their Production, Suspension, a?id Destruction. 347 



any pressure of another elastic fluid as it is in vac2io. Thus 

 the torce of aqueous vapour of 212" is equal to 30 inches of 

 mercury; at 30° below, or 182°, it is of half that force; 

 and at 40^ above, or 232°, it is of double the force : so like- 

 wise the vapour from sulphuric ether which boils at 102°, 

 then supporting 30 inches of mercury, at 30° below that 

 temperature it has half the force, and at 40" above it, dou- 

 ble the force : and so in other liquids. JVLoreover, the force 

 of aqueous vapour of 60° is nearly equal to half an inch of 

 mercury when adiiiitted into a Torricellian vacuum ; and 

 water of the same temperature, confined with perfectly dry 

 air, increases the elasticity to just the same amount. 



'^ 3. The quantity ofany liquid evaporated in the open air is 

 directly as the force of steam from such liquid at its tem- 

 perature, all other circumstances being the same." 



The following is part of the Essay on Evaporation : 



" When a liquid is exposed to the an-, it becomes gradu- 

 ally dissipated in it; the process by which this effect is 

 produced we call evaporation. 



"■ Many philosophers concur in the theorvof chemical so- 

 lution : atmospheric air, it is said, has an affinity for water; 

 it is a menstruum in which water is soluble to a certain de- 

 gree. It is aliovvcd notwithstanding by all, that each liquid 

 is convertible into an elastic vapour in vacuo, which can 

 subsist independently in any temperature ; but as the utmost 

 forces of these vapours are inferior to the pressure of the 

 atmosphere in ordinary temperatures, they are supposed to 

 be incapable of existing in it in the same way as tliey do in 

 a TorricL-llian vacuum : hence the notion of affinity is in- 

 duced. According to this theory of evaporation, atmo- 

 spheric air (and every other species of air tor aught that 

 appears) dissolves water, alcohol, ether, acids, and even 

 metals. Water below 212° is chemicallv combined with 

 the gases ; above 212° it assumes a new form, and becomes 

 a distinct clastic fluid, called steam : whether water first 

 chemically combined with air, and then heated above 212°, 

 is detached Irom the air or remains with it, the advocates 

 <;f the thet)r\- have not determined. This theory has alwa)-s 

 bi'cn considered as complex, and attended with difficulties ; 

 Fo much that M. Pictet and others have rejected it, and 

 •idopted thatw hich admits of distinct clastic vapours in the at- 

 mosphere at all temperatures, uncombined with eith(n-of the 

 principal conslilucut gases, as being much more simple and 

 <»6y oi explication than the other ; though they do not remove 

 tnc grand objection to it, arising from atmospheric pre^■surc." 



"On 



