xeviii Proceedings of the Asiatic Soc. of Bengal. [N.S., XVII, 
curves with it. Thus at 20 km. the proportions of the gases 
resent - 
® 
argon a a ah i 
oxygen os ieaee : ts 
nitrogen 8% 
um none 
But if we pass to 100 km. we have cut out all the argon and 
most of the oxygen, and are beginning to get an appreciable 
quantity of helium. At 200km. there is an atmosphere of 
re helium, and this extends probably unalloyed into outer 
. 
no chance of being retained in the earth‘s atmospher or- 
ding to Jeans,! the hi emperature of 740°C in the outer 
: - When we get to these great heights, the atmosphere 
is almost inconceivably attenuated, and it may interest you 
to see what the actual conditions are, according to the cal- 
culations by Chapman and Milne. 
eatin pina en cael ncnn i Pesto Ett ete 
| Ht. km. | Gr. pere.c. + | Relative density. | Mol. per c ce. | 
| 
| : 
0 | 125 10°. | 1 | 27 10" 
| WEL 8 peel ee a | a | 9 10° 
| -17 -1b | 5 
bie, BOD dl ss FIG A eI i430. | 27 10° 
fo | | 
If we take the density of the air at the ground surface as 
unity, we must think of it as 10- at 800 km.; the atmos- 
phere there is only a hundred-million-millionth part as dense 
! Jeans, Dynamical theory of gases, 1904, p. 323. 
