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 commercial 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 uninterrupted 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 discrimination does not end with a chemical examination of the sand rock. It concerns itself quite as strenuously with the physical structure of the material. As it is needful that all the ingredients of the batch shall be in a state of fine powder, the condition of the silica supplied by Nature is a matter of no small importance. 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 respect 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, Ill., Crystal City, Mo., and southern New Jersey. New deposits in various parts of the country are constantly being announced. 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 generally a carbonate of either sodium or potassium. At the present time the sulphate of soda, or salt-cake, is also frequently employed. The function of the alkali is to furnish one of the metals of the double silicate. Where the carbonate is used, sodium is