266 XXI. BACTERIA YEAST. 



These concern themselves chiefly with the microscopical distinc- 

 tion of bacteria by means of stains, with pure cultures of them, 

 and with their pathological action.. Those who wish to undertake 

 special bacteriological studies should turn to these works; in this 

 chapter our intention is merely to introduce the beginner to. the 

 study of bacteria, to make known to him the botanical side of the 

 problem, to teach him some of the most important methods of 

 staining, and to give him an insight into the technics of pure 

 culture. 



General Notions. We will first of all fix our attention upon 

 certain universally-distributed bacterial forms, in order to obtain 

 some generalised information about the group. We shall not 

 endeavour at first to study any particular species ; we will rather 

 leave to accident what form may be at our disposal. 



We boil some green leaves, say of the lettuce, in a Florence 

 flask, and leave it standing open at a comparatively high tempera- 

 ture. Into another flask, with a little water, we place some 

 peas, killed by steeping in boiling water. At the same time 

 we prepare disks of boiled carrot, turnip and potato, on watch- 

 glasses or object-slides, and place them about in warm, moderately 

 moist, places, some free, others covered with glass bell- jars. 

 Upon the infusion of leaves, after two days (or longer if the 

 situation is cooler), a skin may have been formed, w r hich we 

 will call the pellicle. On the different vegetable disks we 

 see small whitish, rarely coloured, spots of gelatinous substance 

 appear, enlarge, fuse together ; on the potato disks these struc- 

 tures mostly take the form of a white, later on grey, and ulti- 

 mately brown, wrinkled skin. We bring a trace of any one of 

 these spots of jelly into a drop of distilled water on an object-slide. 

 This is best effected by means of a bit of platinum wire, fused 

 into a glass-rod for a handle, and the free end of which is curved 

 round into a small loop. Before use this must be heated red 

 hot in a spirit flame, and used immediately when cold, in order 

 to prevent the inclusion of other bacteria which may be about. 

 Cover the drop with a very thin cover-glass, and examine it 

 with the strongest possible magnification ; we find an enormous 

 number of exceedingly minute bodies, appearing almost dot-like, 

 embedded in the jelly. These bodies show a necklace-like 

 arrangement ; but we also find them singly, or in pairs, or 

 united in large numbers into a string. Embedded in the jelly,. 

 then,vwe have the Coccus-form of some kind of bacteriad. If 



