Laws of Evaporation and Absorption. 337 



meter with a dry cap may be advantageously used as a hygro- 

 meter. Putting v 1 for the velocity of augmentation per minute, 

 when the thermometer with its dry cap is exposed to air whose 

 vapour has the tension p', then p' = av l , where the constant 



a=-= the ratio of the velocity of augmentation, in air satu- 

 rated with vapour, to the tension of that vapour. 



5. The same principle was found to hold true in relation to 

 certain hydrated salts, and other compounds having an affinity 

 for watery vapour. 



Experiment VI. In these experiments the salt was deposited 

 on the bulb of the thermometer by repeatedly dipping it in a hot 

 saturated solution of the salt and then placing the thermometer 

 in the dry-air bottle, where it was allowed to remain until the 

 salt was thoroughly dried. The bulb with the dry salt upon it 

 was then placed in the humid-air bottle, and the velocity of 

 augmentation of temperature per minute, as well as the maxi- 

 mum augmentation, &c, were observed as before. As in the 

 case of the woollen cloth cap, the maximum effect depends on 

 the quantity of the salt deposited on the bulb ; but by using the 

 same quantity in each series of experiments, the results admit of 

 being compared with one another. 



The results of experiment with chloride of calcium were as 

 follows : — 



At the temperature of 19° C, the rate of augmentation per 

 minute v was found to be 4°, and the maximum augmentation 

 E was found to be 13° # 3. The temperature being variable, the 

 rate of augmentation was found to be expressed by the formula 

 v = 6^p, and the maximum augmentation by 



„ 25-3» T 



E= 1 -^r = 4-6xl-057 T . 



The maximum augmentation, in degrees Centigrade, of the 

 following substances was found at mean temperature (15° C.) as 

 follows : — 



Potassa, 10°; chloride of zinc, 7 C *4; nitrate of lime, 7° 

 sulphuric acid, 12 o, 0; oxalic acid, 6°; sulphate of copper, 6° 

 chloride of sodium, 5°; carbonate of soda, 5°; charcoal, 4°*6 

 sulphate of soda, 4°*5 ; nitrate of ammonia, 4°'4 ; oxalate of 

 lime, 4 0, 3; sugar, 3°; plaster of paris, 3°; starch, 2°; tartaric 

 acid, 2°. These numbers probably express the relative force 

 with which the respective substances attract watery vapour. 



Charcoal in ammoniacal vapour gave an augmentation of 

 22° C, or 39°*4 F. ; and plaster of paris in the same vapour 

 gave 20° C, or 36° F. 



