244 



Professor Sir James Dewar 



[Jan. 28, 



sparking coil the lilac-glow of hydrogen fills the tube. When a 

 sponge of liquid air is applied in less than a minute the water vapour 

 is condensed ; the glow disappears ; the tuhe passes through the 

 phosphorescent stage, and finally becomes too highly vacuous for a 

 discharge to pass. On removing the sponge the phenorcena are reversed. 



As concrete examples of the extent of the heat isolation, it can be 

 stated that liquid air in 5-litre vacuum flask loses 12*5 grms. per 

 hour, or 8 per cent per day. If contents weigh 3650 grms., trace is 

 left on the twelfth day. Heat exchange per minute per sq. cm. = 

 0'007 calories. 



Taking radiation law of heat exchange and silver emissivitv, heat 



COOLING ofBOELINC WATER 



5 LITRE, SJLVERED.CUVSS 



VACUUM FIASK 



TIME 



Fig. 3« 



exchange by theory per min. per sq. cm. = 0*015 calories, the 

 observed experimental value being O'Ol. 



1 • 5 litre silvered vacuum vessel lost 9 • 5 grms. per hour, or 15 per 

 cent on first day. Trace left on sixth day. 



Fig. 3 gives the curve for decrement of temperature from the 

 boiling-point of water in a silvered vacuum vessel, showing the same 

 low rate of loss of heat as compared with gain of heat in the former 

 example. 



Some twelve years later a further advance was made by the 

 application of the well-known absorptive power of charcoal. The 

 possibiUty of the occlusion of gases in porous bodies has been a 

 subject of scientific inquiry for more than two hundred years, and 

 has been investigated by a long line of famous chemists. The 

 Discourse on "New Low Temperature Phenomena," in the year 

 1905, was devoted to a consideration of the thermal evolution and 

 absorption of gases by charcoal at low temperatures. A year later 

 the application of this knowledge was made to the production of very 



