282 REPORT— 1880. 



The general results of the investigation are summed up by Profs. 

 Liveing and Dewar thus : — 



' The phenomena attending the compression of the vapours, as well as 

 those of the amalgams of varying percentages, seem to indicate that the 

 width of the D absorption is dependent on the thickness and temperature 

 of the absorption vapour rather than on the whole quantity of sodium 

 present in it. Very minute quantities diffused into the cool part of the 

 tube appear to give a broad diffuse absorption, while a layer of denser 

 vapour of small thickness in the hottest part of the vessel gives but a 

 very narrow absorption. This may, however, be due to the variation of 

 temperature.' 



In a previous paper, Profs. Liveing and Dewar ' had expressed them- 

 selves as follows on the widening of lines : — 



' It is apparent that the expansion of lines so often observed when 

 fresh materials are introduced, must be ascribed to increase in the density 

 of the vapours, not to any increase in temperature. Moreover, the length 

 of the tube, which reaches a very high temperature in the experiments 

 above described, is very short in the lime crucibles, and still shorter in 

 the carbon crucible, so that the reversing layer is also short in many 

 cases.' 



There is one cause, which, as Profs. Liveing and Dewar mention, 

 may have affected the results of the later paper : ' The results of the 

 foregoing experiments may have been complicated by the sodium- vapour 

 which diffused into the cool part of the vessel. We have attempted to 

 overcome this complication by passing down into the bottle, when full, or 

 nearly full, of sodium vapour, a platinum tube, closed at the top with a 

 glass plate and filled with nitrogen, and observing the absorption through 

 this tube. The nitrogen in the tube prevents, for a short time, the entry 

 of the sodium vapour into the tube, and so, by passing the tube to differ- 

 ent depths, the thickness of the layer of sodium through which the 

 observations were made could be varied. It was found, in this way, that 

 a layer of sodium-vapour, about 4 cm. thick, at the atmospheric pressure 

 at the temperature of our furnace, gave the D absorption sharp and very 

 narrow ; but as the sodium diffused into the tube the absorption extended 

 until it produced a broad band with difiTuse edges.' 



In these experiments, the light emitted by the bottom of the platinum 

 vessel, in which the sodium was evaporated served as the source of light, 

 the absorption of which on its passage through the vapour was observed. 

 Now it is clear that, had the vapour been throughout of the same tem- 

 perature with the vessel, the absorption would have exactly counter- 

 balanced the radiation, and no effect would have been produced. The 

 absorption which was produced was, therefore, entirely due only to the 

 vapour in the parts of the tube which were cooler than the bottom. If, 

 therefore, the effect of compression was to di'ive down the vapour into 

 the hotter part of the tube, a thinning out of the absorption would be a 

 necessaiy consequence, and no conclusions as to the effect of pressure 

 can be drawn. On the other hand, it is difficult to see why, even in the 

 compressed tube, the vapour should not have gradually diffused into the 

 cooler parts. The disappearance of the channelled- space spectrum of 

 sodium, however, in the compressed vapour, indicates a higher tempera- 

 ture, and consequently a diminished absorption. 



' Proc. Roy. Sor. xxviii. p. 370. 



