4 



Mr. G. J. Stoney on the Physical 



[Recess, 



moderately to radiate heat, most of which will escape into space. It will 

 thus become a surface of minimum temperature, cooler than the depths of 

 the ocean within, and also for a time than the adjoining parts of the at- 

 mosphere without — just as the surface of the ground becomes a surface of 

 minimum temperature while dew is falling. This surface of minimum tem- 

 perature will draw heat from the warmer bodies on both sides of it, and will 

 thus tend to cool both the atmosphere above and the ocean beneath. 



8. Let us next conceive a particle with a highly emitting surface, and 

 of the same temperature as the surrounding medium, situated in the atmo- 

 sphere a short distance above the ocean. Such a body, owing to its lavish 

 radiation, would quickly fall in temperature far below the bodies around 

 it. If, however, these latter can supply it with heat so fast as to prevent 

 its reduced temperature sinking below the temperature of brilliant incan- 

 descence, it will continue a magnificent spectacle amid the comparative 

 darkness around. Let us now suppose that we have in the neighbourhood 

 of this particle a vapour such that it is gaseous at the ardent temperature 

 of the surrounding medium, but that it is precipitated either as a smoke or 

 mist by the coolness of the radiating particle ; it will, the instant it is so 

 precipitated, begin itself to radiate copiously, and so will tend to maintain 

 the reduced temperature which is the condition of its continuing to blaze 

 forth. The process is the inverse of what takes place in setting a flame 

 alight, and is strictly analogous to it *. If the vapour be of great depth, 

 the upper parts, when precipitated in luminous cloud, will protect the rest 

 of the vapour from that free radiation towards the sky which is the neces- 

 sary condition of the phenomenon. If the blaze had been first communi- 

 cated to a part below the outer layer, it would at first form a cloud in that 

 situation — the radiation from the upper side of this cloud would be least 

 obstructed — the blaze would therefore tend to spread outwards, and would 

 no doubt do so much more swiftly than the cloud could subside. The 

 blaze would therefore soon fly to the upperf surface of the vapour, where 

 alone it could establish itself permanently. Such, then, appear to be 

 the luminous clouds. 



9. Since the great escape of heat takes place from the photosphere, it 

 must be cooler than the contiguous parts of the regions within or of the 

 atmosphere without. And the lowest temperature to which either of these 

 could possibly fall is evidently the reduced temperature of the clouds, a mi- 



* It seems not improbable that as there are substances which will take fire sponta- 

 neously and, when they have done so, will maintain a temperature of ignition — a far 

 higher temperature than they had before — so perhaps there may be vapours in the 

 solar atmosphere capable of spontaneously forming a molecule of liquid or of solid at 

 their own high temperature, which would have but a momentary existence were it not 

 that its instantly beginning to radiate both renders its new state fixed and sets the 

 whole neighbourhood ablaze. 



t It will be shown further on, that a trace of the vapour which forms the clouds 

 may, and probably does, extend far beyond them. But the clouds are at the boundary 

 of the region in which there are; large quantities of the vapour. 



