IX.] YEAST. 379 



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

 contents, which readily diffuse themselves through the sur- 

 rounding fluid. The whole structure is called a 'cell;' the 

 sac being the ' cell-wall,' and the more solid portion of the 

 contents the 'protoplasm.' It appears that a nucleus is 

 also present, but on this point there is still some difference 

 of opinion. 



When yeast is dried and burned in the open air it gives 

 rise to the same kind of smell as burning animal matter, 

 and a certain quantity of mineral ash is left behind. Ana- 

 lysed into its chemical elements, yeast is found to contain 

 Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur, Phosphorus, 

 Potassium, Magnesium and Calcium ; the last four in very 

 small quantities. 



These elements are combined in different ways, so as to 

 form the chief proximate constituents of the Toriila^ which 

 are (i) a Protein compound, analogous to Casein, (2) Cellu- 

 lose, (3) Fat, and (4) Water. The cell-wall contains all the 

 Cellulose and a small proportion of the mineral matters. 

 The protoplasm contains the Protein compound and the Fat 

 with the larger proportion of the mineral salts. 



These Tondce are the 'particles' in the yeast which have 

 the power of provoking fermentation in sugar; it is they 

 which are filtered off from the yeast when it loses its effi- 

 ciency by being strained through porous earthenware ; it 

 is they which form the fine powder to which yeast is reduced 

 by drying, and wliich, from their extreme minuteness, are 

 readily diffused through the air in the form of invisible 

 dust. 



That the Torulce are living bodies is proved by the manner 

 in which they grow and multiply. If a small quantity of 

 yeast is added to a large quantity of clear saccharine fluid 

 so as hardly to disturb its transparency, and the whole is 



