YEAST. 35 



protoplasm of the cells is stained yellowish-brown. This is 

 one of the best tests for proteid matter. The absence of blue- 

 stained particles is proof that the cells contain no starch. The 

 cell-wall is not stained. 



Yeast occurs in the market in a dry or pasty condition as 

 "yeast cakes." Make an emulsion of a bit of one of these cakes 

 in water and examine a drop under high power. Cells similar 

 or identical to those seen in the yeast culture are in abundance. 

 In the pasty cake there is also present rounded bodies many 

 times larger than the yeast cells. These are starch grains, 

 usually of potato starch, which are added to absorb the water 

 of the mass of yeast cells in order to render it semi-solid and 

 capable of being moulded into cakes. 



In brewing industries two well-defined varieties of yeast are 

 used, known respectively as top and bottom yeast. Top yeast 

 is employed in making English ale, stout and porter, fermenta- 

 tion taking place at ordinary summer temperature and pro- 

 ducing carbon dioxide rapidly enough to cause the yeast to 

 collect at the surface of the liquid, hence the name top yeast. 

 Bottom yeast is used in making "lager" beer, and grows quietly 

 at the bottom of the vat at a temperature of about 4 G., which 

 is kept constant by artificial means. Besides this difference in 

 conditions of growth, the two yeasts also differ in form, size and 

 structure when seen under the microscope.- 



Torula is classed among the plants, because it has a cellulose 

 cell-wall and the power of constructing protoplasm (living 

 matter) out of comparatively simple substances, such as am- 

 monium tartrate, which is distinctively a vegetable peculiarity. 

 But though a plant, it contains neither starch nor chlorophyll, 

 and cannot obtain the whole of its food from inorganic com- 

 pounds, thus differing widely from green plants. 



PASTEUR'S SOLUTION. Potassium phosphate, 2 parts; cal- 

 cium phosphate, 0.2 parts; magnesium sulphate, 0.2 parts; am- 

 monium tartrate, 10 parts; cane sugar, 150 parts; water, 838 

 parts. 



