Fermentation and Kindred Phenomena. 75 



appearance in the juice, whilst in proportion its sweetness 

 becomes lessened. Here we have a case of spontaneous fermen- 

 tation, and the reason why wine-making is such an ancient 

 process at once becomes apparent : for the first person who 

 pressed out grape juice and allowed it to remain undisturbed 

 for some time must have been the conscious or unconscious 

 discoverer of fermentation. 



But in the manufacture of other alcoholic beverages, such as 

 beer, &c, the conditions are not so simple, for something must 

 be added to the " sweet wort," or infusion of malt, to cause the 

 fermentation, and that something is "yeast" or "barm." 



Here let me at once say that in all cases of ordinary fermen- 

 tation two things are necessary (1) a solution of sugar (2) yeast. 

 I will explain presently why yeast is not added to grape juice, 

 merely remarking that it is found abundantly in the juice after 

 it has fermented. 



I have here some sugar (not ordinary sugar, but the same 

 sugar which exists in grape juice — hence called " grape" sugar) 

 dissolved in water. To the mixture I have added some yeast, 

 and the whole has been kept at about blood heat for some six 

 hours. 



You observe that the liquid is frothing, or ■' working" as it 

 is called, and it is from that phenomenon that the term fermen- 

 tation is deried \_fervere — Lat. to boil~\. Now this frothing 

 is due to the escape of a gas (as was first noticed by Van 

 Helmont) and that gas, as we can readily demonstrate, is car- 

 bonic acid. 



I have here another experiment proceeding, namely, the 

 distillation of some fermented sugar solution, to show you that 

 spirit has actually been formed. 



Now, as we took nothing originally but sugar and yeast, it is 

 obvious that the spirit has been produced from them ; and as 

 at the end of the experiment we find the yeast in undiminished 

 quantity, whereas some or the whole of the sugar has disappeared 

 (according to the conditions of the experiment) it is obvious 

 that the spirit and carbonic acid have come from the sugar. 

 In fact it has been ascertained that the sugar is decomposed in 



