134 



Profs. Liveing and Dewar 



[Feb. 28, 



metallic vapour reaches, and to prevent further displacement of the 

 vapour, and thus to maintain different lengths of the iron tube full of 

 metallic vapour at a comparatively constant temperature for consider- 

 able periods of time. 



By this means the following observations have been made up to the 

 present time. 



The first metal experimented on was thallium, one of the most 

 volatile of metals. On inserting some pieces into the hot tube, we had 

 at first great difficulty in avoiding oxidation at the upper surface, and 

 we observed no reversal. However, after arranging the current of 

 hydrogen so as to keep the tube free from air, but without any rapid 

 movement of the gas, we succeeded in seeing the characteristic line 

 reversed, and maintained it so for a considerable time. 



The metal indium, closely allied in its behaviour and volatility to 

 thallium, was next examined, and although the amount of this metal 

 at our disposal was not large, we observed the bright blue line 

 reversed. This was most plainly visible when that portion of the 

 vapour which was nearest to the sides of the tube was looked through. 



We had great difficulty in preventing the oxidation of magnesium 

 in the tube, and in using tubes wider than half-an-inch did not 

 succeed in getting any reversal, but with half- inch tubes the b lines 

 were clearly and sharply reversed, also some dark lines, not measured, 

 were seen in the blue. The sharpness of these lines depended on the 

 regulation of the hydrogen current, by which we could cool the upper 

 stratum of vapour at will. 



A piece of metallic lithium was introduced, and gave no results. 

 Sodium was next added in the same tube, and this did not bring out 

 the reversal of the lithium lines. Similarly, chloride of lithium and 

 metallic sodium, introduced together, gave no better results. To a 

 tube containing potassium vapour, some lithium chloride was added, 

 but no lithium line appeared. On adding metallic sodium to this 

 atmosphere, and more lithium chloride, the bright-red lithium line 

 appeared sharply reversed, and remained well defined for a long time. 

 It is worthy of observation that the lithium Hue was only reversed in 

 a mixture of the vapours of potassium and sodium, and it seems highly 

 probable that a very slightly volatile vapour may be diffused in an 

 atmosphere of a more volatile metal, so as to secure a sufficient depth 

 of vapour to produce a sensible absorption. This would be analogous 

 to well-known actions which take place in the attempt to separate 

 organic bodies of very different boiling points by distillation, where a 

 substance of high boiling point is always carried over, in consider- 

 able quantity, with the vapour of a body boiling at a much lower 

 temperature. It is a matter for future investigation how far chemical 

 interactions taking place in a mixture of metallic vapours affect the 

 volatility of a third body, and what relation, if any, this may have to 



