724 Professor Sir James Deiuar [June 11, 



WEEKLY EVENING MEETING, 



Friday, June 11, 1909. 



His Grace The Duke of Northumberland, K.G. P.C. D.C.L. 

 LL.D. F.R.S., President, in the Chair. 



Professor Sir James Dewar, M.A. LL.D. D.Sc. F.R.S. M.R.I., 



Fullerian Professor of Chemistry, Royal Institution. 



Problems of Helium and Radium. 



[abstract.] 



Metallic Vacuum Flasks and Syphons. 



Progress in low temperature investigation is greatly aided by careful 

 attention to questions of method. This observation is specially 

 applicable to vacuum-jacketed arrangements for heat isolation. 

 Silvered glass vessels are most useful, and give high isolation, but 

 are subject to deterioration and collapse, all the more when their 

 form is complicated. Thus the question of the production of good 

 metallic vacuum- jacketed apparatus becomes highly important. If 

 influx of heat by radiation and gas convection is diminished as 

 much as possible, metallic vacuum-jacketed vessels give no out- 

 ward sign of the low temperature of their contents. I have here a 

 lead pipe nearly 100 ft. long (Fig. 1). Liquid air is flowing from 

 the glass vessel A to the lower one B, which is connected to a 

 suction pump at C. The pipe shows no sign of frosting or con- 

 densation of moisture, except at the two ends which dip into the 

 liquid air. The outside tube is the jacket to a smaller inner tube 

 (which is externally covered with a layer of flannel) in which the 

 liquid air is flowing, the annular space between the two tubes being 

 well exhausted. The vacuum is maintained high by some charcoal 

 placed in a small enlargement D at the end of the tube dipping 

 into the liquid air reservoir A. 



I have here a double jacketed vacuum vessel of 3 litres capacity, 

 made of nickel, full of liquid air. A little charcoal placed — as des- 

 cribed in my Friday Evening Address of 1906— in the lower part of 

 the flask, where it is cooled to the temperature of the liquid air, keeps 

 the vacuum between the walls up to the necessary perfection by 

 absorbing adventitious gases. The neck of the flask is made of 

 thin German silver, a badly conducting alloy, and is covered with a 



