The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON reader may ask, and what has platinum got to do with glass? Well, it is generally recognised that bodies expand when heated—they become more bulky as the temperature rises. As a rule, the expansion differs with different substances, but it so happens that platinum expands to the same extent as glass for a given rise of temperature. We may realise the significance of this fact when we remember that it is sometimes necessary to pass a meta] wire through the walls of a glass vessel. This has to be done, for example, in the electric glow lamp, the illuminating power of which is due to a carbon filament raised to a white heat by the passage of an electric current. As the carbon would soon burn away if it were surrounded by air, the little glass globe which protects the filament must be freed from air and then sealed up. The wires, therefore, which carry the current must pass through the walls of the globe, and the question at once arises, what metal should be used for these wires ? Copper, the metal which is so commonly used for electric wiring, would be quite unsuitable, because it does not expand and contract with change of tempera- ture at the same rate as glass. If we passed a copper wire through a piece of glass while the latter is hot and soft, and melted the glass all round the wire, then on cooling, owing to the unequal contraction of the metal and the glass, a condition of strain would be produced, leading finally to the fracture of the glass. A metal is required which expands and contracts at the same rate as glass, and the one metal with this characteristic is platinum. Hence it comes that an electric glow lamp has two little pieces of platinum