12 GLASS glass appear to be in the proportions of chem- ical equivalents results, however, obtained by practice and not by mixtures made with this view. Various causes affect the stability of the combinations and the qualities of the com- pounds. The alkali in window glass powder- ed and moistened is detected by its action upon turmeric paper, and may be partially dissolved out by long continued digestion in boiling water. Atmospheric agents sometimes remove it in part from window panes, leaving a film of silica or silicate of lime. The glass of sta- ble windows is liable to change its appearance, and assume prismatic colors, from the action of the ammoniacal vapors upon the silica. Changes in the degree of oxidation of its metal- lic ingredients, which are sometimes induced by atmospheric causes, are also attended by changes of colors. Long continued cooling has the effect of changing the structure, caus- ing it to lose its transparency and become devitrified. Its ingredients form among them- selves a new arrangement of their particles, and compounds are produced which assume a crystallized structure. By remelting, the vitreous character may be restored, though with a loss of a portion of potash which was volatilized in the devitrification. In ma- king articles of glass, and especially bottles, it is necessary to guard against this tendency to crystallize, and shorten the process of anneal- ing on account of it. Devitrified glass was first described by Reaumur, and has hence been called Reaumur's porcelain. In consequence of the ease with which it may be made into any shape, and its tenacity and refractory na- ture, not unlike porcelain itself, it has been thought that it may be employed as a cheap substitute for this material, especially in many articles used in chemical laboratories. The specific gravity of glass varies with its compo- sition, from 2-4 to about 3'6, although optical glass of greater specific gravity is sometimes made, amounting in some instances to 6. Its density and also its refractive property are in- creased with the proportion of oxide of lead it contains. Brittleness is a quality that limits the alteration of the shape of glass within narrow bounds, after it has cooled; but when softened by heat while it is highly tenacious, no substance is more easily moulded into any form, and it can be blown by the breath into hollow vessels of which the substance is so thin that they may almost float in the air. It may also be rapidly drawn out into threads of several hundred feet in length ; and these have been interwoven in fabrics of silk, producing a beautiful effect. In the soft plastic state it may be cut with knives and scissors like sheets of caoutchouc. It is then inelastic like wax ; but when cooled its fibres on being beaten fly back with a spring, and hollow balls of the material have, when dropped upon the smooth face of an anvil from the height of 10 or 12 ft., been found to rebound without fracture to one third or one half the same height. It has the valuable property of welding perfectly when red hot, and portions brought together are instantly united. When moderately heated it is readily broken in any direction by the sudden contraction caused by the application of a cold body to its surface. It is also divided when cold by breaking it along lines cut to a slight depth by a diamond, or some other extremely hard-pointed body of the exact form suited for this purpose ; and it may be bored with steel drills, provided these are kept slightly moistened with water, which forms a paste with the powder produced. Oil of turpentine, either alone or holding some camphor in solution, is also used for the same purpose. Copper tubes fed with emery also serve to bore holes in glass. Acids and alka- lies act upon glass differently according to its composition, and reference should be made to this in storing different liquids in bottles. Sili- cate of alumina is readily attacked by acids, and bottles in which this is in excess are soon cor- roded even by the bitartrate of potash in wine, and by the reaction the liquor itself is contam- inated. A glass that loses its polish by heat ia sure to be attacked by acids. Oxide of lead when used in large proportion is liable to be in part reduced to a metallic state by different chemi- cal reagents, and give a black color to the glass. All glasses are attacked by hydrofluoric acid. In 1863 a series of experiments showing the action of sunlight on glass was begun, and has since been continued, by Mr. Thomas Gaffield, a merchant of Boston, whose collection of authorities on glass and kindred subjects is more complete than any other in this country. As early as 1 824 Prof. Faraday had noticed a change in color produced in glass containing oxide of manganese when exposed to the sun's rays, and this effect was attributed to the ac- tion of solar light on that ingredient. Mr. Gaflfield's experiments, embracing about 80 dif- ferent kinds of glass, colored and uncolored, of English, French, German, Belgian, and Ameri- can manufacture, have proved that this remark- able phenomenon is not limited to glass con- taining oxide of manganese, but extends to almost every species of glass. That the effect is not due to heat, but solely to the actinic rays of the sun, is shown by the fact that no change of color is produced in the glass when it is exposed to heat ; while on the contrary, after the discoloration has been produced by solar light, the colors thus acquired disappear under the action of heat, and the glass as- sumes its normal color. This process may be repeated indefinitely, the change of color being produced by solar light, and the original col- or restored by heat. It was also shown that the effect was not produced by air or moist- ure. In some specimens the change was more easily effected than in others; in some days were sufficient, in others years were required ; but in almost all the change was produced. "It is very interesting," says Mr. Gaffield, " to witness any one of these series of specimens, showing, as in one of white plate, a gradual
