THE ORGANISMS IN YEAST. 217 



siderably as to their exact nature and classification. This, then, is, 

 briefly speaking, what has been done in the investigation of ferment- 

 life, and I will now detail certain methods of procedure which may 

 conveniently be employed in such enquiries, and point out the 

 grounds on which their value depends. 



In the first place, in order to study the Hfe-history of any 

 animal or vegetable organism, we need to obtain satisfactory and 

 uncontaminated specimens, which, as regards many species, espe- 

 cially of the moulds, may be readily accomplished by the means 

 already described as employed by M. Pasteur. But where it is 

 desired to study the principal organism in yeast — the "yeast- 

 plant " itself — the most convenient way is to cultivate ordinary 

 yeast in successive solutions of cane-sugar, until microscopic exa- 

 mination shows that such other organisms as were at first present 

 have disappeared, when the weakened yeast-cells may be re-invigo- 

 rated, and a pure crop obtained by another culture in well-boiled 

 malt-wort, which supplies the needful protein matter, as well as the 

 carbo-hydrates of the sugar-solution. Often, commercial yeast, as 

 obtained from the brewery, may be utilised directly for many pur- 

 poses of study ; but it should be noted that, whilst, in many 

 instances, such yeast is almost wholly free from foreign organisms, 

 in other cases, and especially where the liquid drainings can be 

 had, foreign organisms are found in very large amount. Where it 

 is wished to make a rough separation of the more minute cell-Hfe, 

 it may easily be done by means of filter-paper, the pores of which 

 admit the passages of such cells as the Bacterium lactis and similar 

 minute species. Having obtained the necessary supply of material, 

 the next thing required is to cultivate it, or breed it, under favour- 

 able circumstances, so that its development may be watched and 

 the changes that take place noted. 



In the study of the higher plants, the botanist avails himself 

 for this purpose of the aquarium, the greenhouse, and the garden ; 

 but in the case of these minute plants which are now under con- 

 sideration, not only must a suitable soil, so to speak, be provided 

 for their nutriment, but they must be protected in an effectual 

 manner from the intrusion of those germs of cell-life, which are 

 practically omnipresent, and from which, therefore, both the 

 nutrient liquids and the circumambient air must be absolutely 



