ASSIMILATION OF CARBON 55 



long the solution in the flasks remained unchanged and free from microorganisms 

 for an indefinite period of time. The air that entered the flasks during cooling 

 was filtered through the cotton plugs, in which all the germs that it originally 

 held were left behind. Since the spores of some bacteria withstand a single, 

 though long-continued boiling, this operation must sometimes be repeated 

 several times, and even under pressure, in order to kill all organisms originally 

 present. Pasteur carried out a number of his experiments in glass flasks espe- 

 cially arranged with two necks (Fig. 32). One of the necks bore a short piece 

 of rubber tubing, which was closed by a bit of glass rod. The other neck was 

 drawn out into a narrow tube, bent twice upon itself. Both were open during 

 the boiling of the liquid. While boiling was still going on the wide tube was 

 plugged, after which boiling was stopped and the apparatus was cooled, air 

 entering through the narrow tube. The solution re- 

 mained unchanged indefinitely, since all spores con- 

 tained in the entering air were caught in the narrow 

 bend of the tube. However, if the glass stopper was 

 momentarily removed, thus allowing a very small 

 number of microorganisms to enter the flask, then 

 the solution immediately began to decompose. De- 

 composition is brought about in such an experiment 

 as a result of the rapid multiplication of the micro- 

 organisms that have been introduced. 



To demonstrate conclusively that the theory of 

 spontaneous generation is untenable, it remained 

 still to prove that microorganisms and their spores 

 really do occur in the air in great abundance. This 



question was also worked out by Pasteur in the most PlG p a steur flask 



exact manner. He took a series of flasks, filled to a 



third of their volume with nutrient solution, brought the contents to boiling 

 and then sealed them by fusing the glass of their narrow necks. The flasks 

 were then placed in positions where he wished to investigate the air, and the 

 sealed ends were then broken off, thus allowing air to enter. The flasks were 

 then resealed. If the air entering a flask was free from germs, than the liquid 

 remained unchanged, but if the entering air contained microorganisms or their 

 spores, then decomposition began. In this way Pasteur proved that the air of 

 deep cellars and high mountains is most nearly pure. It need not be con- 

 cluded, however, that the air is absolutely free from organisms in those cases 

 where the liquid remains unchanged in such experiments; it is quite possible 

 that spores may be contained in the air but they may be unable to develop in 

 the particular nutrient medium chosen. 



Many exact investigations have now been made upon the distribution of 

 microorganisms in the air. The table given below presents the average results 

 from ten years of observation (1 885-1 894) upon the number of microorganisms 

 in a cubic centimeter of air in the Park of Montsourie. In the same table are 

 shown the corresponding numbers, averages from ten years of observations, 



