These complex issues being satisfactorily settled the exploitation of the erstwhile waste, or rubbish, is energetically pursued. It has now become a potentially valuable by-product, and, accordingly, must be worked for all it is worth. Firmly entrenched upon the market development is vigorously pursued, often to culminate in the quondam waste, now an established by-product, being lifted to such a position of commercial eminence as to dispute premier recognition with the staple in the production of which it is incurred. In more than one instance the by-product has even eclipsed the primary product, or at least attained a level of equal importance, while occasionally the staple has even suffered virtual deposition to rank as little else but a by-product. There are even some cases on record where the manufacture of the staple has been abandoned, at all events for a time, because the by-product, the former incubus of the industry has become invested with such far-reaching importance as to demand the concentration of effort upon its production. Waste--by-product--staple: such constitutes the brief evolution of more than one of the world’s leading lines of trading. Many instances of remarkable topsy-turvydom in this connection might be cited. Possibly one of the most impressive illustrations in this respect, although the transposition is not yet quite complete, is offered by coal-gas. When Clayton first demonstrated the practicability of extracting illuminating gas from coal commercialism feverishly set to work to exploit the gas, and gas only. But the gas proved to be associated with a variety of substances which threatened the very future of Clayton’s discovery. Ammonia fumes poisoned the atmosphere of the room in which the gas was burned to the grave danger of the health--even lives--of the occupants according to the cynics, critics, and caricaturists of the day. The tar carried in suspension in the gas was every whit as exasperating because it condensed in the mains to choke them. Ammonia and tar became the bane of life to the gas-engineers of the period, harassing them to the verge of endurance, while the elimination of the two deleterious substances involved the expenditure of enormous sums of money and prodigious thought.