method to start the process by pouring upon the shavings some warm vinegar.  Unless in this way the shavings become charged with the vinegar-holding bacteria the alcohol will not undergo oxidation during its passage over them, and after the bacteria thus introduced have grown enough to coat the shavings thoroughly the acetic-acid production is much more rapid than at first.  If vinegar is allowed to trickle slowly down a suspended string, so that its bacteria may distribute themselves through the string, and then alcohol be allowed to trickle over it in the same way, the oxidation takes place and acetic acid is formed.  From the accumulation of such facts it has come to be recognised that all processes for the commercial manufacture of vinegar depend upon the action of bacteria.  While the oxidation of alcohol into acetic acid may take place by purely chemical means, these processes are not practical on a large scale, and vinegar manufacturers everywhere depend upon bacteria as their agents in producing the oxidation.  These bacteria, several species in all, feed upon the nitrogenous matter in the fermenting mass and produce the desired change in the alcohol.

This vinegar fermentation is subject to certain irregularities, and the vinegar manufacturers can not always depend upon its occurring in a satisfactory manner.  Just as in brewing, so here, contaminating bacteria sometimes find their way into the fermenting mass and interfere with its normal course.  In particular, the flavour of the vinegar is liable to suffer from such causes.  As yet our vinegar manufacturers have not applied to acetic fermentation the same principle which has been so successful in brewing—­namely, the use, as a starter of the fermentation, of a pure culture of the proper species of bacteria.  This has been done experimentally and proves to be feasible.  In practice, however, vinegar makers find that simpler methods of obtaining a starter—­by means of which they procure a culture nearly though not absolutely pure—­are perfectly satisfactory.  It is uncertain whether really pure cultures will ever be used in this industry.

Lactic acid.

The manufacture of lactic acid is an industry of less extent than that of acetic acid, and yet it is one which has some considerable commercial importance.  Lactic acid is used in no large quantity, although it is of some value as a medicine and in the arts.  For its production we are wholly dependent upon bacteria.  It is this acid which, as we shall see, is produced in the ordinary souring of milk, and a large number of species of bacteria are capable of producing the acid from milk sugar.  Any sample of sour milk may therefore always be depended upon to contain plenty of lactic organisms.  In its manufacture for commercial purposes milk is sometimes used as a source, but more commonly other substances.  Sometimes a mixture of cane sugar and tartaric acid is used.  To start the fermentation the mixture is inoculated with a mass