and of the Atmosphere, 29 



My reasons for concluding that clouds consist of minute drops, 

 are as follows. Prof. Henry^s experiments demonstrate that the 

 film of a soap-bubble contracts on the included air with great 

 force considering the thinness of the film*. Now the amount 

 of condensation to which the enclosed air is subjected by the 

 film is inversely as the diameter of the sphere, that is, if the 

 thinness of the film is always the same. Supposing a little 

 bubble to be formed in the air, like one of those imagined to 

 form cloud, then, from the very small diameter of such a vesicle, 

 the included air would be strongly condensed, perhaps to the 

 extent of two or three atmospheres or more. Oxygen and nitrogen 

 being soluble in water, and the solubility of gaseous matter in 

 water increasing rapidly with the pressure, it follows that the 

 particles of water will take up air at the interior surface of the 

 film ; and on these particles diffusing themselves, and so arriving 

 at the outer surface of the film, they will liberate a quantity of 

 air corresponding to the diminished pressure. So that from this 

 species of exosmose alone, the bubble would soon become a 

 homogeneous drop. I conclude, therefore, that vesicles so small 

 as the particles which form clouds cannot long exist, if ever they 

 are formed. With regard to the ease with which water takes up 

 air under a high pressure, I may mention, that if water be 

 shaken for a few minutes with air condensed to the extent of 

 about six atmospheres, then on allowing the water to escape into 

 the air by a quiet stream, and receiving it into a glass vessel, a 

 considerable efiervescence can be perceived. 



Dr. Waller could find no evidence of vesicular structure by the 

 aid of the microscopef. 



I think the sky has an important action in producing the 

 morning and evening red. Now the highest clouds are evidently 

 situated below the region of the sky ; and the blue colour of the 

 sky deepens to an observer ascending a mountain. From this I 

 conclude that the firmament consists of a distinct and very distant 

 orb of the atmosphere, containing a great number of particles of 

 water so small as to give a blue of the first order by reflexion. 

 When the sun is near the horizon, its rays traverse a much 

 greater thickness of this orb than at other times, and consequently 

 the direct light of the sun becomes sensibly reddened by inter- 

 ference on transmission through the sky. The orb of the sky 

 can, I think, hardly be regarded as being in a state of rest with 

 respect to the earth, for it may be moved, and perhaps heaped 

 up at times by winds; the sky may also be subject to tidal, and 

 other motions caused by the expansions and contractions of a 

 variable temperature, by which motions the curvature of the 



* Phil. Mag. vol. xxvi. p. 541. 



t Ibid. vol. xxviii. p. 99, and vol. xxix. p. 103. 



