436 THE POPULAR SCIENCE MONTHLY. 



now chosen — the " soda " of the markets, as it happens to be the 

 cheaper. Our forefathers used potash. As none of these sub- 

 stances 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 com- 

 paratively 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 de- 

 velopment 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 in- 

 dustries 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 depend- 

 ent upon England for our supply of alkali, but there is a promis- 

 ing 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 abun- 

 dant 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 de- 

 posits of lead ore in the Mississippi Valley, and the large output 

 of the metal from the silver smelters of the West, make the sup- 

 ply of the oxide quite up to the demand. 



