I.] YEAST. 3 



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

 sac being the 'cell- wall' and the contents the * protoplasm.' 



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 Torula, 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 Torultz 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 which, from their extreme minuteness, are 

 readily diffused through the air in the form of invisible 

 dust. 



That the Torultz 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 

 kept in a warm place, it will gradually become more and 

 more turbid, and, after a time, a scum of yeast will collect, 

 which may be many thousand, or million, times greater in 

 weight than that which was originally added. If the Torula 

 are examined as this process of multiplication is going on, it 



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