relative to Black, Watt, and Cavendish. 491 



The production of volatile salts in combustion, by an analo- 

 gous process of combination, seems likewise to have been ap- 

 prehended by him, where he represents "other parts of the 

 combustible," not capable of the aerial form, as nevertheless 

 so "mixing and uniting with the parts of the air," as "to 

 make a coagulum or precipitation, as one may call it, which is 

 separated from the air," but being light and volatile is carried 

 up by its motion, till the agitation that kept it rarefied ceases, 

 and it condenses into " a certain salt which may be extracted 

 out of soot:" and the view thus expressed appears from the 

 Registry to have been corroborated at one of these sessions 

 by Boyle, who observed that " vegetables reduced in the open 

 air yield store of volatile salt like that of hartshorn and other 

 animal bodies, whereas in common distillations he had not 

 found them to yield a grain." 



Hook produced evidence also before the Society of that 

 sameness of effect, by which he identified the particular ingre- 

 dient in the air that supports combustion with one of the 

 fixed constituents of nitre. To this purport he " made an ex- 

 periment with charcoal enclosed in a glass, to which nitre being 

 put, and the hole suddenly stopped up, the fire revived, al- 

 though no fresh air could get in,"— and another "of gun- 

 powder burning without air." 



It is curious to remark that a similar experiment was made 

 some fifty years before by the Cabbalist and Rosicrucian an- 

 tagonist of Kepler and Mersenne, Fludd ; who in proof " that 

 the substance of saltpetre is nothing else but air congealed 

 by cold*," relates that he filled an egg with it, mixed with 

 sulphur and quick lime, and closing the aperture with wax 



medium between the first candle and the eye caused the darkness to inter- 

 mix with the light, so as to exhibit the appearance of the heterogeneous 

 jet d'eau. This jet deau I suppose to be nothing else but the mixture of the 

 air with the parts of the candle which are dissolved into it in the flame. 

 The reason why this mixed body, which certainly is otherwise heavier than the 

 air, and so ought to descend, doth notwithstanding ascend with great swift- 

 ness, is first from the ascent of the flame in the middle, and next from the 

 extraordinary rarefaction of the same by the nearness and centrality of the 

 flame and heat, whereby it is made much lighter than the ambient air." 



* " Videmus salis petrae substantiam nihil aliud esse quam a'erem frigore 

 congelatum, cui si accedit sulphuris aliqua portio, licet exigua, admodum 

 strepitum ingentem edit, fulguraque artificialia emittit." — Utriusque Cosmi 

 Historia, vol. i. tract. 1. lib. 7. cap. 6. De fulmine et tonitru, 1617. 

 " In 2da demonstratione, candela in fundo vasis alicujus aqua repleti affi- 

 gitur, cujus flamma per orificitim phialae ingrediens, depresso ejus orificio 

 ad angulos rectos cum candela in vasis aqua', sursum attrahet tantam aquae 

 proportionem quantam aeris in phiala inclusi consumpserit; aer enim nutrit 

 igncm, et nutriendo consumitur; ae ne vacuum admittatur, aqua, hoc est 

 tertium elementum, locum possidet aeris comesti." — Ibid, tract, 2. part. 1. 

 lib. 3. Reg. 6. 



2 L2 



