It was evident both to the superintendent and to Bob that his distress was real, and although neither of them understood it Mr. Hennessey cut short his explanation. Try as they would the strange interruption left a jarring note behind it, and to ease the tenseness the older man stepped forward and, taking from a rack near by one of several glass tubes filled with yellow liquid, held it up to the light. "You see much must still be done to this stuff before it comes out white," he said. "We squeeze the liquid through a series of filter bags and also send it through other filters filled with black bone coal." "What is black bone coal?" Bob demanded. "Bone coal is a product made by burning and pulverizing the large bones left at the abbatoirs until a coarse-grained black powder not unlike emery sand is made; if this is not allowed to become too fine with using it is an excellent sugar filter. In fact, strangely enough, nothing has ever been found to take its place, and it has become a necessary but expensive agency employed in every sugar refinery. Quantities of it are used; in our refinery alone we have about a hundred bone coal filters and each one holds thirty tons of black bone coal. That will give you some idea how much of it is needed. We get nothing back on it, either, for in the process of using it becomes finer, and after that it is good for nothing unless, perhaps, to be made into cheap shoe-dressing. Unlike many of the other industries sugar refining has no by-products; by that I mean nothing on which the manufacturer may recover money. On the contrary in the leather business, for example, almost every scrap of material can either be utilized or sold for cash; odds and ends of the hides go into glue stock, small bits of leather are made into heel-taps or hardware fittings. But in refining cane-sugar there is nothing to be turned back into money to reimburse the manufacturer for his outlay. What isn't sugar is dead loss." The three now moved on and saw how the heated juice traveled by means of pipes from one vat to another, and how it constantly became thicker and clearer. "One of the greatest dangers to successful sugar making is fermentation," observed Mr. Hennessey. "Sugar must continually be stirred by revolving paddles to keep it from fermenting; we also are obliged to take the greatest care that our vats and all other receptacles are clean, and that the plant is immaculate. Frequently we wash down all the walls with a solution of lime in order that the entire interior of the refinery may be quite fresh." "I didn't dream it was so much work to make white sugar," ventured Bob, a little awed. "Our maple-sugar making was much simpler."