HEAT OR CALORIC. 105 



is kept cold, and vapor will be condensed in dew or frost if the cold 

 be considerable. Camphor, carbonate of ammonia, and other vola- 

 tile solids give off vapor so rapidly, that when placed in equilibrio in 

 balances, they are soon found to lose weight. 



(C.) The cause of natural evaporation is caloric. It produces 

 from, water, at every temperature, an elastic invisible vapor, whose 

 elasticity increases with the temperature, and which sustains a corres- 

 ponding column of mercury. Dalton and Gay Lussac have fully es- 

 tablished this position. The theory, formerly so prevalent, that evap- 

 oration depends on the solution of water in air, is no longer tenable 

 as the sole and sufficient cause, but it is still very possible,* that va- 

 por may be dissolved in air. The lower the boiling point of a fluid, 

 the more readily it evaporates. 



(d.) It has already been stated, (p. 87 J that the force of vapor is 

 the same at the boiling point for every fluid ; it equals thirty inches 

 of mercury, and is the same, in all cases, for an equal number of 

 degrees above and beloiv ebullition.-\ This is a curious fact ; per- 

 haps it would have hardly appeared probable, for instance, that the 

 vapor of ether at its boiling point, 98, of water at 212, and of 

 mercury itself at 656, should each exert a power capable of sus- 

 taining in a tube, a column of that metal thirty inches in altitude. 



EFFECTS OF NATURAL EVAPORATION. 



(e.) Evaporation produces cold because heat must be absorbed to 

 form vapor. The evaporation of ether under the receiver of the air 

 pump freezes water in contact with it, or having only a thin vessel 

 between ; so a stream of ether falling upon a thin glass tube, freezes 

 water contained in it. 



The sensation of cold in coming out of a bath, especially if warm, 

 is owing to the absorption of heat to form vapor. The formation of 

 vapor is a cooling process ; it goes on extensively, and thus regulates 

 natural temperature. In the hottest climates, evaporation from ex- 

 tensive surfaces of water, mitigates the heat, but where there is little or 

 no water, as in the great African desert, the heat becomes intolerable. 



Excessive degrees of heat have been occasionally endured by hu- 

 man beings in consequence of evaporation from their own surfaces. 



" Sir Joseph Banks and Sir Charles Blagden, breathed for some 

 time an atmosphere in a room prepared by Dr. Fordyce, which 



* Nor is it impossible or even highly improbable, that water may be, to a certain ex- 

 tent soluble in air, as there is obviously an affinity between the atmospheric gases 

 and water ; but the fact, if admitted, will not account for all the phenomena, without 

 admitting the formation of vapor at all temperatures. It is even said that vapor 

 formed at atmospheric temperatures, has the same amount of heat as that formed at 

 the boiling point; the latent heat increasing as the sensible heat is diminished. 



t See Dalton's tables. 



14 



