Chap. 2.] Arguments to prove Fire a Subftance. 95 



reality of which has never been doubted. Charcoal, 

 in its natural ftate, contains within its pores a large 

 quantity of air ; but if charcoal is heated, this air is 

 expelled by the fire, which afiumes its place, and oc- 

 cupies the pores of the charcoal. The burning of 

 lime alfo, which deprives it of a great part of its 

 weight by expelling the fixable air, demonftrates that 

 fire, as a fubflance, enters into the pores of the lime, 

 and forces out thofe other fubftances which are leaft 

 intimately combined with it. 



jdly. All the evidence of our fenfes, and many in- 

 dubitable experiments, prove that light, which many 

 fuppofe to be fire in a projectile ftate, is a fubftance. 

 Boerhaave concentrated the rays of the fun in a very 

 ftrong burning-glafs, and by throwing them upon the 

 needle of a compafs, the needle was put in motion by 

 the force of the rays, as it would have been by a bla-ft 

 of air, or a ftroke from fome other body. But this 

 experiment was purfued with Hill fuperior fuccefs, by 

 a late ingenious philofopher *. He conftruded an 

 inftrument, in the form of a fmall vane or weather- 

 cock. It confifted of a very thin plate of copper, of 

 about one inch fquare, which was attached to one of 

 the fineft harpfichord wires, about ten inches long. 

 To the middle of the wire was fixed an agate cap, 

 fuch as is ufed for the fmalleft mariners' compafies, 

 after the manner of which it was intended to turn j 

 and the copper plate was balanced on the other fide by 

 a grain of fmall fhot. The inftrument weighed ten 

 grains j and to prevent its being affected by the vibra- 

 tions of the air, it was inclofed in a glafs box. The 

 rays of the fun were thrown upon the plate of copper 



* Mr. Mitchell. See a fuller defcription of his inftrument and 

 experiments, in the Phil. Tranf. and Piicftley's Opt it?, p. 387. 



from 



