126 FHILOSOPHICAL TRANSACTIONS. [aNNO 1757. 



known, will sufficiently explain the ascent of vapours ; and then, that some 

 kinds of vapours are not endiunl witli more or loss than their natural share of 

 electric aether. The immonso raivtiution of explosive bcxlies by heat, depends 

 either on the escape of air before condensed in them, or on the expansion of 

 the a^nstitiKMit parts of those bcxlies. Where air is emitted, it camiot be con- 

 densed ag-ain into the same bulk by cold ; but the expansion of the heated 

 parts of bodies, as soon as that heat is withdrawn, ceases to exist. 



Nitre comes umlor the tirst of these elasso> : \n detonation it emits threat 

 quantities of air, not atlerwards (H>iulensible to the like space. This may be 

 seen bv firing a few i^rains o{ gunpowder in an unblown bladder, or in a vessel 

 ne;u*l) full of water with its mouth inverted. The same is true of all the solid 

 parts of animals and vegxHables, when subjected to tire; as ap{)ears i'rom the ex- 

 periments oi' Dr. Hales. But o\' water the contrarj- is evident. In the steam- 

 engine, a jet of cold water instantly eondenses that immense mretaction ; which 

 could nt>t be, if it was constituted t>f esc\if.K\l elastic air. And though this steam 

 nui>t be aeknowUnlgtHl to lui\e >ome properties of air, sueh as ventilatina^ a fire 

 or that a tajvr blown out by it is capable of being again lighttxl innnediately, 

 and that without a crackling noise, which occurs when touched w ith w ater, this 

 does not in the least invalidate our opinion, though it has certainK conduced 

 very nuich to propag-ate the former one : since from this way of reasoning, the 

 whole must be air, and we should have no water at all in vapour. 



From considering this power of expansion, which the constituent parts of 

 some Ixxlies aa]uiR' by heat ; many things before utterly inexplieable, Ixvame 

 easily understocxl. Sueh as, why when bismuth and zinc are fuseil too;ether, 

 and set to ctxil, the .ine, whieh is sjxvitieally heavier, is touml above the bis- 

 muth: Why the butf covering of intiamnuitory blcxxl, the scum of heated milk, 

 the sedative Silk of borax, which are all sjx.vitically heavier than the liquids in 

 which thev an* tornuxl, aa^ still foruKxl at tlieir surface. How IxMizoin, sul- 

 phur, and e\eu the jKMulerous bcnly mereury, may be raised into \ajxHir, ajjain 

 to be condensed unaltenxl ? And lastly, how water, whose parts appear tKnn 

 the iBolipile to Ix^ capable o( immeasurable exjxmsion, should by heat alone be- 

 come sjxvitically lighter than thceotninon atmosphen\ without having Rx\)urse 

 to a shell inclosing air, or other assistant n\u hiner\ ; and when raiseii, to sup- 

 port theu\ tlixiting, perhaps many days in the atmosj)here. 



But before we prvxxxsl to this Ind \\in of our task, it will be necessary pre- 

 viously to consider, tirst, how small a degrtv o( heat is ixx|uired to detach or 

 raise the vapour of water tixnn its parent fluid. In the coUlest day, or even the 

 coldest niglit of winter, when the weather is not tt\)st\ or vit\ damp, wet linen 

 or paji«p will become dry in the ct>urse of a few hours. A greater tlt^ree of heat 

 must indeeil cause a quicker evaponUion. But vx^re it not for tiie pressure of 



