed the air through a saline solution. Various substances 

 were found. Miguel gives the following : — spores of algte, fungi, 

 pollen grains, fragments of tissue, starch grains, and particles of 

 silex, but all agree that nothing was found relating to Bacteria 

 germs. Cohn accounts for this by supposing that they passed 

 through his saline solution in the bottles and escaped without 

 being wetted. That Bacteria, or their germs, are not present in 



