HYDROCARBONACEOUS SUBSTANCES. 57 



sure to air and light, it undergoes an alteration, in which some of its 

 tartaric acid is replaced by carbonic acid. In this condition it will par- 

 tially precipitate on boiling, even without the presence of sugar. It 

 should, therefore, be kept in bottles which are quite full and protected 

 from the light ; and, whenever a suspected fluid is to be examined, a 

 small portion of the test-liquor should be previously boiled, to make 

 sure that it has not undergone decomposition. Although by exposure, 

 at a summer temperature, Fehling's liquor may become altered at the 

 end of a week, yet if protected from the light, in carefully closed and 

 full bottles, it can be kept unchanged for several years. 



Thirdly, one of the most marked properties of glucose, available as 

 a test, is its capacity for fermentation. If a small quantity of beer- 

 yeast be added to a glucose solution, and the mixture kept at a tem- 

 perature of 25 C., after a short time it becomes turbid. It then begins 

 to liberate carbonic acid, which is partly dissolved in the liquid and 

 is partly given off in the form of gas bubbles, which rise to its surface. 

 From this circumstance the process has received the name of " fermen- 

 tation" 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 and car- 

 bonic acid. The transformation is expressed as follows : 

 GLUCOSE. ALCOHOL. CARBONIC ACID. 

 C 6 H 12 6 = 2C 2 H fi O + 2C0 2 . 



When this change is complete, the fermentation stops and the liquid 

 becomes clear, its turbid contents 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 capable of exciting fer- 

 mentation in other saccharine liquids. 



If, instead of pure glucose, we employ the expressed juices of certain 

 fruits, like those of the grape, which contain albuminoid matters in 

 addition to glucose, fermentation begins after a certain period of ex- 

 posure, and goes on with the same phenomena as before. This is the 

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

 fermentation of fluids containing glucose or a similar saccharine sub- 

 stance. 



The fermentation of glucose is due to the action of a colorless micro- 

 scopic fungus, known as Saccharomyces. This plant consists of cells, 

 which multiply by a process of budding-, but do not produce filaments, 

 nor any compound vegetable fabric. The species present in beer-yeast 

 is the " Saccharomyces cerevisiaa." Its cells are usually rounded in 

 form, sometimes oval (Fig. 3). They vary in size, the greater num- 

 ber having a diameter of about 10 mmm. They have a thin investing 

 integument, which incloses a finely granular semi-solid substance, often 

 containing rounded cavities or vacuoles filled with fluid. The cells are 

 mostly isolated, but occasionally two of them may be seen adhering 

 together. There is a small amount of intercellular liquid, containing 

 albuminous matter and mineral salts. 



