324 ^'HILOSOPHICAL TKANSACTIONS. [aNNO 172g. 



that fire is a particular substance, or distinct element; which has never yet been 

 proved by experiment or observation ; and which Mr. Hales, in his Vegetable 

 Statics, has shown to be an ill grounded opinion ; making it very plain that, 

 in chemical operations, those bodies which had been thought to become heavier 

 by particles of fire adhering to them, were only so by the adhesion of particles 

 of air, &c. which he has shown to be absorbed in great quantities, by some 

 bodies, while it is generated, or reduced from a fixed to an elastic state, by 

 others ; nay, that it may be absorbed and generated successively by the same 

 body, under different circumstances. 



2dly. If we should allow the abovementioned supposition, the difficulty will 

 still remain about the production of rain by the separation of the fire from the 

 water; for Dr. Niewentyt ascribes this effect to two different causes. First, to 

 Condensation, (Sect. 23,) Saying, '* That when contrary winds blow against 

 the same cloud, and drive the watery particles together, the fire that adhered to 

 them gets loose, and they, becoming then specifically heavier, precipitate and 

 fall down in rain." Then in the very next Sect, he ascribes it to rarefaction, 

 when he says, " That when a wind, blowing obliquely upwards, causes a cloud 

 to rise into a thinner air, i. e. specifically lighter than itself, the fire which, 

 by sticking to the particles of water rendered them lighter, extricates itself 

 from them, and ascending by its lightness, the water becomes too heavy, not 

 only to remain in this thin and light air, but even in a thicker and heavier near 

 the earth, and so will be turned into a descending dew, mist, or rain, or 

 snow, or the like, according as the watery vapours are either rarefied or 

 compressed." 



The first of these causes of rain is contrary to experience : for when two 

 contrary winds blow against each other, over any place of the earth, the baro- 

 meter always rises, and we have fair weather. For then, as Dr. Halley says, 

 in Philos. Trans. N° 183, the air being accumulated above, becomes specifi- 

 cally heavier about the clouds, which, instead of falling into rain, as Dr. Nie- 

 wentyt supposes, ascend up into such a part of the atmosphere, as has the air 

 of the same specific gravity with themselves. 



And if the falling of rain might be attributed to the second of these causes, 

 then every time a cloud is encompassed with air specifically lighter than itself, 

 rain must necessarily follow ; whereas one may often see the clouds rise and 

 fall without rain, even when the barometer shows the weight of the air to be 

 altered. For that happens only when, by the great diminution of the specific 

 gravity of the air about the cloud, it has a great way to fall ; in which case, 

 the resistance of the air, which increases as the square of the velocity of the 

 descending cloud, causes the floating particles of water to come within the 



