touching his Theory of Light and Colours. 221 



usually happens an ebullition. This is one way of transmuting 

 gross compact substance into aerial ones. Another way is by 

 heat ; for as fast as the motion of heat can shake off the par- 

 ticles of water from the surface of it, those particles, by the 

 said principle, will float up and down in the air, at a distance 

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

 make that substance we call vapour. Thus I suppose it is, 

 when the particles of a body are very small (as I suppose 

 those of water are), so that the action of heat alone may be 

 sufficient to shake them asunder. But if the particles be much 

 larger, they then require the greater force of dissolving men- 

 struums to separate them, unless by any means the particles 

 can be first broken into smaller ones. For the most fixed 

 bodies, even gold itself, some have said will become volatile, 

 only by breaking their parts smaller. Thus may the volatility 

 and fixedness of bodies depend on the different sizes of their 

 parts. And on the same difference of size may depend the 

 more or less permanency of aerial substances, in their state of 

 rarefaction. To understand this, let us suppose A B C D to 

 be a large piece of any metal, E F G H the limit of the inte- 

 rior uniform aether, and K a part j?itr. 5. 

 of the metal at the superficies AB. 

 If this part of particle K be so 

 little that it reaches not to the 

 limit EF, it is plain that the aether 

 at its centre must be less rare than 

 if the particle were greater; for 

 were it greater, its centre would be 

 further from the superficies AB, 

 that is, in a place where the 



aether (by supposition) is rarer; the less the particle K there- 

 fore, the denser the aether at its centre ; because its centre 

 comes nearer to the edge AB, where the aether is denser than 

 within the limit E F G H. And if the particle were divided 

 from the body, and removed to a distance from it, where the 

 aether is still denser, the aether within it must proportionally 

 grow denser. If you consider this, you may apprehend how, 

 by diminishing the particle, the rarity of the aether within it 

 will be diminished, till between the density of the aether with- 

 out, and the density of the aether within it, there be little dif- 

 ference; that is, till the cause be almost taken away, which 

 should keep this and other such particles at a distance from 

 one another. For that cause explained in the fourth and fifth 

 suppositions, was the excess of density of the external aether 

 above that of the internal. This may be the reason then 

 why the small particles of vapours easily come together, and 



