Chap. 5-] Subjtances which eafily evaporate. 133 



The general procefs which bodies not highly in- 

 flammable undergo when fubjected to the a6Hon of 

 fire, is firft to be reduced to a fluid (late, and then to 

 a ftate of vapour. There are, however, fome matters 

 which are converted into vapour without at all afiiim- 

 ing the fluid form, fuch as camphor, fal ammoniac, 

 arfenic, &c. Thefe, when expofed to fire, fly, ofF in 

 vapour, without being melted j which vapour, on con- 

 denfation, becomes a folid mafs again. Thefe fub- 

 flances may therefore be faid to have their vapourific 

 point below that of their fluidity -, and the reafon of 

 this appears to be, that their particles have a ftronger 

 attraction for the matter of fire than for each other. 

 In fad, we find that thefe fubftances may be reduced 

 to a fluid form by confining them in clofe veflels, 

 where they may be forced to endure a greater degree 

 of heat than under the preflure of the atmofphere. 

 Camphor at leaft has in this manner been rendered 

 fluid ; and there is no reafon why fal ammoniac, and 

 all the volatile alkalies, might not be reduced to the 

 fame ftate j but from the great elafticity of the vapour, 

 the procefs has not been completed for fear of burfting 

 the veflels. 



There are fome bodies which have never hitherto 

 been reduced to a ftate of vapour. Thofe earthy fub- 

 ftances which have been rendered fluid, have never, by 

 any degree of heat, been rendered volatile, and there 

 are fome earths which have never been even brought 

 into fufion. Some of the metals, particularly gold 

 and filver, were thought formerly to be abfolutely fix- 

 ed. Mr. Boyle expofed a fmall quantity of each for 

 two months to the heat of a glafs-houfe furnace, and 

 at the end of that time he found them not altered ; 

 they have fince, however, been compelled to emit very 

 fenfible vapours by the more intenfe heat of a burning- 

 K 3 glafs: 



