78 THE WONDERFUL CENTURY 



the water had evaporated sufficiently to saturate the air 

 the vessels were slightly cooled, when a dense cloud was 

 at once formed in the one while the other remaind quite 

 clear. These experiments, and many others, showed 

 that the mere cooling of vapor in air will not condense 

 it into mist clouds or rain, unless particles of solid 

 matter are present to form nuclei upon which condensa- 

 tion can begin. The density of the cloud is proportion- 

 ate to the number of the particles; hence the fact that 

 the steam issuing from the safety-valve or the chimney 

 of a locomotive forms a dense white cloud shows that 

 the air is really full of dust particles, most of which are 

 microscopic but none the less serving as centres of con- 

 densation for the vapor. Hence, if there were no dust 

 in the air, escaping steam would remain invisible; there 

 would be no clouds in the sky ; and the vapor in the at- 

 mosphere, constantly accumulating through evaporation 

 from seas and oceans and from the earth's surface, would 

 have to find some other means of returning to its source. 

 One of these modes would be the deposition of dew, 

 which is itself an illustration of the principle that vapor 

 requires solid or liquid surfaces to condense upon ; hence 

 dew forms more readily and more abundantly on grass, 

 on account of the numerous centres of condensation it 

 affords. Dew, however, is now formed only on clear cold 

 nights after warm or moist days. The air near the sur- 

 face is warm and contains much vapor, though below the 

 point of saturation. But the innumerable points and ex- 

 tensive surfaces of grass radiate heat quickly, and becom- 

 ing cool, lower the temperature of the adjacent air, which 

 then reaches saturation point and condenses the con- 

 tained vapor on the grass. Hence, if the atmosphere at 



