176 Mr, T. Henry on the Weight acquired by 



joint affcion of the above caufes, weighed more 

 when hot, than when cold. 



Having thus endeavoured to Hiew, the infufR- 

 ciency of the explanation given by Mr. White- 

 hurft, I will venture, with the greateft diffidence, 

 to propofe the following query. May not the 

 increafe of weight, acquired by heated iron, 

 and copper, during cooling, be afcribed to 

 the calcination, and confequent abforption of 

 air, continuing to proceed after the removal of 

 the mafs of metal from the fire, the abforption 

 of air in particular, in the firft ftages of the 

 cooling, perhaps, with increafed rapidity? In fup- 

 port of this conjecture, the following fa6ls may 

 be adduced. Firft; That fome metals, parti- 

 cularly copper*, are found to calcine more 

 rapidly, in a moderate degree of heat, than in 

 one more intenfe. Secondly; That, the calces 

 of fome metals, as that of lead, have been ob- 

 ferved to increafe in weight by long expofure 

 to the air, and that they now afford by proper 

 treai;menf, more air, than could have been ob- 

 tained from them, previous to fuch expofure. 

 Thirdly; We fhall find by examining Dr. Roe- 

 buck's account of his experiments, that the 

 weight continued to increafe long after the caufe 

 afllgned by Mr. Whitehurft muft have ceafed 

 to act. The cylinder, which was repeatedly 



* Macquer's Inft. of Chemillry, vol. I. 



weighed 



