mechanical Condenfation and Rarefaclkn of Air. 63 



is, 1-J-f.th of the whole elaftic mafs was converted into water; 

 then the heat given out would be fufficient to raife the tem- 

 perature of the remaining mafs of air and vapour 6 or 8 ; ; 

 which fufficiently accounts for the fmall difference obferved 

 in the refults upon warm vapoury air and cold dry air. Hence 

 vapour, far from producing the change of temperature in 

 queftion, tends to diminifli the effect. 



If any doubt remained with me refpecting the real change 

 of temperature that takes place in the operations related above, 

 it was completely removed by the refults of the two following 

 experiments. 



Exp. X. Tnclofcd a fmall graduated glafs tube, of -^th of 

 an inch internal diameter, and ten inches long, with a fhort 

 column of mercury in it, in the large receiver ; the tube was 

 fealed at one end and open at the other, fo that a portion of 

 air of given capacity was confined by the mercurial column, 

 which was near the open end of the tube, and fubject to rife 

 or fall by any variation of elafticity of the air on either fide, 

 being a proper manometer : then doubled the denfity of the 

 air in the receiver, and, opening the ftop-cock, the mercu- 

 rial column foon ran up to its former ftation; but inftantly 

 turning the cock again, the mercurial column returned or 

 fell down gradually for five or ten feconds, to the amount 

 of nearly j-^th of the whole aerial column, and then became 

 ilationary. Again opening the cock, a quantity of air rufhed 

 out, and the mercury relumed its original ftation. Thefe 

 effects were always the fame, on a repetition of the experi- 

 ment. 



Exp. XT. Let the mercurial column of the manometer 

 down by a wire to 'th of the length of the tube from the 

 fealed end ; then exhautted |ths of the air from the receiver, 

 which was feen by the mercury riling to the top of the tube; 

 and upon opening the cock the mercury fell to its former 

 ftation, but then fuddenly turning the cock, the mercury 

 gradually rofe, for the fpace of five or ten feconds, to more 

 than T '- th of its original height above the ftationary point, 

 and remained there till the cock was opened; after which it 

 re fumed its proper ftation. 



The phaenomena in the two laft experiments can be ex- 

 plained only on the following principle : — The air in the re- 

 ceiver and in the manometer is fubject to a like degree of 

 rarefaction and condenfation in thofe experiments, or very 

 nearly fo. When the equilibrium of heat in the air is dif- 

 turbed by the operations of condenfation and rarefaction, it 

 is reftored in the manometer inftantly by reafon of the con- 

 tiguity of the glafs to the air; but in the large receiver it re- 

 quires 



