EVAPORATION. 



167 



in the rate of evaporation is subject to the law of continuity, or undergoes a 

 slow, gradual, and continued diminution, the determination of its actual limit, 

 if it has one, by experiment or observation must obviously be exceedingly diffi- 

 cult, if, indeed, it be within the bounds of possibility. Such a limit, therefore, 

 if it exist, must rather be sought for by the operation of the reason on facts 

 known, than by the operation of the senses on facts to be observed. A system 

 of reasoning, applied with great ingenuity by Dr. Wollaston to fix the limits of 

 the atmosphere, has been applied by Faraday to show that an actual limit must 

 exist, for a similar reason, to the operation of the evaporating principle. Dr. 

 Wollaston argued that the tendency of the molecules of the atmospheric air to 

 repel each other being known by direct observation to be subject to a continual 

 diminution, in proportion as the distances between the molecules increased, or, 

 in other words, in proportion to the rarefaction of the air, and the same mole- 

 cules being admitted, in common with all other matter, to be subject to the laws 

 of gravitation, it follows inevitably that, when the actual weight of the mole- 

 cules becomes equal to their mutual repulsion, then, these two forces balancing 

 one another, the molecules will rest altogether like the particles of a liquid. This 

 must happen, therefore, on the top of the atmosphere, where it is possible to con- 

 ceive a body whose specific gravity is less than the specific gravity of air in that 

 state of rarefaction in which the repulsion of its molecules equals their weight to 

 float on the surface exactly in the same manner, and for the same reason, as a ship 

 floats on water, or, to come to a closer analogy, for the same reason that we see a 

 balloon float between two strata of air when, bulk for bulk, it is lighter than that 

 on which it presses, and heavier than that immediately above it. Now, admitting 

 that the tendency to evaporation depends on the energy of the repelling force 

 produced by the presence of heat having a tendency to drive off the stratum of 

 particles which rest on the surface of the liquid, it will follow that gravity will, 

 at length, balance or prevail over the repulsive force, and will prevent the par- 

 ticles from flying off or evaporating. Immediately before the liquid attains this 

 state, the repulsive principle exceeds the gravitating one by so exceedingly 

 small an amount, that the quantity of evaporation, though not exactly nothing, 

 may be conceived to be so extremely small as to be utterly inappreciable by 

 any direct sensible observation. Such is Faraday's reasoning, to prove that 

 there exists a limit in all bodies to the action of the evaporating principle, and 

 that this limit is very low in those bodies that fuse at low temperatures, and 

 that it may be high in bodies which fuse at very high temperatures. 



If it be admitted that the evaporating principle has no limit of this nature, it 

 will follow that the atmosphere must always be impregnated with the vapors 

 of all bodies, whether solid or liquid. It is difficult to imagine this to be the 

 case, without supposing a great variety of chemical effects to be produced by 

 such a confusion of substances, having such an indefinite variety of physical 

 relations one to another. It seems much more probable that the less vaporiza- 

 ble substances at common temperatures are below the vaporizing limit, and 

 that the atmosphere contains suspended in it chiefly the vapor of water, with 

 slight and occasional admixtures of the vapors of the more volatile bodies. 



The elevation of the average temperature of the air has a double effect on 

 the rate of evaporation. By raising the temperature of water, it has a tendency 

 to increase the rate ; but by causing an increased quantity of vapor to be sus- 

 pended in the air, it has, on the other hand, a contrary effect. The difference 

 between the extreme tension due to the temperature and the tension of the va- 

 por actually suspended is, perhaps, greater in warm than in cold weather, be- 

 cause in cold weather the atmosphere is nearer its point of saturation than in 

 warm weather. Hence the rate of evaporation is probably greater in summer 

 than in winter. 



