now chosen—the "soda" of the markets, as it happens to be the cheaper. Our forefathers used potash. As none of these substances are furnished directly by Nature, the supply is subject to more rapid and more extreme fluctuations than in the case of silica. Before the Napoleonic wars, and indeed until within comparatively recent times, the chief source of the alkaline carbonates was the ashes of wood and sea-weed. Whole forests were burned, and vast piles of sea-weed were annually collected and reduced to ashes to gain the alkalies. To-day at many a country hearth the wood ashes are carefully put aside for the annual soap-making. Our earlier glass-makers were thus dependent upon the coastman or forester for their supply of alkali, and it can readily be seen that this dependence was a large determining factor in the development of the industry. The poor quality and the uncertain supply were an inconvenience particularly felt in France, where war so often cut off the foreign commerce. To protect French industries from these international hazards, as well as to secure a better supply at all times, the French Government offered a prize for the invention of a process by which soda could be made directly from common salt. The Leblanc soda process was the result. It was published in 1792. By means of the new process any nation which possessed salt springs or brines—and there are few without them—was enabled to make its own soda. The process came into use but slowly, though its effect has been very far-reaching, since it transferred the soda manufacture from the wilderness to the laboratory. In the development of glass-making in America these improvements were quite without influence until within the last half century or so. At the present time we are still largely dependent upon England for our supply of alkali, but there is a promising increase in the home manufacture. The large production of salt in Michigan and New York yields an assured supply of the crude material within comparatively short distances of the glass-making centers, while recently invented processes have greatly improved upon the method of Leblanc.
The third constituent of ordinary glass, limestone, is so abundant and so free from impurity that it is scarcely a determining factor in the development of the industry. One stone is almost as good as another. There is a tendency toward the increased use of lime in modern glass-making, but it is a tendency which may be indulged at very slight expense.
In the finer grades of tableware and decorative products lead takes the place of lime as the second base in the silicate, but with this material, again, America is well supplied. The immense deposits of lead ore in the Mississippi Valley, and the large output of the metal from the silver smelters of the West, make the supply of the oxide quite up to the demand.