relative to Black, Watt, and Cavendish. 485 



certain atmospheres of aether to surround the particles of 

 bodies, and describing a pressure of elastic forces, which vary- 

 ing with the distance produces cohesion at small distances, 

 and repulsion at greater, he deduces among other conse- 

 quences this — " that the particles of vapours, exhalations, and 

 air, do stand at a distance from one another, and recede as 

 far from one another as the pressure of the incumbent atmo- 

 sphere will let them : for I conceive," he says, " the con- 

 fused mass of vapour, air, and exhalations, which we call the 

 atmosphere, to be nothing else but the particles of all sorts of 

 bodies of which the earth consists, separated from one another 

 and kept at a distance by the said principle." 



He then proceeds to distinguish the three different ways 

 which nature has of " transmuting gross compact substances 

 into aerial ones" — vaporisation — volatility — and the libera- 

 tion of fixed air, and to propose a theory to explain the dif- 

 ferences. From the hypothesis, to which I before alluded, of 

 a double repulsive force, producing unequal degrees of aethe- 

 rial pressure, he deduces different spheres of cohesion and re- 

 pulsion for different bodies, and their particles, in proportion 

 to their density and size : small particles are easily detached, 

 and easily condensed; and this is the condition of volatile 

 substances, and of liquids — " when the particles of a body are 

 very small, as I suppose," he says, " those of water are, the 

 action of heat may be sufficient to shake them asunder ; " and 

 " as fast as the motion of heat can shake them off, those par- 

 ticles, by the said principle, will float up and down at a dis- 

 tance from one another, and from the particles of air, and 

 make that substance we call vapour." " But if the particles 

 be much larger, they then require the greater force of dissol- 

 ving menstruums to separate them." Thus he comes to the 

 chief object of this letter, which was to illustrate the theory of 

 gases — of the substances, that is, then recently discovered to 

 be more durably jixed, and more durably aerial, than vapours 

 or volatile effluvia. For this purpose, having assumed that 

 the essence of such substances is, that their constituent parti- 

 cles are relatively larger and denser, and therefore, by hypo- 

 thesis, more elastic than others in the aerial, and more cohe- 

 sive in the fixed condition, he brings in the doctrine of che- 

 mical affinities, elective and mediate, to liberate them from 

 their close state of cohesion, and force them out of the proxi- 

 mate sphere of compression into the remoter one of repulsion. 

 And thus, as subsidiary to a wild play of philosophical fancy, 

 were those great principles laid down, which experience has 

 subsequently verified, and on which the whole fabric of the 

 chemistry of solids, liquids, and gases, has been built. 



