6% On the Heat and Cold produced by the 



Jew fecondsy by the receiver and unrounding bodies, to their 

 own temperature. 



Kip. VII. On condenfed Air. — Took a large fpherical glaf3 

 receiver, the capacity of which was fomething more than 

 twice that of the former (above one gallon), and fufpended 

 a thermometer in the centre of it, of a larger bulb than that 

 before ufed ; the receiver had a brafs cap and {top-cock 

 adapted to it : then doubled the denfity of the air within it 

 by a condenfer. The thermometer rofe 2° or more. Let out 

 the air fuddenly, and the thermometer immediately funk each 

 time from g° to 3°.$ ; at the fame time an exceedingly denfe 

 milt was produced in the receiver, which foon fubfided. 



Sufpecting that aqueous vapour, which always exifts in 

 the atmofphere, and is liable to atmme the liquid or aerial 

 form according to circumftances, might be the principal 

 agent in the production of heat and cold by condenfation 

 and rarefaction, I thought that an increafe of it might pro- 

 duce a greater effect, and that cold air, which contains lefs 

 vapour, might have a lefs effect:. The reverfe however was 

 the fact, as appears by the following 



Exp. VIII. and IX. In a cold morning lad winter when 

 the air was clear and the thermometer without Hood at 20 , 

 I took the receiver and condenfer into the open air, and let 

 them (land for 15 minutes to acquire its temperature; then 

 repeatedly condenfed the air to a double denfity, and fuddenly 

 liberated it again. On a medium of five trials, the mercury 

 fell 3^.3 on opening the cock. The vapour precipitated 

 was whiter than ufual, and not nearly fo denfe. 



Again, took the receiver and condenfer into a dyer's Move 

 where the temperature was about 100 ', and the air abounded 

 with vapour in a tranfparent ftate : after fome time, condenfed 

 the air and liberated it as before, when on a medium of five 

 trials the mercury funk only 3 , and a very copious mill was 

 precipitated, fo denfe that one could but juft diliinguifh the 

 degree of the thermometer through it. 



Thefe experiments mow that the greater the quantity of 

 vapour condenfed, the lefs is the change of temperature ; and 

 that confequently, if air was entirely free from vapour, the 

 change of temperature would be a maximum. Indeed this 

 is clearly confident with the known law, that, when vapour 

 is condenfed, heat is given out. /• ny procefs to cool the 

 air muft be retarded by the condenfation of part of the vapour 

 it contains. Suppofe, for inftancc, that a portion of the at- 

 mofphere contained ^th of its weight of aqueous vapour, and 

 that *ths of this vapour were condenfed by 50° of cold; that 



is, 



