of sour milk or decaying cheese, or both, such a mixture always containing lactic organisms.  To be sure, it also contains many other bacteria which have different effects, but the acid producers are always so abundant and grow so vigorously that the lactic fermentation occurs in spite of all other bacteria.  Here also there is a possibility of an improvement in the process by the use of pure cultures of lactic organisms.  Up to the present, however, there has been no application of such methods.  The commercial aspects of the industry are not upon a sufficiently large scale to call for much in this direction.

At the present time the only method we have for the manufacture of lactic acid is dependent upon bacteria.  Chemical processes for its manufacture are known, but not employed commercially.  There are several different kinds of lactic acid.  They differ from each other in the relations of the atoms within their molecule, and in their relation to polarized light, some forms rotating the plane of polarized light to the right, others to the left, while others are inactive in this respect.  All the types are produced by fermentation processes, different species of bacteria having powers of producing the different types.

Butyric acid.

Butyric acid is another acid for which we are chiefly dependent upon bacteria.  This acid is of no very great importance, and its manufacture can hardly be called an industry; still it is to a certain extent made, and is an article of commerce.  It is an acid that can be manufactured by chemical means, but, as in the case of the last two acids, its commercial manufacture is based upon bacterial action.  Quite a number of species of bacteria can produce butyric acid, and they produce it from a variety of different sources.  Butyric acid is a common ingredient in old milk and in butter, and its formation by bacteria was historically one of the first bacterial fermentations to be clearly understood.  It can be produced also in various sugar and starchy solutions.  Glycerine may also undergo a butyric fermentation.  The presence of this acid is occasionally troublesome, since it is one of the factors in the rancidity of butter and other similar materials.

Indigo preparation.

The preparation of indigo from the indigo plant is a fermentative process brought about by a specific bacterium.  The leaves of the plant are immersed in water in a large vat, and a rapid fermentation arises.  As a result of the fermentation the part of the plant which is the basis of the indigo is separated from the leaves and dissolved in the water; and as a second feature of the fermentation the soluble material is changed in its chemical nature into indigo proper.  As this change occurs the characteristic blue colour is developed, and the material is rendered insoluble in water.  It therefore makes its appearance as a blue mass separated from the water, and is then removed as indigo.