Theory of Comhujiion. 497 



muft not be caft too thick, or it will never take the parabolic figure intended to be given to 

 it. The beft proportion I have found for this purpofe is, a metal of four inches and a half 

 diameter, and eighteen inches focus, fhould be four-tenths of an inch thick at the edge of 

 it : the back of the mirror fiiould be convex to ftrengthen it, and to caufe it to fpring and 

 adhere to the poliftier uniformly. Its convexity {hould be equal to its concavity on the face, 

 that the metal may be every where of an equal thicknefs. The handle (hould be made of 

 lead of the fame convexity and concavity as the metal, its thicknefs about double that of the 

 metal, and its diameter three-fourths of that of the fpeculum •, it fhould have a hole in the 

 middle, with a copper or iron fcrew on it, fo as to put it together with the mirror, to which 

 it is fattened with pitch on a collar lathe, in order to fmooth and finilh the edge of the 

 metal, which may be done by holding a fine file to it when in the lathe at the firft, and 

 afterwards one of the above mentioned ftones. 



(To be coticluded in our next, J 



v.. 



Early Developement of the Antiphlogijlian Theory of Combujlion. By Robert Hooke. 



To Mr. NICHOLSON. 



London, January 7, 1800.. 



SIR, 



M, 



ATCH pains have with juftice been bellowed on the publication and developement of 

 the difcoveries of John Mayow ; but I think the prior claims of another man, of no lefs 

 ability, feem to be overlooked. The Traclatus qtiinque Medlco-Phyfici of Mayow were pub- 

 liftied at Oxford in 1,674, and the licence of Lord Brouncker, firft prefident of the Royal 

 Society, for printing Hooke's Micrographia, bears date Nov. 23, 1664, which itfelf appeared 

 in 1675. I fend the following extraft from page 103 of that work, and have no doubt but 

 it will prove acceptable to the readers of your ufeful Mifcellany. 



I am, &c. R. B.. 



From the experiment of charring of coals (whereby we fee that, notwithftanding the 

 great heat and the duration of it, the folid parts of the wood remain, whilft they are preferved 

 from the free accefs of the air undiffipated) we may learn that which has not, that I know 

 of, been publilhed or hinted, nay, not fo much as thought of, by any } and that, in fliort, 

 is this : 



Firft, that the air in which we live, move, and breathe, and which encompalTes very many 

 and cherifties moft bodies it encompaffes, that this air is the menjlruum, or univerfal dif- 

 folvent of d\\ fulphtireous bodies. 



Secondly, 



