In another direction the expansive properties of glass are of importance wherever glass is rigidly attached to metal. At the present time this is done in several industrial products, such as incandescent electric lamps and “wired” plate glass. In certain varieties of incandescent lamps, metallic wires are sealed into the glass bulbs, and the only metal available for this purpose, at all events until recently, has been platinum, whose coefficient of expansion is low as compared with most metals, and whose freedom from oxidation when heated to the necessary temperature makes it easy to produce a clean joint between glass and metal. More recently the use of certain varieties of nickel steel has been patented for this purpose, since it is possible to obtain nickel steel alloys of almost any desired coefficient of expansion from that of the alloy known as “invar,” having a negligibly small expansion compared with that of ordinary steel. By choosing a suitable member of this series a metal could be obtained whose coefficient of expansion corresponds exactly with that of the glass to which it is to be united. The oxidation of the nickel steel when heated to the temperature necessary for effecting its union with the glass presented serious difficulties to the production of a tight joint, and several devices for avoiding this oxidation have been patented. In the incandescent electric lamp, although the joint between glass and metal is required to be perfectly air-tight, the two bodies are only attached to one another over a very short length. In wired plate glass, however, an entire layer of wire netting is interposed between two layers of glass, the wire being inserted during the process of rolling. Here a certain amount of oxidation of the wire is not of any serious importance, as it only appears to give rise to a few bubbles, whose presence does not interfere with the strength and usefulness of the glass; but any considerable difference of coefficient of expansion will produce the most serious results on account of the great lengths of glass and metal that are attached to each other. This factor has been neglected by some manufacturers, with the result that much of the wired glass of commerce is liable to crack spontaneously some time after it has left the manufacturer’s hands, while there is also much loss by breakage during the process of manufacture.