Leather preparation.—­The tanning of leather is a purely chemical process, and in some processes the whole operation of preparing the leather is a chemical one.  In others, however, especially in America, bacteria are brought into action at one stage.  The dried hide which comes to the tannery must first have the hair removed together with the outer skin.  The hide for this purpose must be moistened and softened.  In some tanneries this is done by steeping it in chemicals.  In others, however, it is put into water and slightly heated until fermentation arises.  The fermentation softens it so that the outer skin can be easily removed with a knife, and the removal of hair is accomplished at the same time.  Bacterial putrefaction in the tannery is thus an assistance in preparing the skin for the tanning proper.  Even in the subsequent tanning a bacterial fermentation appears to play a part, but little is yet known in regard to it.

Maceration of skeletons.—­The making of skeletons for museums and anatomical instruction in general is no very great industry, and yet it is one of importance.  In the making of skeletons the process of maceration is commonly used as an aid.  The maceration consists simply in allowing the skeleton to soak in water for a day or two after cleaning away the bulk of the muscles.  The putrefaction that arises softens the connective tissues so much that the bones may be readily cleaned of flesh.

Citric acid.—­Bacterial fermentation is employed also in the ordinary preparation of citric acid.  The acid is made chiefly from the juice of the lemon.  The juice is pressed from the fruit and then allowed to ferment.  The fermentation aids in separating a mucilaginous mass and making it thus possible to obtain the citric acid in a purer condition.  The action is probably similar to the maceration processes described above, although it has not as yet been studied by bacteriologists.

Benefits derived from the products of bacterial life.

While bacteria thus play a part in our industries simply from their power of producing decomposition, it is primarily because of the products of their action that they are of value.  Wherever bacteria seize hold of organic matter and feed upon it, there are certain to be developed new chemical compounds, resulting largely from decomposition, but partly also from constructive processes.  These new compounds are of great variety.  Different species of bacteria do not by any means produce the same compounds even when growing in and decomposing the same food material.  Moreover, the same species of bacteria may give rise to different products when growing in different food materials.  Some of the compounds produced by such processes are poisonous, others are harmless.  Some are gaseous, others are liquids.  Some have peculiar odours, as may be recognised from the smell arising from a bit of decaying meat.  Others have peculiar tastes,