fome Experiments made -with an Air-pump. 627 
20,000, and fometimes no more than 500; it likewife 
differs very much with the box wood, which may per- 
haps be owing to different degi'ees of heat and moifture. 
From thefe experiments it is evident, that there, arises 
an elaftic vapour from the leather dreffed in allum and 
foaked in oil and tallow, and alfo from the piece of box 
wood, when the weight of the atmofphere has been 
partly taken off by the adfion of the pump ; and that this 
vapour prefTes upon the furface of the quickfilver in the 
tube of the long barometer-gage, and of that in the cif- 
tern of the fhort one ; and that, confequently, the tefti- 
mony of both thefe gages muft be influenced by this 
vapour, as well as by the fmall remainder of common 
air : but as it is the nature of the pear-gage not to give 
its teftimonv till the remaining air contained in it is 
preffed, fo as to become of the fame denfity of the atmo- 
fphere; and as this vapour cannot fubfiff in the form of 
vapour under that preffure, this gage is not at all in- 
fluenced by it, but indicates the remaining quantity of 
permanent air only. 
Seeing thus what a confiderable quantity of vapour 
arofe from the compound of leather, allum, oil, and tal- 
low, my next objedt was to find out from which of thofe 
fubftances it chiefly arofe ; how far I have fucceeded will 
appear by the following experiments. 
4 M 2 
Sub- 
