3/8 ELEMENTARY BIOLOGY. [CHAP. 



the filter allows the fluid part of the yeast to pass through 

 into the solution of sugar. 



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 (i) 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 con- 

 siderable 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 -g-g^lh to 

 T oVuth of an inch (on the average about ^oVo tn )- 



The Torultz are either single, or associated in heaps or 

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

 ing 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 



