determining the Vapour-Density of Metallic Vapours. 551 



with temperatures as ordinates and densities as abscissae, is 

 shown in fig. 3 (p. 552) ; the dotted curve shown is that for Hg 

 at temperatures where the density of the vapour corresponds 

 to that of Xa ; the temperatures for this latter curve are 

 indicated on the curve. 



Table I. 



Temp. 



Density. 



o 

 308 



00000009 



373 



0000002 



376 



000000035 



380 



0*00000043 



385 



000000103 



387 



0-0O000135 



390 



0-00000160 



395 



0-00000270 



100 



100,H)350 



406 



00000480 



408 



0-00000543 



412 



000000590 



418 



0-00000714 



420 



0-00000750 



"While the densities were not obtained much below 365°, 

 it will be noticed that at this temperature — which cannot be 

 far from that commonly employed in vacuum-tube work * — 

 the density of the Na vapour increases at about the same rate 

 as that of Hg at 85°, while at points slightly above this the 

 Na curve increases much the more rapidly. This fact, taken 

 together with its low atomic weight, might well account for 

 the peculiarities observed by Professor Michelson. 



As was stated above, the experiment was undertaken solely 

 to determine the densities within the range between 350° and 

 450°, and indeed the use of a glass bulb precludes the 

 possibility of anything being done above 500° ; with a 

 porcelain bulb it would be comparatively easy to attain any 

 desired temperature below 1700°. 



The ease with which the temperature of the electric bath 

 could be regulated at once suggested the desirability of 

 making a series of determinations on the vapour-density of 

 Hg, and with the slight alteration in the form of the bulb 



* Professor Michelson assumes that the temperature of the heating- 

 box, 350", is that of the vapour also. There appears to be some doubt as 

 to the legitimacy of this assumption, owing to the very considerable 

 heating produced by the discharge itself. 



