Jome Experiments made with an Air-pump. 623 
“ renheit’s fcale, it turns into vapour as foon as the. 
“ preffure is no greater than that of three quarters of an 
“ inch of quickfilver, or about one-fortieth of the ufual. 
“ preffure of the atmofphere ; but when the heat is only 
“ 4.1 °, the preffure mult be reduced to that of a quarter 
“ of an inch of quickfilver before the water turns into 
“ vapour. It is true, that water expofed to the open air 
“ will evaporate at any heat, and with any preffure of 
“ the atmofphere;. but that evaporation is intirely owing 
“ to the adfion of the air upon it: whereas the evapora- 
“ tion here fpoken of is performed without any affiftance 
“ from the.air. Hence it follows, that when the receiver 
“ is exhaufted to the above-mentioned degree, the moif- 
“ ture adhering to the different parts of the machine 
“ will turn into vapour and fupply the place of the air, 
“ which is continually drawn away by the working of 
“ the pump, fo that the. fluid in the pear-gage, as well as 
“ that in the receiver, will confift in good meafure of var 
“ pour. Now letting the air into the receiver, all the 
“ vapour within the pear-gage will be reduced to water, 
“ and only the real air will remain uncondenfed; confe- 
“ quently the pear-gage flrews only how much real air 
“ is left in the receiver, and not how much the preffure 
“ or fpring of the included fluid is diminifhed, whereas 
«• the 
