VOL. XXXVI.] PHILOSOPHICAL TRANSACTIONS. 32g 



top, till only the mouth remains under water; then, as the steam condenses by 

 the cold air on the outside of the bell, the water will rise up into the bell at p 

 quite to the top, without any bubble above it; which shows that the steam 

 which kept out the water was not air. 



N. B. This experiment succeeds best when the water has been first purged of 

 air by boiling, and the air-pump. 



We know by several experiments made on the fire engine, in Captain Savery's 

 way, where the steam is made to press immediately on the water, that steam 

 will drive away air, and that in proportion to its heat, though in the open air it 

 floats and rises in it like smoke. 



Now if the particles of water, turned into steam or vapour, repel each other 

 strongly, and repel air more than they repel each other; aggregates of such 

 particles, made up of vapour and vacuity, may rise in air of diff^erent densities, 

 according to their own density depending on their degree of heat, without hav- 

 ing recourse to imaginary bubbles formed in a manner that is only supposed, 

 and not proved, as has been already shown. 



Dr. D. owns indeed, that if the watery particles had no repellent force, they 

 must precipitate in the same manner that dust will do, after it has been raised 

 up; but we have too many observations and experiments to leave any doubt of 

 the existence of the repellent force abovementioned. And it cannot be shown 

 by any experiment, how large the moleculae of vapour must be, which exclude 

 air from their interstices, and whether those moleculas vary in proportion to the 

 degree of heat, by an increase of repellent force in each watery particle, or by 

 a further division of the particles into other particles still less; but in general 

 we may reasonably affirm, that the rarity of the vapour is proportionable to the 

 degree of its heat, as it happens in other fluids (see Phil. Trans. N" 270) and 

 that, though the different degrees of the air's rarefaction are also proportion- 

 able to the heat, the same degree of heat rarefies vapour much more than air. 



Now to show that what has been said will account for the rise of vapours and 



the formation of clouds, we must only consider ; whether that degree of 



heat, which is known to rarefy water 14000 times, being compared with several 

 of those degrees of heat in summer, autumn, and winter, which are capable of 

 raising exhalations from water or ice; the rarity of the vapours, estimated by 

 the degree of heat, will appear to be such, that the vapour will rise high 

 enough in winter, and not too high in summer, to agree with the known phas- 

 nomena. 



That the efl^ects are adequate to the causes in this case, the Doctor thinks 

 can be made out in the following manner, viz. The heat of boiling water, ac- 

 cording to Sir Isaac Newton's table (Phil. Trans. N° 270) is 34; the mean heat 



VOL. VII. U u 



