THEOKIES OF FERMENTATION. 233 



of spontaneous generation. Tfyey fell back on the theory that 

 the powerful treatment the air had undergone had so altered 

 its composition that it was no longer able to produce life. 



Schroeder and Dusch took up the subject in 1853-1861, 

 Avith the object of proving that air containing all its gaseous 

 constituents, unaltered, may be allowed to react on boiled 

 fermentable material without effect. It is unnecessary to 

 expose the air to any vigorous chemical treatment with strong 

 reagents, if it is first separated as far as possible from solid 

 particles. For this purpose they made use of a filter of cotton 

 wool, through which the air was led before it came into contact 

 with the boiled organic mixtures. Boiled meat and meat- 

 broth, as well as malt-wort, were unaltered when filtered air 

 was introduced into the flasks. On the other hand, the 

 experiments did not succeed with milk or the yoke of egg 

 stirred up in water and boiled. A completely decisive proof 

 could not be furnished by Schroeder until, in 1861, he succeeded 

 in sterilising this substance. About this time Pasteur had 

 begun a number of his epoch-making researches, in which the 

 principle of sterilisation was clearly established. 



It appeared clear, therefore, to Schroeder that in certain 

 cases before filtration the air must contain something that 

 could bring about fermentation and decay. Whether these 

 are " floating, microscopic, organised germs in the air, or a 

 chemical substance, as yet unknown, which is separated by 

 contact action and fixed on the cotton wool, must remain to 

 be determined." It also appeared to be probable, after 

 correction of the unsuccessful experiment, " that lower in- 

 fusorial ferments exist, produced and separated either from 

 living plant cells or from living animal tissue, which are 

 capable of exercising certain organic functions and trans- 

 formations/' Mention must also be made of the experiments 

 begun by Hoffmann in 1860 regarding decay and fermentation. 

 He boiled the organic matter in a flask with a long drawn-out 

 neck bent several times at an acute angle. The subsequent 

 inflow of air during cooling deposited dust by gravity so that 

 none could fall into the liquid.* The result was exactly that 

 obtained by filtration ; the liquid remained unaltered. Not- 



* A similar arrangement was made use of by Chevreul. 



