164 EXPERIMENTS ON HEAT AND COLD. 



diately funk each time from 3° to 3°. 5 ; at the fame time an 

 exceedingly denfe mitt was produced in the receiver, which 

 foon fubtided. 



Sufpecting that aqueous vapour, which always exifts in the 

 atmofphere, and is liable to affume the liquid or aerial form 

 according to circumftanoes, might be the principal agent in 

 the production of heat and cold by condenfation and rarefac- 

 tion, I thought that an increafe of it might produce a greater 

 effect, and that cold air, which contains lefs vapour, might 

 have a lefs effect. The reverfe however was the fa£t, as ap- 

 pears by the following, 



EXPERIMENTS 8 and 9. 

 Exp. 8 and 9. In a cold morning laft winter when the air was clear and 

 did not'arife tne thermometer without ftood at 20°, I took the receiver and 

 from the water condenfer into the open air, and let them ftand for 15 minutes 

 J r pen in 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 

 For the change cock. The vapour precipitated was whiter than ufual and 

 r'tlT not nearly fo denfe. 



were moft abun- Again, took the receiver and condenfer into a dyer's flove 

 daat. 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 9 , and a very copious mift was 

 precipitated, fo denfe that one could' but juft diflinguilh the 

 degree of the thermometer through it. 



Thefe experiments fliew 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 



This might be change of temperature would be a maximum. Indeed this is 



inferred from clearly confiftent with the known law, that when vapour is 



•ry, condenfed, heat is given out. Any procefs to cool the air 



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



contains. Suppofe for inftance that a portion of the atmof- 



phere contained -^ of its weight of aqueous vapour, and that 



\ of this vapour were condenfed by 50* of cold ; that is, r ~^ 



of the whole elaftic raafs was converted into water ; then the 



heat given out would be fufficient to raife the temperature of 



the remaining mafs of air and vapour 6 or 8°, which fuffi- 



ciently 



