Although numerous accurate and extended determinations have been made of the refractive indices of gases which exist as such at ordinary temperatures, few have been made with the vapours of elements or compounds which, at room temperatures, are solids and possess a low equilibrium vapour pressure in that state. Notable among the latter are the determinations made by Prof. R. W. Wood with sodium vapour and those made by Messrs. C. Cuthbertson and E. Parr Metcalfe with the vapours of sulphur, phosphorus, mercury, zinc, cadmium, selenium, and tellurium. In the work of both of these investigations an interference method was adopted. In Wood’s a Michelson interferometer was used and in Cuthbertson and Metcalfe’s one of the Jamin type. For the investigation of Cuthbertson and Metcalfe the glass interferometer tubes ordinarily used with Jamin interferometers were wholly unsuitable. These, as is well known, are provided with plane parallel plate glass ends ground in and luted with shellac or a wax having similar properties, and when heated to 300°C. or higher, the shellac chars and the tubes frequently crack, soften or become distorted. For these reasons Cuthbertson and Metcalfe had recourse to interferometer tubes made of fused silica which were made, with great skill, by Messrs. Heraeus, of Hanau. The ends of the tubes through which the interfering rays passed were plates of quartz ground optically flat, fused into the tubes and again polished. In their manufacture such tubes are, however, difficult to make. Failures are frequent and consequently the tubes are expensive. On this account investigators have been deterred hitherto from studying the refractivities of the vapours of metals and other substances, having moderately high melting points and vapourising temperatures.
On the refractive indices of Gaseous Potassium, Zinc, Cadmium, Mercury, Arsenic, Selenium, and Tellurium
