1868.] 



Constitution of the Sun and Stan 



18 



31, since its atomic vapour- volume is half that of the foregoing gases. 

 Similarly in arsenic we must take 150 instead of 75 ; on the other hand, in 

 cadmium and mercury we must halve the atomic weights, and take 56 and 

 100 instead of 112 and 200. In the case of sulphur, each molecule of its 

 vapour has a mass represented on the same scale by the atomic weight of 

 sulphur, viz. 32, if the vapour be observed at temperatures above 1000° 

 Centigrade ; but at some lower temperature it seems to contract to one-third 

 of its former volume, since at 500° Centigrade, and under, it is found to be 

 thus shrunk. The mass of each molecule has become three times what it 

 had been before, and is therefore represented at low temperatures by 96. 



23. Let us now consider what it is that puts a limit to the atmosphere. 

 Let us first suppose that it consists of but one gas, and let us conceive a 

 laver of this gas between two horizontal surfaces of indefinite extent, so 

 close that the interval between them is small compared with the mean 

 distance to which molecules dart between their collisions, but yet thick 

 enough to have, at any moment, several molecules within it. Molecules 

 are constantly flying in all directions across this thin stratum. Some of 

 them come within the sphere of one another's attraction while within the 

 layer, and therefore pass out of it with altered direction and speed. Let 

 us call these the molecules emitted by the layer. If the same density and 

 pressure prevail above and below the layer, the molecules which strike 

 down into it will, on account of gravity, arrive with somewhat more speed 

 on the average than those which rise into it. Hence those molecules 

 which suffer collision within the stratum will not scatter equally in all 

 directions, but will have a preponderating downward motion, so that of the 

 molecules emitted by the stratum more will pass downwards than upwards. 

 This state of things is unstable, and will not arrive at an equilibrium until 

 either the density or the temperature is greater on the underside of the 

 layer. If the density be greater, more molecules will fly into the stratum 

 from beneath than from above ; and if the temperature be greater the mole- 

 cules will strike up into it, both more frequently and with greater speed. 

 In the earth's atmosphere it is by a combination of both these that the 

 equilibrium is maintained : both the temperature and the density decrease 

 from the surface of the earth upwards. 



24. We have hitherto taken into account only those molecules which, 

 after a collision, have arrived at the stratum from the side on which 

 the collision took place. But beside these there will be a certain number 

 of molecules which, having passed through the stratum from beneath, 

 fall back into it without having met with other molecules, either by reason 

 of the nearly horizontal direction of their motion, or because of its low 

 speed. The number of molecules that will thus fall back into the stratum 

 will be a very inconsiderable proportion of the whole number passing 

 through the stratum, so long as the temperature and density are at all like 

 what they are at the surface of the earth. In the lower strata of the at- 

 mosphere, therefore, the law by which the temperature and density de- 



