320 Prof. De la Rive's Description 



less containsabout 10 or 11 per cent, of some alkali, the nature 

 of which I have not been able to ascertain. 



Art. XXII. — Description of a New Hygrometer, depending 

 on the Affinity of Acids for Water. * By Professor Aug. 

 de la Rive. 



In the course of some researches on the different degrees of 

 heat, occasioned by the affinity of acids for water, I was led 

 to recognize in that phenomenon a very exact indication of 

 the degree of humidity of the atmosphere. 



If we plunge the ball of a thermometer into a concentrated 

 acid, such as the nitric acid, but particularly the sulphuric 

 acid, it will be seen that, as soon as the ball is withdrawn 

 from the acid and exposed to the open air, the thermometer 

 will rise considerably. This phenomenon is owing to the con- 

 densation of the aqueous vapours produced by the affinity 

 exerted on these by the thin stratum of acid which adheres to 

 the ball. The heat produced is very considerable with sul- 

 phuric acid, because in this case there are two sources of ca- 

 loric, 1st, That which proceeds from the condensation of the 

 vapour ; and, 2d, That which is owing to the mixture of the 

 water and the acid. 



Having noticed that the quantity of heat, indicated by the 

 thermometer when taken out of the acid, varies with the hu- 

 midity of the air, other circumstances remaining the same, 

 I sought to determine whether or not these variations of heat 

 might serve to measure different degrees of humidity. 



Every hygrometer, or apparatus for measuring variations in 

 the humidity of the air, ought to possess the following quali- 

 ties. 



1. To agree with itself, or, on the return of the same state 

 of the humidity of the air, to indicate the same degree of its 

 scale. 



2. That its variations be proportional to those of humidity, 



* This paper is a translation and abstract of a Memoir read to the Na- 

 tural History Society of Geneva, on the 21st April 1825, and appeared 

 in the Bill. Univ. Avril 1825. 



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