1920-21.] Evaporation of Liquid Air in Vacuum Flasks. 99 
the metal which, with the sole exception of silver, has the lowest emissivity. 
The German-built vessels now in this country are of brass, in the manu- 
facture of which alloy, in its finest grade, the Germans are unrivalled. 
Dewar, before the war, built satisfactory vacuum flasks of nickel. 
A first-class 50-lb. container loses, by tranquil evaporation, about 
2‘5 lbs. of liquid air per day. The average loss of these vessels is probably 
about 3*5 lbs. per day. 
Causes of Evaporative Loss. 
Leaving out of account certain minor and negligible causes of heat- 
transfer, there may be said to be three ways by which heat from the 
outside atmosphere may reach liquid air stored in a 
vacuum flask. Stated in their order of importance 
for a good flask, these are : 
(1) By radiation from the relatively warm outer 
vessel to the cold inner vessel ; 
(2) By conduction across the vacuum space ; and 
(3) By conduction down the neck of the flask. 
When the vacuum is failing, conduction across 
the vacuum becomes responsible for a greater heat 
transfer than radiation. 
Let be the absolute temperature of the hotter 
outer globe, and 0 2 that of the colder inner globe. 
Let us deal with the three processes of heat-transfer in the order 
given above. 
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(1) Radiation . — We are here concerned with heat passing from a 
relatively hot spherical surface (whose temperature was caused to assume 
different values in the course of the experiments) to a similar cold surface 
whose temperature remains constant, the latter being the boiling-point of 
the liquid the vessel contains. Variation in the temperature of the hotter 
surface involves variation in the dominant wave-length of the radiation, 
and it is necessary to inquire whether this does not, in its turn, involve 
a change in emissivity. 
The dominant wave-length, in microns, \ m , and the absolute temperature 
of the hotter surface, 0 V are connected by the expression \ m Q 1 = 2950. In 
the case of the three-litre gilding-metal flask (fig. 2), whose evaporation 
rates were determined (see below) at external temperatures of 10°, 44°, 70°, 
and 100°C., \ m assumes the values 10*4, 9*4, 8-6, and 7‘9 respectively. The 
gilding metal contained 95 per cent, of copper; and although it will 
eventually be seen that its emissivity is considerably higher than that of 
