The ovens and apparatus required are considerably more expensive, but, since this industry has developed in this country during the last twenty-two years to a point where one-quarter of all of the coke manufactured is made by the by-product process, there can be no doubt that it is a profitable proposition and that eventually the wasteful beehive ovens will be a thing of the past. Practically all of the types of by-product coke ovens in use have been developed in Germany or Belgium, where circumstances forced earlier conservation of resources than in this country. The three best known types are the Semet-Solvay, the Otto Hoffman, and the Koppers—the latter a recent arrival. They differ mainly in details of construction and operation. [Illustration: MACHINE FOR PUSHING COKE FROM OVENS ] In a general way a “battery” of coke ovens consists of from 40 to 80 long narrow brick-walled chambers placed closely side by side with heating flues or “checker-work” between them. The fire for the baking process is in these flues, which are interconnected, and the heat developed is sufficient to drive off the moisture and volatile substances of the coal in the narrow chambers just on the other side of the brick walls. Charging is done by a “lorry” as in the beehive process. After from seventeen to twenty-four hours at a red heat, the coke is “pushed” from the ovens, one after another, by an electric ram which enters at one end. The 30 × 7 × 1½ foot block of glowing coke emerges from the other end, where, breaking under its own weight into good-sized pieces, it falls into a steel car on a track just beneath. A spray of water quenches it and it is taken to the storage bins to be sorted. Rich coal-gas is the main by-product. That which comes off during the first seven hours is the richest and has the greatest illuminating or “candle” power. After washing free from dust, tar, ammonia, etc., the gas is usually run into holders or tanks from which it is distributed for use for illuminating or for heating purposes. That which comes off during the latter part of the coking period has much less of those constituents which give illuminating value. It has good heat value, however, and as fuel is required for keeping the ovens up to the coking temperature, this poorer gas from the coking chambers is switched into and burns in the flues between the coking chambers as mentioned. Thus the larger part of the gas is sold to customers, usually in the city near which the ovens have been located, and the poorer part is utilized in heating the ovens and the steam boilers which run the plant. [Illustration: QUENCHING CAR AWAITING ITS LOAD ]