fonu Experiments made with an Air-pump, 625 
and the barometer-gages indicated a degree of exhauftion 
nearly 600; the air was then let into the receiver, the 
pear-gage indicated a degree of exhauftion, but very 
little more than 600 alfo. The near agreement of the 
pear-gage with the barometer-gages in this laft experi- 
ment, in which I had been fo careful to exclude the 
moifture as much as poflible, feemed to prove beyond a 
doubt, that their difagreeing in the firft and fecond ex- 
periments mull have been owing (as Mr. cavendish fup- 
pofed) to the moifture which in them had not been fo 
carefully excluded. But I began now to fufpedt alfo, that 
there might arife a vapour from fome moifture that 
might be contained in the leather foaked in oil and tal- 
low, or in the wooden foot which was cemented to the 
glafs cup, both ufed in the firft and fecond experiments : 
thefe fufpicions induced me to try the following experi- 
ments, 
EXPERIMENT IV. 
A piece of leather dreffed in allum, knowh by the 
name of white Iheep-fkin, of about four inches diame- 
ter, which had been foaked in oil and tallow about a year 
ago (fuch as was ufed to place the receiver on in the firft 
and fecond experiments') was put into the receiver; the 
Vol. LXVII. 4 M pump 
