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I will express the meaning at once. "WTien I speak of a sterilized 

 infusion, I mean one which has been thoroughly boiled, and is 

 enclosed in a vessel which has been perfectly purified in all its 

 parts. If all these precautions are properly observed, an organic 

 infusion may be certainly kept free fi-om the presence of organisms 

 or of decomposition for an indefinite period. At present we are 

 of com'se supposing that the air is absolutely excluded. 



I shall ask you now to leave the infusions for a time while we 

 deal with the other Jialf of the question, \iz., the part played by 

 the atmosphere. For our knowedge of this portion of the subject 

 we are largely indebted to the researches of Professor Tyndall. 

 His first question was, is decomposition caused by the mere 

 contact of air, from whatever locality it may be obtained ? To 

 determine this point he prepared fifty flasks of stjrilized infusions. 

 Twenty-five of these he opened in a hay loft, and the remaining 

 twenty-five on the edge of an Alpine clifi", afterwards closing them 

 and placing the whole in a warm chamber. After a few days he 

 found the whole of the hay-loft series in a state of decomposition, 

 and full of organisms, while the whole of the Alpine series remained 

 pure as before. Now we know that the chemical composition of 

 the atmosphere is remarkably constant, therefore, to what is this 

 strange variation due ? Tyndall's experiments with the electric 

 ray, by which he showed that the air, in most situations, is full 

 of solid particles, are, no doiibt, famdiar to you all. He also 

 demonstrated that the same method is applicable to liquids, and 

 so delicate is the test that he was able to show the presence in 

 water of minute portions of mastic, which eluded a magnifying 

 power of 1200 diameters, and also by a comparative experiment 

 that the particles contained in a highly infecting liquid were even 

 smaller still. By means of the electric ray, you will also recollect, 

 that he showed that cotton wool had the property of filtering out 

 all these minute particles from air drawn through it. He next 

 proceeded to apply these results to the investigation of the 

 question before us. He constructed an air-tight box, vrith 

 glass sides and ends, and after coating the whole of the interior 

 with glycerine, he closely fitted into the bottom a number of glass 

 tubes, containing organic infusions, the tubes opening freely into 

 the interior of the box. C'n fii-st passing the electric ray through 

 this arrangement, he found the air in the box, as usual, fidl of 



