324 Prof. De la Rive's Description 



quantity of vapours in the atmosphere, there ought to he a 

 quantity sufficient to saturate the stratum of acid adhering to 

 the bull) of the thermometer, and consequently to develope 

 the same quantity of heat. 



But we must remark, that there is a struggle between the 

 force of affinity of the acid for the vapour, and the tendency 

 which the water possesses to remain under that form of va- 

 pour, — a tendency which increases as the quantity of vapour in 

 the atmosphere diminishes. Hence it follows, that the great- 

 er the humidity, the more facility will the water have to con- 

 dense the vapour, the more rapid also will the condensation 

 be, and, consequently, the more considerable will be the heat 

 developed. The thermometer will not stop, therefore, till 

 the cooling occasioned by the difference of temperature be- 

 tween the air and the thermometer bulb shall compensate the 

 quantity of heat produced by the condensation of the vapour, 

 and the term at which that limit takes place will depend on 

 the greater or less degree of the humidity of the air. 



When the thermometer ceases to rise, we must say that its 

 stopping is owing to the equilibrium which is then establish- 

 ed between the heat produced and the cooling, which ought to 

 take place at this temperature ; but still the acid does not 

 cease to condense the vapour. We may prove this by sus- 

 pending the thermometer to a sensible balance, when it will 

 be seen that its weight increases by the condensation of the 

 aqueous vapour round the bulb, even after the thermometer has 

 ceased to rise. It is easy also to measure exactly the quantity 

 of water which is attracted by a known quantity of sulphuric 

 acid. For this purpose, the thermometer is first weighed in 

 its natural state ; — it is then weighed after it is plunged in 

 the acid, by placing its bulb in a receiver dried by the muri- 

 ate of lime. The difference between these two weights gives 

 that of the thin stratum of acid adhering to the bulb. The 

 receiver is then removed, and exposed to the open air, and 

 condenses on the acid which envelopes it a certain quantity 

 of water, the weight of which it is easy to appreciate, as I 

 have often ascertained. 



Of all the acids, the sulphuric acid is, without doubt, the 

 most proper for these experiments, on account of its great af- 



