THE PREPARATION OF FARM PRODUCTS. 2/9 



upon nearly every farm. The farmer puts aside his cider, or 

 perhaps weak wine, and allows it to remain unmolested for 

 months. During this time the alcohol is, in part, slowly con- 

 verted into vinegar by the formation of acetic acid. In all of 

 these processes the vinegar-maker is, consciously or uncon- 

 sciously, making use of bacteria. 



The formation of acetic acid is essentially an oxidizing 

 process. The acid may be produced from quite a long list of 

 organic compounds, but in practice ordinary alcohol always 

 serves as its basis. Various weak alcoholic solutions are 

 used, such as cider, weak wine, beer, etc., each locality natu- 

 rally using as the source of its vinegar the alcoholic solution 

 which is cheapest and most easily obtained. That the process 

 is essentially an oxidation of the alcohol has long been recog- 

 nized. Even near the beginning of the nineteenth century it 

 was known that vinegar-making was essentially a union of 

 oxygen with alcohol. It is common to express the reaction 

 as occurring in two stages as follows : 



C 2 H 6 O + O = C,H 4 O -|- H 2 O 



Alcohol Aldehyde 



C 2 H 4 + 0= C 2 H 4 2 



Acetic Acid 



Such a simple oxidation can be brought about by a purely 

 chemical process. As long ago as 1821 Davy discovered 

 a peculiar property of platinum black, or finely divided 

 platinum. When alcohol is mixed with this substance the 

 mixture becomes very hot and an active union with oxygen 

 takes place which results in the production of acetic acid. 

 This is a purely chemical, and not a vital phenomenon. 



For a time it was believed that the natural formation of 

 vinegar, by the so-called mother of vinegar, was a similar 

 process. This brownish mass was thought to serve, like 

 the platinum black, to condense the oxygen, and even when 

 Pasteur began his studies, he made the erroneous conclusion 



