20 PRINCIPLES OF BACTERIOLOGY 



Air thus robbed of its living organisms did not produce decomposition; 

 whereas when no such precautions were taken with the air admitted 

 the boiled solutions quickly fell into putrefaction, and living organisms 

 were found to be present. 



Schwann in 1839 obtained similar results in another way; he deprived 

 the air admitted to his boiled liquids of micro-organisms by passing it 

 through a tube which was heated to a temperature high enough to 

 destroy them. To this investigator is also due the credit of having 

 discovered the specific cause the yeast plant, or saccharomyces cerevisice 

 of alcoholic fermentation, the process by which sugar is decomposed 

 into alcohol and carbonic acid. 



Again, it was objected to these experiments that the heating of the 

 air had perhaps brought about some chemical change which hindered 

 the production of fermentation. Schroeder and von Dusch in 1854 

 then showed that by a simple process of filtration, which has since 

 proved of inestimable value in bacteriological work, the air can be 

 mechanically freed from germs. By placing in the mouth of the flask 

 containing the boiled solutions a loose plug of cotton, through which 

 the air could freely circulate, it was found that all suspended micro- 

 organisms could be excluded, and that air passed through such a filter, 

 whether hot or cold, did not cause fermentation of boiled infusions. 



Similar results were obtained by Hoffmann in 1860, and by Chevreul 

 and Pasteur in 1861, without a cotton filter, by drawing out the neck 

 of the flask to a fine tube and turning it downward, leaving the mouth 

 open. In this case the force of gravity prevents the suspended bacteria 

 from ascending, and there is no current of air to carry them upward 

 through the tube into the flask containing the boiled infusion. 



Tyndall later showed (1876), by his well-known investigations upon 

 the floating matters of the air, that in a closed chamber, in which the 

 air is not disturbed by currents, all suspended particles settle to the 

 bottom, the superincumbent air being optically pure, as is proved by 

 passing a ray of light through it. He demonstrated that the presence 

 of living organisms in decomposing fluids was always to be explained 

 either by the pre-existence of similar living forms in the infusion or 

 upon the walls of the vessel containing it, or by the infusion having 

 been exposed to air which was contaminated with organisms. 



These facts have since been practically confirmed on a. large scale 

 in the preservation of food by the process of sterilization. Indeed, 

 there is scarcely any biological problem which has been so satisfactorily 

 solved or in which such uniform results have been obtained; but all 

 through the experiments of the earlier investigators irregularities were 

 constantly appearing. Although in the large majority of cases it was 

 found possible to keep boiled organic liquids sterile in flasks to which 

 the oxygen of the air had free access, the question of spontaneous 

 generation still remained unsettled, inasmuch as occasionally, even 

 under the most careful precautions, decomposition did occur in such 

 boiled liquids. 



This fact was explained by Pasteur in 1860 by experiments showing 



