1920-21.] Evaporation of Liquid Air in Vacuum Flasks. 103 
The charcoal (plumstone) is in this instance held in a copper tube, F, passing 
through the inner globe, the ends of the tube being covered with gauzes. 
The spheres are respectively 8§ ins. and 7-J ins. in diameter, and their 
surfaces are 220 and 177 sq. ins. or 1419 and 1152 sq. cms. in area. The 
neck is unusually short and wide in comparison to the size of the 
bottle, being 3J ins. long, of which only 2f ins. (6 cms.) are surrounded 
by vacuum. The bore of the neck is f- in. and the metal 0*024 in. in 
thickness; there is 0047 sq. in. or 0*303 sq. cm. of metal in a cross-section 
of the tube. 
Before any of the following observations were made, the flask held 
liquid air for twelve hours It was never allowed to boil empty during the 
fortnight over which the test extended. The same weight of liquid air 
(5 lbs.) was put in, and about 2 lbs. of air were allowed to evaporate on 
each “ run.” The exact losses were ascertained by weighing. The lowest 
of the external temperatures recorded below was obtained by standing the 
flask in a cellar, and the others by immersing it, up to the base of the neck- 
screw, in a water-bath kept at constant temperature. The average 
composition of the liquid air was 50 per cent, oxygen, 50 per cent, nitrogen, 
and was found from samples of the liquid drawn from the flask at the 
beginning and end of a “run.” This mixture boils at —191° C. The 
ascertained losses at different external temperatures are given in Table II. 
Temperature measurements of the air passing up the neck were made 
during the first few tests in order to find out if neck-loss was serious. They 
showed that, (a) with the short neck in question, this source of loss could 
not be neglected, and ( b ) the neck -loss was proportionately less important 
as the outside temperature rose. 
Table II. — Metal Flask : Evaporation Losses at Different External 
Temperatures. 
Period of Test. 
Hours. 
i 
Temperature of 
Outer Globe. 
Absolute 
Temperature, 
Outer Globe, j 
0i. 
Absolute 
Temperature, 
Inner Globe. 
0 2 . 
Evaporation 
Loss, grams 
per hour. 
(a) 21 ) 
( b ) 161 ( 
(c) 14 A 
10° C. 
283° 
82° 
42*5 
(d) 111) 
(a) 131 1 
(b) 131 } 
13 
44° C. 
317° 
82° 
l 
61*3 
70° C. 
343° 
82° 
72-0 
91 
100 ° c. 
373° 
82° 
97*6 
