96 PHYSIOLOGY OF NUTRITION. 



quantities of starch are present as impurities. On addition of 

 Iodine, the protoplasm of the yeast cells becomes brownish in 

 colour. Dilute potash solution dissolves the contents of the yeast 

 cells, the cell wall remaining undissolved. If we take some yeast 

 from very actively fermenting fluid, it is seen, on microscopic 

 examination, that the daughter cells developed by sprouting at 

 first remain associated with their mother cells, but later become 

 isolated. 



Two glass vessels are procured of about 150 c.c. capacity. Into 

 a we pour 100 c.c. of a fluid consisting of 85 parts of water, 15 

 parts of grape sugar, 0'2 parts of KH 2 P0 4 , 0'02 parts of Ca 3 

 (P0 4 ) 2 , 0-02 parts of MgS0 4 , and 1 part of NH 4 N0 3 . Into b 

 we pour 100 c.c. of another fluid, having the same composition as 

 that in a, except that it contains no sugar. To each we now add 

 2 c.c. of yeast water (see Appendix), and then put them into a 

 thermostat at a temperature of 25-30 C., shaking at frequent 

 intervals. In a fairly active fermentation quickly sets in, and the 

 fluid becomes turbid, owing to the propagation of the yeast. The 

 solution b remains clear, since it contains no sugar, and conse- 

 quently no source of carbon for the yeast, and the fungus is unable 

 to propagate itself. 1 If, after allowing the fermentation to pro- 

 ceed for sometime, we filter the fluid a, and collect the yeast on a 

 previously dried and weighed filter, we can accurately determine 

 the amount of yeast formed. If we estimate the amount of sugar 

 in the fluid a before the addition of yeast, and again after the fer- 

 mentation has gone on for a good time (see Section III.), we shall 

 find that, in consequence of the fermentation, and owing of course 

 to the vital activity of the yeast fungus, much sugar has disap- 

 peared. The sugar is expended chiefly in the production of Carbon 

 dioxide and alcohol, and in the growth of the yeast fungus. 



A solution a is prepared which consists of 85 parts of water, 15 

 parts of grape sugar, 0'2 parts of Potassium phosphate (KH 2 P0 4 ), 

 0-02 parts of Calcium phosphate (Ca 3 2P0 4 ), 0'02 parts of Mag- 

 nesium sulphate, and 1 part of pepsin. A solution b is similar in 

 composition, except that the grape sugar is replaced by cane 

 sugar (sugar candy). On adding a good supply of yeast, active 

 fermentation is quickly set up in both fluids, especially if they are 

 kept warm (25-30 C.). The yeast inverts the cane sugar, and 

 the fermentable material thus produced rapidly undergoes decom- 

 position into alcohol, Carbon dioxide, etc. 



If fluids similar in composition to those just described are 



