The electrical properties of glass are of much greater practical importance, glass being frequently used in electrical appliances as an insulating medium. The insulating properties of glass, as well as the property known as the specific inductive capacity, vary greatly according to the chemical composition of the material. Generally speaking, the harder glasses, _i.e._, those richest in silica and lime, are the best insulators, while soft glasses, rich in lead or alkali, are much poorer in this respect. In practice, particularly when the glass insulator is exposed to even a moderately damp atmosphere, the nature of the glass affects the resulting insulation or absence of insulation, in another way. Almost all varieties of glass have the property of condensing upon their surfaces a decided film or layer of moisture from the atmosphere, and, as we have seen above, glasses differ very considerably in the degree to which they display this hygroscopic tendency. The softer glasses are much more hygroscopic than the hard ones, and the resulting film of surface moisture serves to lessen or even to break down the insulating power of the glass, the electricity leaking away along the film of moisture. In the case of appliances for static electricity, where very high voltages have to be dealt with, an endeavour is sometimes made to avoid this leakage by varnishing the surface of the glass with shellac or other similar substance, and this proves a satisfactory remedy up to a certain point. Quite recently a variety of glass has been brought forward which is peculiar in having a comparatively low electrical resistance, so that for certain purposes it can be used as an electric conductor. Although interesting in itself, this glass is not very likely to prove useful even for the limited number of applications that could be found for an electrically conducting glass, since it is very rich in alkali, and is, therefore, likely to be unstable chemically, even under the action of atmospheric agencies alone. The most valuable and in many ways the most interesting of the properties of glass--its transparency--has not been dealt with as yet, and all mention of this subject has been postponed to the end of the present chapter, because the whole subject of the optical properties of glass will be dealt with more fully in the chapter on optical glass (Chap. XII.), so that a very brief reference only need be made to the matter here.