2 ELEMENTARY BIOLOGY. [l. 



If the saccharine fluid is boiled, so as to destroy the 

 efficiency of any yeast it may accidentally contain, and then 

 allowed to come in contact only with such air as has been 

 passed through cotton wool, it will never ferment. But if it 

 is exposed freely to the air, it is almost sure to ferment sooner 

 or later, and the probability of its so doing is greatly increased 

 if there is yeast anywhere in the vicinity. 



These experiments afford evidence (1) that there is some- 

 thing in yeast which provokes fermentation, (2) that this 

 something may have its efficiency destroyed by a high tem- 

 perature, (3) that this something consists of particles which 

 may be separated from the fluid which contains them by a 

 fine filter, (4) that these particles may be contained in the 

 air ; and that they may be strained off from the air by 

 causing it to pass through cotton wool. 



Microscopic examination of a drop of yeast shews what 

 the particles in question are. 



Even with a hand-glass, the drop no longer appears 

 homogeneous, -as it does to the naked eye, but looks as if fine 

 grains of sand were scattered through it ; but a considerable 

 magnifying power (5 — 600 diameters) is necessary to shew the 

 form and structure of the little granules which are thus 

 made visible. Under this power, each granule (which is 

 termed a Torula) is seen to be a round, or oval, transparent 



body, varying in diameter from 2^*^*°i=oon*^ °^ ^^ ^^^^^ 

 [on the average about oTTaa ^h j . 



The TorulcB are either single, or associated in heaps or 

 strings. Each consists of a thin-walled sac, or bag, containing 

 a semi-fluid matter, in the centre of which there is often a 

 space full of a more clear and watery fluid than the rest, 

 which is termed a 'vacuole.' The sac is comparatively tou^h, 

 but it may be easily burst, when it gives exit to its contents. 



