238 



Large globes, each containing about a cubic foot of space, fur- 

 nished with thermometers and hygrometers, were made to communi- 

 cate, respectively, with reservoirs of perfectly dry air, and of air re- 

 plete with aqueous vapour.* The cold, ultimately acquired by any 

 degree of rarefaction, appeared to be the same, whether the air was 

 in the one state or the other; provided that the air, replete with 

 aqueous vapour, was not in contact with liquid water in the vessel 

 subjected to exhaustion. When water was present, in consequence 

 of the formation of additional vapour, and a consequent absorption of 

 caloric, the cold produced was nearly twice as great as when the air 

 was not in contact with liquid water; being nearly as 9 to 5. 



Under the circumstances last mentioned, the hygrometer was mo- 

 tionless; whereas, when no liquid water was accessible, the space, 

 although previously saturated with vapour, by the removal of a por- 

 tion of it together with the air which is withdrawn by the exhaus- 

 tion, acquires a capacity for more vapour; and hence the hygrome- 

 ter, by an abstraction of one-third of the air, revolved more than 

 sixty degrees towards dryness. But when a smaller receiver (after 

 being subjected to a diminution of pressure of about ten inches of 

 mercury, so as to cause the index of the hygrometer to move about 

 thirty-five degrees towards dryness) was surrounded by a freezing 

 mixture, until a thermometer in the axis of the receiver stood at three 

 degrees below freezing, the hygrometer revolved towards dampness, 

 until it went about ten degrees beyond the point at which it rested 

 when the process commenced. 



It appears, therefore, that the dryness produced by the degree of 

 rarefaction employed is more than counterbalanced by a freezing 

 temperature. 



As respects the heat imparted to the air above mentioned, the fact, 

 that the ultimate refrigeration in the case of air replete with vapour, 

 and in that of anhydrous air, was equally great, and that when water 

 was present the cold was greater in the damp vessel, led to the idea, 

 that the heat arising under such circumstances could not have much 

 efficacy in augmenting the buoyancy of an ascending column of air: 

 but when, by an appropriate mechanism, the refrigeration was mea- 

 sured by the difference of pressure at the moment when the ex- 

 haustion was arrested, and when the thermometer had become sta- 



* The hygrometers were constructed by means of the beard of the avena 

 sensitiva or wild oat, also called animated oat. 



