1920-21.] Evaporation of Liquid Air in Vacuum Flasks. 101 
channel of discharge of the cold gas leaving the vessel. Much of, and in 
many cases all of, the heat flowing down the neck-tnbe enters the uprising 
current of air in that tube, and never reaches the liquid in the flask. The 
evaporation loss due to neck conduction thus depends upon the rate of 
discharge of gas from the flask as well as upon the dimensions of the neck, 
and in a poor flask with a high total evaporation loss the neck-loss will 
be actually as well as relatively less than in a good flask of the same 
dimensions. 
Other things being equal, the evaporation rate of a large vessel, though 
proportionately less, is, in grams per hour, actually more than in a small 
vessel It therefore follows that as the size of the flask is increased the 
neck may be shortened, or alternatively, made stouter, without the evapora- 
tion rate due to neck conduction being affected. Heat-transfer along the 
neck is studied experimentally in a later part of the paper. 
In the cases examined, the temperature of the gas issuing from the 
mouths of metal vacuum flasks containing liquid air lay between —4° C. 
and —30° C. As the inner globe is made of so good a conductor as copper, 
gilding metal, or brass, its temperature may be regarded as uniform at all 
points of the sphere, that temperature being the boiling-point of the liquid. 
It is therefore apparent that the heat transferred to the inner globe by 
radiation and conduction across the vacuum space is all absorbed in giving 
latent heat to the gas boiled off, and that the neck alone is responsible for 
heating the evaporated gas from the boiling temperature to that at which 
it discharges into the outer air. 
Evaluation of the Effects of Radiation and Conduction 
for a Glass Flask. 
The writer’s method of analysing the tranquil evaporation-rate of a 
vessel holding liquid air or oxygen, so as to apportion the amount of loss 
due to the three several causes set forth above, is indicated by the present 
example, which consists of a simplification of the general problem in 
that the transference of heat down the glass neck, and in opposition 
to the upward flow of cold gaseous oxygen, must have been altogether 
negligible. 
Dewar filled a glass vacuum flask with liquid oxygen (boiling-point, 
— 185° C.) and measured its evaporation rate when the flask was immersed 
in liquids maintained at different temperatures, with the results stated in 
Table I.* At that time (1893) the vacuum was obtained by washing out 
* Sir James Dewar, “Liquid Atmospheric Air,” Proc. Roy. Inst., xiv (1893), p. 1. 
