[ 364 3 



moft detrimental to the pump's performance) is more impreg- 

 nated with an acid principle ; unlefs we may fuppofe that mere 

 water diffolves other metals as well as iron : And this with 

 what has preceded, will perhaps account for the different powers 

 of the machine in different ftates of the air ; and for its general 

 failure in the perfedion to be expeded from the foregoing 



theory *. 



The 



• 'I'he lower plate of the piflon was made of iron ; and water or moift vapour 

 will difliilvc or decompofe iron ; by which folution inflammable air is always emitted : 

 whether this might have taken place in the prefent inftance I know not, nor whe- 

 ther mere moifture in the air, free from any acid, could aft on the brafs as on iron, 

 when aflifted by fri£lion, or could difpofe the oil or ointment to adl; on it, and to 

 produce by a kind of calcination any decompofitioii of either the oil, or metal or 

 water : what is certain is, that air is generated in greater quantity by fome fuch de- 

 compofition in damp weather, than it is when the air is dry, and this both in fummer 

 and winter. We are told that iron, without heat, and zinc, charcoal and oils, with 

 heat, will decompofe water, and feparate inflammable air. Could the fricllon of the 

 pifton produce the requifite heat ? we are alfo told that if water were decompofed 

 into its conflituent parts, viz. inflammable air and dephlogiflicated air, this latter is 

 or contains an acid principle, which, when uncombined with any other fubftance, 

 would aft on moft metals ; and fo the damp air loaded with moifture only, and not 

 with any adventitious acid in the atmofphere, may be conceived to produce efl^efts 

 different from thofe of dry air. The paflage of the eleftric matter however, in the 

 atmofphere, will convert a part of the air into nitrous acid, which diflblves moft 

 metals; may not this abound more in the air in moift weather ; moifture in the air 

 being generally, if not always, the effeft or confequence of eleftricity ? and may not 

 this be the caufe of the rufting of metals by damp air, and of the above-mentioned 

 effefts in the pump-barrel .' This I believe to be the cafe, becaufe I found that in 

 certain ftates of the air, much aqueous vapour was introduced by it into the rec'' 

 without generating any confiderable quantity of fluid which was peirmanently elaftic, 

 and becaufe the difpofition of iron, &c. to ruft is not always the fame when the 

 moifture and temperature of the air is the fame. 



