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, as may be realized in the gamy taste of meat which is in the incipient stages of putrefaction. By purely empirical means mankind has learned methods of encouraging the development of some of these products, and is to-day making practical use of this power, possessed by bacteria, of furnishing desired chemical compounds. Industries involving the investment of hundreds of millions of dollars are founded upon the products of bacterial life, and they have a far more important relation to our everyday life than is commonly imagined. In many cases the artisan who is dependent upon this action of microscopic life is unaware of the fact. His processes are those which experience has taught produce desired results, but, nevertheless, his dependence upon bacteria is none the less fundamental. BACTERIA IN THE FERMENTATIVE INDUSTRIES.