of a Neiv Hygrometer. 325 



finity for aqueous vapours. This affinity is such, that, even 

 in a receiver dried by muriate of lime, I have seen the ther- 

 mometer rise from 15° to 18° centig., that is to say, 3°, but 

 it cannot rise higher. 



The nitric acid produces also heat by the condensation of 

 aqueous vapours, but in a much less degree. A thermometer, 

 for example, whose bulb had been plunged into that acid, 

 rose only in extreme humidity from 131° to 17^° centig., that 

 is, 4°, and exposed to an average humidity, it rose only 3°, 

 the temperature of the air being 1 4°. 



The hydrochloric acid presents a singularity when it is em- 

 ployed for the same purpose. A thermometer whose bulb 

 has been plunged in that acid begins by descending, and this 

 rises higher than the part from which it set out. At the 

 temperature of 15°, for example, the thermometer descends 

 at first to 14°, and then rises to 17°: but when placed in a 

 receiver dried by muriate of lime, the thermometer descended 

 from 15° to 12°, and did not rise higher than the point 

 from which it set out. From this it seems to follow, that 

 the first tendency of the water contained in the hydrochlo- 

 ric acid is to evaporate, — a tendency to which it yields en- 

 tirely when the air is very dry, — and that afterwards the 

 acid, become more concentrated by the privation of the water 

 which is evaporated, tends to take some of it again, by con- 

 densing the aqueous vapours of the atmosphere, and conse- 

 quently producing heat ; or may not the phenomenon be ow- 

 ing to the disengagement of the hydrochloric acid gas, which, 

 in quitting the water with which it was united, tends to pro- 

 duce cold, a cold which is soon more than compensated by 

 the condensation of the water attracted by the acid round the 

 bulb ? However this may be, the phenomenon seems to pre- 

 sent a kind of contradiction which it is not easy to solve. 



I shall not enter at present into any further details respect- 

 ing the heat produced by the affinity of acid for aqueous va- 

 pours, trusting that I shall resume the subject, and treat it 

 more profoundly, when I have finished the experiments on 

 this subject with which I am at this moment occupied; 



