90 Fermentation and Kindred Phenomena. 



of manufacturing for themselves albuminoid bodies out 

 of simple compounds. The schizomycetes then thrive in liquids 

 containing the necessary materials for their growth, but they 

 also frequently live on solids, such as potatoes, white of egg, &c. 

 Oxygen (in the free state) is essential to the lives of some, but 

 not of all. 



It is necessary for me to say a few words about the methods 

 which have been devised for cultivating organisms in the pure 

 state, and indeed our present knowledge of most of the really 

 important facts connected with them depends very much upon 

 the introduction of accurate methods for the purpose. For 

 if we consider for one moment that the spores of countless 

 species swarm in the air, and are present, unless suitable pre- 

 cautions are taken, in water and in animal and vegetable 

 matter, we can easily see that the isolation and cultivation of one 

 particular species is a difficult task : for how exclude the others ? 

 An agriculturist would be puzzled to know how to raise a crop 

 of corn without any weeds whatever if he were not permitted to 

 remove the weeds as they appeared. The Bacteriologist is 

 called upon to do this, but the weeding operation is denied him. 

 How has this difficulty been overcome ? It is evidently essential 

 in the first place to have a soil suitable for the growth of the 

 organism we want to cultivate, and this soil must be free from 

 other organisms or their spores ; next, the organism we wish 

 to cultivate, or its spore, must be introduced into the soil ; and 

 thirdly, the soil and the organism (or its spore) must be placed 

 under suitable conditions for the growth of the latter ; the 

 experiment being so conducted that no adventitious organisms 

 can make their entrance into the vessel in which the cultivation 

 is proceeding. 



We require then : — 



(i) A suitable nourishing medium, which must be sterile (i.e. t 

 free from organisms). 



(2) The pure organism, free from other species. 



(3) Suitable conditions as to temperature, &c, for the growth 

 of the organism. 



Now, in the laboratory various " soils" are made use of. 



