SUGAR. 



65 



carbonic acid, which is partly dissolved in the liquid and partly rises 

 in the form of gas bubbles to its surface. It is this circumstance which 

 has given to the process the name of " fermentation" or boiling. At the 

 same time the sugar is gradually destroyed and alcohol appears in its 

 place. Finally the whole of the glucose is decomposed, having been 

 converted principally into alcohol, C 2 H 6 0, and carbonic acid, CO.,. 

 Then the fermentation stops and the liquid becomes clear, its turbid con- 

 tents subsiding to the bottom as a whitish layer. This layer is itself 

 found to consist of yeast, which has increased in quantity over that 

 originally added, and is itself capable of exciting fermentation in another 

 saccharine liquid. 



If, instead of a solution of pure glucose, we employ the expressed 

 juices of certain fruits, like those of the grape, which contain nitro- 

 genous albuminoid matters in addition to glucose, fermentation begins 

 after a certain period of exposure to the air, and goes on with the same 

 phenomena and results as before. This is the natural source of all the 

 vinous and alcoholic fluids used by man ; namely, the fermentation of 

 some fluid containing glucose or a similar saccharine substance. 



The alcoholic fermentation of glucose is due to the vegetative 

 action of a microscopic fungus, known as Saccharomyces. This plant 

 consists entirely of cells which 



multiply by a process of bud- Fig. 6. 



ding, but do not produce fila- 

 ments, nor any compound ve- 

 getable fabric. The species 

 which is present in beer-yeast 

 is the " Saccharomyces cerevi- 

 sise." Its cells are usually 

 rounded in form, sometimes 

 oval (Fig. 6). They vary in 

 size, but the greater number 

 have an average diameter of 10 

 mmm. They have a very thin 

 investing integument, which 

 incloses a finely granular semi- 

 solid substance, often with one 

 or two rounded cavities or 

 vacuoles filled with fluid. The 

 cells are mostly isolated, but 

 occasionally two of them may 



be seen adhering to each other. There is also a small amount of inter- 

 cellular liquid, containing albuminous matter and various mineral salts. 

 When a little of the yeast is added to a solution containing glucose, 

 the cells of the yeast-plant after a short time begin to multiply by bud- 

 ding. The buds increase rapidly in size, and, when the young cell has 

 nearly attained the size of its parent, it usually separates and begins an 



SACCHAROMYCES OEREVISI^E, in its 

 quiescent condition ; from deposit of beer- 

 yeast, after fermentation. 



