[ 3(^3 ] 



even new, and lubricated with the cleaneft oil, yet by a fingle 

 ftroke, the oil will be tinged of a greeniili colour from its 

 having corroded or difTolvcd a portion of the brafs. Brafs is a 

 compofition of copper and zinc, either of which, being diffolved 

 by the vitriolic acid, emits inflammable air; and diflblved by the 

 aerial or vegetable acid, will (I fuppofe) produce fome kind of 

 permanently elaftic air; here fuch folution of the brafs is ma- 

 nifeft, and why, at every ftroke of the pifton, air ftiould be ge- 

 nerated ; and alfo why I have often found more of this lir to 

 be produced in the pump (and the degree of rarefadion it could 



* 



effed to be lefs) when it was clean than when dirty ; as the 

 oil being, when foul, more faturated, could diflblve lefs of the 

 brafs ; and alfo that oil proved a worfe fubftance to lubricate 

 the pifton than hog's-lard, becaufe (as I apprehend) the fluidity 

 of the oil allowed every particle of it fuccefllvely, to come in 

 contad with the barrel, and to diflTolvc more of the metal ; 

 which could in a lefs degree happen with the lard, from its 

 ftiffhefs ; neither is it hard to conceive that the folution of the 

 metal, and confequently the quantity of faditious air produced 

 by it, ftiould be more copious in a moift atmofphere than in a 

 dry one ; when it is confidered that /ales nott agunt n'lfi foluti ; 

 and that all metals are in damp weather moft apt to tarnifh 

 and contrad ruft, which is a fuperficial folution of them. I 

 believe alfo that air made moift not by what is called veficular, 

 but by diflxilved vapour (in which latter ftate I found it to be 



Z z 2 moft 



