14 



Mr. G. J. Stoney on the Physical 



[Recess, 



crease will not be appreciably affected by molecules thus falling back. 

 But in those regions where the atmosphere is both very cold and very 

 attenuated, where accordingly the distance between the molecules is great 

 and the speed with which they move feeble, the number of cases in which 

 ascending molecules become descending without having encountered others 

 will begin to be sensible. From this point upwards the density of the 

 atmosphere will decrease by a much more rapid law, which will within a 

 short space bring the atmosphere to an end. 



Not, however, before the density has sunk immeasurably below what can 

 be reached in our laboratories. If there be a unit-eighteen * of molecules 

 in every cubic millimetre of the air about us, there will remain about a 

 unit-fifteen in every cubic millimetre of the best vacuums of our air-pumps. 

 The molecules are still closely packed, within about an eighth-metre of one 

 another ; i. e. there are about 60 of them in a row as long as a wave of 

 orange light. This accounts for our atmosphere's spreading to the height 

 at which meteors betray its presence, which is far beyond the height at 

 which we can detect it by any ordinary means. 



25. If an atmosphere consist of a mixture of gases (for example, of un- 

 combined nitrogen and hydrogen), the boundary of each gas will be at a 

 different height. "Where the nitrogen is no longer able to maintain itself, 

 the molecules of hydrogen, with a velocity \/l4 (or nearly 4) times as great, 

 can still spread far beyond it. It is also to be observed that the nitrogen 

 will reach a greater height in consequence of the presence of the hydrogen 

 than it could alone, since an ascending molecule of nitrogen has more 

 chance of escaping the fate of falling back without having encountered 

 another molecule if there be molecules of hydrogen to be met with as well 

 as molecules of nitrogen. In this way a substance of which there is but 

 little in the atmosphere may ascend nearly to the full height to which it 

 would rise if present in abundant quantity. 



Thus the vapour of sodium, which, as we shall find, is present in the 

 sun's atmosphere as a mere trace, seems nevertheless to reach nearly the 

 full height assigned to it by the mass of its molecules, through the assist- 

 ance afforded to it by the abundant atmosphere of hydrogen, which extends 

 much further. In the same way the vapour of water is probably borne to 

 the limits of the earth's atmosphere, although but a minor constituent ; and 

 the trace of carbonic acid which terrestrial air also contains, is probably 

 supported to a height nearly as great as it would reach if present in much 

 greater quantity. Where, then, as in the sun's atmosphere, the lightest 

 constituent is abundant, ail the other gases which enter into its composi- 

 tion, will range to heights which stand in the order of the masses of their 

 molecules, whether they be present in large or in small quantities. And 

 where, as in the earth's atmosphere, there is but a trace of the lightest 



* See Phil. Mag. 1868, vol. xxxvi. p. 141. A unit-eighteen is a convenient name for 

 the number expressed by 1 with eighteen 0s after it — that is, for a unit multiplied by 

 10 18 . Similarly an eighth-metre is to be understood as a metre divided by 10 8 . 



