THE GLASS INDUSTRY. 435 



centers as Pittsburg and southern New Jersey may shift during 

 the course of a decade or so, and bring about a migration of 

 the industry, similar to the migrations which so many of our 

 industries have undergone. It requires a nice technical and com- 

 mercial judgment to strike a balance which shall equally satisfy 

 all these requirements. So it has come about that in certain 

 branches of the industry, and notably in the manufacture of 

 plate glass, the record until quite recently is an almost uninter- 

 rupted record of financial disaster. 



A word, then, in regard to these elements, beginning with the 

 first, the crude materials. 



In many respects the most important ingredient is silica, 

 since every true glass is a silicate of two or more metals. Sands 

 and sandstones are its commercial representative. They are found 

 the world over, but not of equal purity. Much of this material 

 is quite unfit for the glass-maker's use, on account of the iron 

 and other impurities which it contains. Here we reach at once 

 a determining cause in the habitat of the industry. But the dis- 

 crimination does not end with a chemical examination of the 

 sand rock. It concerns itself quite as strenuously with the phys- 

 ical structure of the material. As it is needful that all the in- 

 gredients of the batch shall be in a state of fine powder, the con- 

 dition of the silica supplied by Nature is a matter of no small im- 

 portance. If Nature has already done the grinding, and sandstone 

 and quartz ledge have relapsed into the form of a sand bank, so 

 much the better for our purpose ; or, if the choice be between 

 two sand rocks of unequal hardness and tenacity, the softer and 

 more easily reducible rock will be the available one. In this re- 

 spect America is exceedingly well off. Her sands are among the 

 finest in the world. Both English and French writers on glass 

 declare them to be superior to their own supplies. They are as 

 abundant, too, as they are excellent. The best deposits in New 

 England are those of Berkshire County, Mass. In Pennsylvania 

 the sands of Juniata and Fayette Counties are extensively mined. 

 Other notable localities are in Hancock County, W. Va., Fox 

 River, 111., Crystal City, Mo., and southern New Jersey. New de- 

 posits in various parts of the country are constantly being an- 

 nounced. The importance of this wealth of sand to the glass- 

 maker will readily be appreciated when it is remembered that the 

 average glass contains from sixty to seventy per cent of silica. It 

 is indeed the very foundation of the material. 



The next most important constituent is the alkali, which is gen- 

 erally a carbonate of either sodium or potassium. At the present 

 time the sulphate of soda, or salt-cake, is also frequently em- 

 ployed. The function of the alkali is to furnish one of the metals 

 of the double silicate. Where the carbonate is used, sodium is 



