6 HISTORICAL OUTLINE 



of sugar in a watery extract of yeast, by inverting the flasks over a mercury bath 

 and admitting the air through the mercury seal. Here his results, as regards the 

 occurrence of fermentation, were altogether uncertain, and his conclusions lost 

 much of their force. Helmholtz (1844) confirmed certain of Schwann's observa- 

 tions. Schultze (1836) had already obtained similar results by admitting to his 

 flasks air which had been drawn through strong potash solutions or through con- 

 centrated sulphuric acid. Schroeder and Dusch (1854) showed that the active 

 principle could be removed from the air by drawing it through cotton- wool. This 

 last method was a real advance, since the incoming air had not been subjected 

 to high temperatures, nor to strong chemical reagents. Unfortunately another 

 element of doubt was introduced. Schroeder and Dusch relied, for their pre- 

 liminary sterilization, on a short period of heating to the boiling-point. They 

 experimented with four kinds of material — water containing meat, malt of beer, 

 milk, and meat without the addition of water. With the first two materials their 

 results were quite uniform : the fluids remained unaltered. With the last two 

 materials fermentation usually occurred. They concluded that there were two 

 kinds of decomposition, associated with the presence of living organisms, the 

 one spontaneous, needing only the presence of oxygen, the other requiring some 

 additional principle, which could be removed from the air by filtration through 

 cotton-wool. 



This, then, was the position when Pasteur began his investigations in 1859. 

 In a series of admirable memoirs, starting in 1860 and continuing for more 

 than four years, he went over the ground already covered, added new and 

 illuminating experiments of his own devising, and terminated the controversy by 

 clear and decisive demonstrations. He showed that the material removed from 

 air by passage through cotton-wool, or through similar filters, contained organized 

 particles which were neither crystals nor starch granules, but which were similar 

 in appearance to the spores of moulds. By introducing these particles into flasks 

 of sterilized organic material, he demonstrated that they were capable of giving 

 rise to the growth of numerous kinds of living organisms. Using other methods, 

 he showed that the air in different situations differed in its content of these germs ; 

 that they were numerous in the streets of cities, less numerous in the air of country 

 uplands, rare in the quiet air of closed and uninhabited rooms or cellars, where 

 the dust had deposited and remained undisturbed, and very rare in the pure air 

 of the high Alps, above the level of human habitation. He showed that Schwann's 

 failures were due to his use of mercury, from the surface of which his fluid had 

 acquired the germs, which had settled on it from the air. He showed that the 

 failures of Schroeder and Dusch were due to the inadequate sterilization of iheir 

 material. 



He also showed that certain animal fluids, such as blood or urine, known to 

 be eminently liable to undergo putrefaction, could be collected in such a way as 

 to remain permanently unaltered. 



The controversy with Pouchet, Joly and Musset, which continued from 1860 

 to 1864, did not lead to the collection of many new facts, except those with regard 

 to the unequal distribution of micro-organisms in the atmosphere ; but a later 

 dispute with Bastian, who became a veteran in the dwindling army of the supporters 

 of spontaneous generation, was more fertile, because it caused Pasteur to reconsider 

 some of his ideas, and to elaborate the technical methods which he had partially 

 developed during his re-investigation of the results obtained by Schroeder and 



