Lead of glass is much less liable to such changes, but where in such glass decay has once set in, the whole mass may be converted into a white horny substance. In other cases the surface of a piece of clear white glass will become gradually filled with a series of minute intersecting fissures, which in time may penetrate the whole mass. When this change has been fully developed we get a true crackle-glass, not to be confounded with the frosted glass of Venice mentioned in Chapter XIII. This fissuring of the glass-mass in its various stages may be traced in many of the specimens of Venetian, Netherlandish, and English glass at South Kensington. When fully developed the effect is at times very beautiful. The tints of coloured glass may, it would seem, change in the course of time. Colourless glass also, from which the greenish shades derived from protoxide of iron have been removed by the addition of binoxide of manganese, is above all liable to assume in the course of time a purple tint under the action of sunlight. Again, if sulphur be present in glass, as is the case where sulphate of soda has been employed as a source of the alkali, the soda salt may be reduced by any protoxide of iron that is present. The sulphide of sodium and the sesqui-oxide of iron thus gradually formed will both of them tend to give a yellowish tint to the glass.[6] Changes of this nature may occasionally have come about in the stained glass of the windows of our Gothic churches—the flesh-tints, which we know were produced in early days by manganese, may in the course of time have become of a more pronounced purple hue. CHAPTER II THE PRIMITIVE GLASS OF THE EGYPTIANS AND SYRIANS From a technical point of view the history of glass might be divided into three periods—periods, it is true, of very unequal length and relative importance.