104 On the Suspension of Clouds. [Feb. 



mise, the presence of this gas in foggy atmospheres, on the tops 

 of lofty mountains, and on other places where humidity was 

 present, could not fail of being detected. 



Mr. Luke Howard, on the contrary, thinks he explains the 

 suspension of vesicular vapour in the atmosphere by supposing 

 its particles to be similarly electrified, and, consequently, to 

 repel each other ; but this repulsion, although by preventing 

 their condensation it might retard, would not altogether prevent 

 their descent ; since a globule of water, however minute, must 

 still be heavier than the circumambient air, and, consequently, 

 gravitate to the earth as certainly as, although less rapidly 

 than, a volume of the same fluid, however considerable its 

 dimensions. 



The suspension of the clouds must, therefore, be considered 

 unaccounted for by either hypothesis, nor am I aware of any 

 other that possesses superior claims to attention, and, accordingly, 

 in that part of your System of Chemistry which treats on meteo- 

 rology, and which I regret to find omitted in the late edition, 

 no explanation of this phenomenon is proposed. 



It is the obscurity which seems to hang over the subject, 

 rather than any confidence I feel in my own speculations, that 

 has induced me to propose the following hints, which, if with- 

 out value in themselves, may yet be of some service in directing 

 the attention of your better informed readers to a neglected 

 department of science.* 



The quantity of moisture which the air is capable of dissolving 

 must depend partly on the temperature and partly on the density 

 of that fluid. 



Near the earth's surface, the high temperature of the air 

 causes it to preserve a larger portion of aqueous vapour in an 

 elastic form : in the most elevated regions, owing to the extreme 

 rarity of the air, atmospheric pressure scarcely opposes any 

 check to the natural tendency of water to assume an aeriform 

 state, so that it remains in the form of vapour independent, 

 perhaps, of any chemical solution. 



Hence as we ascend in the atmosphere, the quantity of mois- 

 ture held in solution by it must go on diminishing, until we 

 arrive at that point beyond which the effect of the diminished 

 pressure predominates over that of its dullness, so that from 

 this point upwards the quantity of aqueous vapour goes on 

 increasing.']- 



This medium point is the region of the clouds, which are 

 known by actual observation to occupy a certain height in the 

 atmosphere, above which they rarely extend.;]: 



» It is much to be regretted that no systematic work on meteorology has np- 

 peared at least in this language. 



+ See Professor Leslie's late work on the Relations of Heat and Moisture. 



J This will appear more clearly from the following plan: the numbers giveu 

 are of course supposititious: 



