°%2 Prof, De la Hive's Description 



then, taking the ratio of these two numbers of degrees, we find 

 the exact ratio between the tension of the vapour existing in the 

 air, and the total tension at the same temperature. 



The thermometer, for example, marks 12° cent, when immers- 

 ed in sulphuric acid ; exposed to the air, it rises to 25|°, that 

 is 13i°; placed in a vacuum, in which the air is at extreme 

 humidity, at the same temperature of 12°, it rises to 27°, that 

 is 15°; the ratio of 13h° to 15°, or 90 to 100, expresses that 

 of the tension of the vapour existing in the air to the total 

 tension at 12° of temperature. If we now seek, in the table 

 of Gay Lussac, the degree of the hygrometer corresponding 

 to the tension 90, we shall find 95° 43, and the hygrometer 

 in the above experiment indicated 95° 50. 



Several other experiments have given me analogous results ; 

 but if, in place of operating at the temperature of 12°, we 

 make the experiment at another temperature, for the same 

 degree of the hygrometer of Saussure the thermometer will 

 rise as much more as the temperature is elevated ; which arises 

 from this, that the absolute quantity of aqueous vapours in 

 the air increases with the degree of heat. The number of 

 degrees which the thermometer will rise for extreme humidi- 

 ty will vary also ; and, consequently, we must determine it 

 for each degree of the thermometer in order to know the ratio 

 required at every temperature. I have made the determina- 

 tion for temperatures sufficiently remote ; and it appears to me 

 that we may, without sensible error, content ourselves with 

 determining the number of degrees which the thermometer 

 rises in the case of total humidity, for two extreme points 

 such as 0° and 20° centig., and divide the difference equally 

 between the intermediate degrees. A great number of expe- 

 riments, made at different temperatures in the way above ex- 

 plained, have given me satisfactory results. Let us suppose, 

 then, a thermometer, on the scale of which is marked, oppo- 

 site to each degree, the number which shows how much, at 

 this degree of temperature, the thermometer taken from the 

 sulphuric acid rises when exposed to extreme humidity ; then, 

 if we wish to know the humidity of the air, we divide the 

 number which expresses the rise of the thermometer by that 

 which is marked at the end of the degrees on the scale, and 



