the Radiation of Heated Gases. 467 



The Sodium Radiation. 



Having thus far failed to produce discontinuous spectra by 

 external heating, I next tried what could be done with 

 metallic vapours. Sodium was the metal chosen for the 

 initial experiments, the powerful absorption produced by the 

 vapour of this element on " D " light seeming, on KirchhofF's 

 hypothesis, to give the best chance of success at the very 

 moderate temperatures I could command with a single large 

 Bun sen flame. 



The form of apparatus used in the earlier experiments was 

 designed with a view to excluding, as far as possible, from 

 the tube in which the sodium was to be heated any gaseous 

 substances that might be expected to react chemically with 

 the vapour of the metal : the emission phenomena produced 

 under these conditions beirg then compared with that pro- 

 duced when traces of oxygen or moisture were purposely 

 allowed to remain in the neutral gas in which the sodium was 

 volatilized. In the diagram (Plate VII. fig. 1) A and B are 

 two similar gas-holders ; a rubber tube leads from A to a 

 couple of wash-bottles S', S", containing strong sulphuric 

 acid ; from S" a long tube of hard glass, P, containing a 

 little phosphorus leads into the drying-tube C, which is 

 packed with calcium oxide and calcium chloride — the former 

 to remove carbonic acid, and the latter traces of water which 

 may remain in the gas used after passing the sulphuric-acid 

 bottles. The drying-tube connects on to the porcelain heating- 

 tube H through a metal T-piece, one end of the T having a 

 glass plate carefully cemented in so that one may look along 

 tbe inside of the heating-tube, to which the T is connected by 

 rubber tube tightened with wire, both connexions being also 

 buried in sealing-wax. At the other end of H, which is covered 

 in the centre by a fireclay arch, is a second T of glass, one limb 

 connecting with a glass gland or stuffing-box, G, with pierced 

 rubber ends, and filled with mercury. A long steel rod passes 

 through the gland, the end being flattened to a spoon-shape ; 

 this can be pushed along to the centre of H, or drawn out 

 past the entrance of the side tube of the T. This side tube 

 is closed by a perforated rubber stopper, through which a 

 small glass tube passes bearing a small reflecting prism 

 cemented to the end, which is thus closed up ; but in order 

 to allow of the escape of the gases, so that a current may be 

 set up in the apparatus, a hole is blown in the side of this 

 tube near the prism. The outer end of the tube is connected 

 by rubber tubing to another wash-bottle S'" containing sul- 

 phuric acid, and from this again a tube leads to the gas- 

 holder B. Thus the entire apparatus forms a closed circuit 

 and has no inlet or outlet. The gas-holders have each a 



