380 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



a tube for purposes of demonstration, a tube of thin steel is preferable, 

 as it can be used over and over again. In this case it is necessary to 

 lay a thick pad of wet asbestos paper along the top 

 of the tube, since the more uniform heating which 

 results from the better conductivity of the steel is 

 detrimental to the formation of the non-homo- 

 geneous cylinder of vapor, which only takes place 

 when the temperature gradient between the top 

 and bottom of the tube is very steep. The plate 

 glass ends are fastened on with sealing-wax, a 

 small glass tube being sealed in in the manner 

 described in my previous paper, through which 

 the air is exhausted. See Figure 4. 



Glass tubes were used in making the measure- 

 ments, as there was no occasion for cooling the 

 tube and reheating it and the temperature gradient 

 takes care of itself in this case. The sodium was 

 first melted in a small iron crucible and poured 

 ^ into an improvised sodium press ; a piece of ap- 

 g paratus so easily made and so useful that a brief 

 o description of it may not be out of place. A steel 

 '^ cylinder is bored with a hole two or three cms. in 

 diameter, to within a few millimeters of its end. 

 A 3 mm. hole is then drilled through the end at 

 the centre of the larger hole. A solid steel piston 

 is turned so as to fit easily into the bore of the 

 larger cylinder. Tlie molten sodium is poured in, 

 allowed to cool, and the piston inserted. The 

 whole is then put into the jaws of a large vice, 

 and the sodium wire squeezed out directly into 

 the dispersion tube. In this way the formation 

 of oxide is prevented and the sodium is freed from 

 the hydrocarbon in which it is kept. The glass 

 plate is immediately cemented on and the tube 

 exhausted on a mercurial pump. An ordinary 

 hand pump would doubtless answer every purpose, 

 as the vacuum does not need to be below a few 

 millimeters. I see no reason why a well made 

 steel tube should not be used year after year in class demonstration, 

 though I have not tried keeping one so long. AVith the tube once 



