260 ON THE 60J|ST1TUTI0N OF MIXED GASES, ^C. 



as a meclianical agent ; and this has directed the attention 

 more efpeeially to high temperatures. But it will appear 

 from what follows that the progrefs of philofophy is more im- 

 mediately interefled in accurate obfervations on the force of 

 Reference to fteam in low temperatures. Different authors have pubiifhed 

 ing ikam/*^ " ^^counts of their experiments on the force of fleam : I have 

 on a former occafion (Meteorological Effays, page 13 !•) given 

 a table of forces for every 10° from 80° to 212°. The author 

 Encyl. Britt. of the article «' Steam," in the'^ncyclopedia Britannica, has 

 Betancouit. done the fame from 32° to 280°: and M. Betancourt, in the 

 " Memoirs des fcavans etrangeres" for 1790, fee Hutton's 

 Math. Diftion. page 755) has given tables on the fubjed, 

 both for vapour from water and fpirit of wine, alfo from 32° 

 to 12S0°* But thefe two authors, having afl'umed the force 

 of vapour from water of 32° to be nothing, are effentially 

 wrong at that point and in all the lower parts of the fcale; 

 and in the higher part, or that above 212°, they determine the 

 force too much : owing as I apprehend to a quantity of air, 

 which being difengaged from the water by heat and mixing 

 with the fteam, increafes the clafticity. — In a queftion of fuch 

 moment it feemed therefore defirable to obtain greater ac- 

 curacy. 

 The author's My method is this : I take a barometer tube perfe6lly dry, 



method. A and fill it with mercury juft boiled, m&rking the place where 

 of the fluid is '*^ '^ flationary; then having graduated the tube into inches and 

 put into the tenths by means of a file, I pour a little water (or any other 

 WmeteTHeat ^'<^1"^^ ^^^ fubjed of experiment) into it, fo as to moiflen the 

 is applied by the whole infidc J after this I again pour in mercury, and, care- 

 evternai contaa |^^j| jj^ygrting the tube,, exclude all air: the barometer by 



of water. The -^ , ^ ■' 



fail of the mer- ffauding fome time exhibits a portion of water, &c. of -g- or —^ 



*^"5y '^^^^ '^^ ^f ^" '"tf» upon the top of the mercurial column ; becaufe 



being lighter it afcends by the fide of the tube ; which may 



• now be inclined and the mercury will rife to (he top manifefl- 



inga perfect vacuum from air. I next take a cylindrical glafs 



tube open at both ends, of 2 inches diameter and 14< inches in 



length ; to each end of which a cork is adapted, perforated in 



the middle fo as to admit the barometer tube to be puflied 



through and to be held faft by them ; the upper cork is fixed 



two or three inches below the top of the tube and is half cut 



away fo as to admit water, &c. to pafs by ; its fervice being 



merely to keep the tube ficady. Things being thus circum- 



ftanced 



