SYSTEMATICALLY SELECTED YEASTS. 11 



and Schwann proved that yeast consists of living cells, and 

 that it is these which bring about alcoholic fermentation. In 

 1765 Spallanzani opposed the doctrine of spontaneous 

 generation, according to which, living organisms were able to 

 develop out of dead nature without eggs, seeds or germs. 

 The experiments of Schwann and his followers showed that 

 various substances which readily ferment and putrefy, can be 

 preserved unchanged when they are boiled, and care taken 

 that the air with which they subsequently come into contact 

 is freed from its germs. Thus the technique of sterilisation 

 was established both as regards practice and science. In 

 1839 Schwann showed that yeast cells perish when they are 

 acted upon by certain chemical compounds, and he thus laid 

 the foundation for the doctrine of antiseptics, which sub- 

 sequently became of such importance. When the acetic acid 

 bacteria were then discovered by Kiitzing, Turpin, in conse- 

 quence of this and of his own yet very imperfect investiga- 

 tions, gave utterance to the following doctrine : " No 

 decomposition of sugar, no fermentation without the physio- 

 logical activity of vegetation." 



These fertile ideas were followed up by Pasteur, and it 

 was mainly due to his epoch-making researches that the new 

 doctrine gained general recognition. On this occasion we 

 have only to deal with his 'Etudes sur la biere,' which 

 appeared in 1876. In this he points out the disagreeable 

 changes (diseases) to which beer may become exposed when 

 it is attacked by bacteria, and since these are readily dis- 

 tinguishable by their form from yeast cells, he recommended 

 the employment of the microscope in breweries ; he further 

 advised that the fermentation be conducted in such a manner 

 that organisms could not gain admission from without. The 

 groundwork was thus laid for important improvements 

 in the brewing industry. The reason why they could not 

 then be carried out was that the yeast question still awaited 

 solution, and in the end everything depended upon this 



