172 A TEXTBOOK OF BOTANY [Ch. IV, 8 



we use the carbon dioxide released by Yeast for raising bread, 

 not, it is true, because it is carbon dioxide, but because it is 

 a gas which can be released conveniently all through the 

 dough, from which, along with the alcohol, it is driven out 

 by heat in the baking. Far more extensive is our use of 

 fermentation for production of alcohol. Thus the fermenta- 

 tion of grape juice yields wine, and of germinating barley, 

 beer; and any germinating grains can be fermented in like 

 manner. Fermentation, however, can never yield alcohol 

 stronger than that which inhibits the growth of the Yeast, 

 viz. not over 15 per cent ; and for stronger solutions we must 

 resort to distillation. This process depends on the fact that 

 the boiling point of alcohol is lower than that of water (78° 

 against 100° C), on which account it is possible, by suitable 

 regulation of the heat, to cause most of the alcohol to evapo- 

 rate without much water, the vapor being collected and con- 

 densed. In this way are obtained the stronger alcoholic 

 liquors, — whisky, rum, gin, brandy, etc., which consist of 

 approximately 50 per cent alcohol, and 50 per cent water, 

 together with various flavoring and other substances, some- 

 times naturally occurring in the distillate and sometimes 

 artificially added. The strong alcohol used in medicine and 

 the arts (about 95 per cent) is obtained by repeated distilla- 

 tion, while pure, or absolute, alcohol is secured only by use of 

 chemical methods which remove the last traces of water. 



Fermentation is a phase or form of respiration, and a 

 special form of fermentation is decay. Everybody now knows 

 that decay is caused principally by the activity of micro- 

 organisms, Molds and Bacteria especially. Accordingly we 

 can preserve organic substances indefinitely by any methods 

 which inhibit the action of those organisms, as do heat, poi- 

 sons, and strong solutions of sugar and salt. When t^ese 

 Molds or Bacteria reach suitable solutions of organic sub- 

 stance, whether in plant tissues or animal bodies, they act 

 precisely as does the Yeast in the solutions of sugar, — viz. 

 deprived of an adequate air supply (some kinds indeed never 



