It has been noticed that the glass found in the windows of old churches and in other places where it has been exposed to the prolonged action of the air and of moisture, has gradually become rough on its surface, and has lost to a considerable extent its transparency. This, which would be a defect in glass for the glazing of ordinary windows, where transparency is desired, is rightly regarded as a beauty in glass which is to be used for the ornamentation of windows. Many reasons have been offered in explanation of this apparently peculiar property of ancient glass; and that which appears to be correct is, that glass is a mechanical mixture of different silicates, some of which may be soluble in water, and others insoluble. The old window glass, whose manufacture will be more fully described by-and-by, was made in a less perfect manner than modern appliances enable glass manufacturers now to produce the same article, so that the silicates composing the old glass were not as intimately mixed as those used in modern glass. By the slow action of air and moisture, portions of the soluble silicates have been dissolved out, and hence we frequently find a sort of honeycomb appearance on the surface of ancient glass, as well as a thin film, which, by refraction of light, causes an opalescence when viewed by reflected light. Efforts have of late been made to produce a similar effect by employing different methods in the process of manufacture, but without complete success. The fact, however, that such changes have taken place in this less perfectly fused glass, tends to show, that if one silicate can be dissolved out, there cannot be _chemical_ union between all the silicates. If a piece of modern window glass be heated in water under pressure in a closed vessel, it will present somewhat the appearance of ancient glass, for a considerable quantity of soluble silicate will be dissolved out from it. The object in dwelling on this matter here, is to induce makers to attend more to the chemical composition of their glass, for, doubtless, much more satisfactory results would be obtained both as to the quality of the material and the cost of its production, if thoroughly scientific investigations were conducted by a competent chemist. MANUFACTURE OF GLASS. The first object in glass making is to obtain suitable materials. The sand which is employed for window glass differs from that which is required for flint glass, in that the latter should be as pure as possible. The maker can correct the impurities in the window glass sand, provided they be not present in too great quantities; but it is far more difficult, in the case of flint glass, to chemically counteract the influence of those substances which might impair its tint. So that the manufacturer would rather pay large prices for his sand, than trust to expedients which in their application might fail, and thus cause a greater loss.