Of the by-products of gas-making, ammoniacal liquor was, until recently, the only commercial source of ammonia. Coal-tar, formerly thrown away as worthless, is today the source of innumerable substances of immense value to science and the industries. From coal-tar are obtained benzine, toluene, xylene, phenol (carbolic acid), naphthalene, anthracene, etc., and these more direct products are combined with one another or with other chemicals to produce coloring matters, explosives, perfumes, flavoring materials, sweetening substances, disinfectants, medicines, photographic developers--in short, a little of everything. The total number of coal-tar products runs into the thousands, and is constantly being increased by fresh discoveries. In Germany, just before the war, the industries engaged in making these products (no longer _by_-products, but far more important than coke and gas) were capitalized at $750,000,000. One firm made no less than 1,800 coal-tar dyes, besides 120 pharmaceutical and photographic preparations. PREPARED BY THE EDITORIAL STAFF OF THE MENTOR ASSOCIATION ILLUSTRATION FOR THE MENTOR. VOL. 6, No. 6, SERIAL No. 154 COPYRIGHT, 1918. BY THE MENTOR ASSOCIATION, INC. [Illustration: SMOKE PROBLEM--SCENE IN PITTSBURGH BEFORE AND AFTER THE SMOKE CURE] _THE STORY OF COAL_ _The Smoke Problem_ FIVE Smoking is a costly and injurious habit. This is not the beginning of an appeal on behalf of the anti-cigarette crusade, but the introduction to a few facts in regard to the far-reaching effects of smoky chimneys. The smoke nuisance is as old as the use of coal. In the fourteenth century a man was executed in London for befouling the air of that city with the fumes of “sea-coal,” as ordinary mineral coal was once called in England, because it was brought to London by sea. Under Queen Elizabeth a law was passed forbidding the burning of coal while Parliament was in session, as the legislators believed their health was likely to be impaired by the smoky air of the city. What would these bygone gentlemen say if they could see modern London enveloped in one of its famous “pea-soup” fogs--the color and denseness of which are entirely due to coal-smoke?