_Substitutions of One Capital Good for Others._--It is not merely in the realm of consumption that the demand for a particular good may increase greatly in consequence of cheapness. The same thing happens in the realm of production, but here the substitution of one thing for others is an even more prominent cause of the increased use of the particular commodity. Aluminum and copper are rivals as carriers of electrical power, with the advantage at present somewhat in favor of copper. As soon as the cost of making aluminum shall be reduced by a moderate fraction it will become the cheaper material for such uses and, unless there is a fall in the price of copper, will thrust itself into use for trolley wires and other conductors of electricity. The possession of an enormous market by the one or the other material depends on their relative costs, and these may easily so change as to transfer most of the demand from the one material to the other. A further fall in the cost of aluminum would make it available for sheathing the hulls of ships and would bring it into general use for many household implements, while a sufficient fall would make it a leading building material and give it a limitless market for the framing and finishing of substantial structures. In these various uses it would substitute itself, not only for copper, but for steel, stone, wood and other materials, and the change would be extensive enough to give it an enormous market without requiring a correspondingly great reduction in its cost. Lowering the cost of aluminum by a third might, by merely making it the favorite carrier of electricity, multiply the present use of it by ten, and lowering it by two thirds might multiply the present use of it by a hundred. If this should take place, saving labor would be anything rather than expelling it from its position in the aluminum-making group. When less labor came to be needed for making a ton of the metal, more labor would be used in the industry that makes it. So long as the substitution caused by the cheapening of aluminum affected copper only it might be a serious matter for the producers of copper; but when it came to replacing in some degree steel, stone, brick, wood, and other materials, the effect would be so diffused and subdivided as to create small disturbances in any one of these industries.