ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 223 



only a very strong- upward wind would sustain the largest. A hail- 

 stone of 2.58 inches in diameter would be kept at a height of about 

 15,000 feet by an upward blast of hurricane force, 100 miles an hour. 

 Drops can never reach the size of a hailstone, for the resistance of the 

 air has the effect of breaking them up. The smallest drops would take 

 about six hours and forty minutes in falling from a cloud 10,000 feet 

 high, but we know that this scarcely ever, if ever, happens. In reality 

 the smallest drops falling on the earth are nearly always derived from 

 a slight elevation and very small drops falling from a great height 

 would, except in an extraordinarily saturated state of the air, evapo- 

 rate in their course. Ordinary small raindrops take about six minutes 

 or somewhat less in falling through 10,000 feet. 



Eain drops are perfectly globular in form. This we know in two ways — 

 first, from the rainbow, which can only arise from the regular disper- 

 sion of white light by transparent globules; and, secondly, by means 

 of instantaneous photographs. The sphere is the figure of smallest 

 volume which can be assumed, and consequently we find that free 

 liquids under the influence of cohesion, surface tension, or gravitation, 

 are always spherical. 



Since a raindrop is an aggregation of cloud particles it contains a 

 number of solid particles or invisible motes, and generally a very small 

 quantity of sea salt. Besides this "dust" it attaches to itself soluble 

 gases contained in the air, the result chiefly of animal life, of decom- 

 position of organic matter, and of manufacturing processes. Thus, 

 ammonia, nitric acid, hydrochloric acid, sulphurous acid, and a little 

 air and carbonic acid, are found in rainwater. Braudes found an aver- 

 age of 26 kilograms of residue in every million of rain evaporated, 

 the amount being greatest in January (65) and least in May (8). The 

 residual substances were chlorine, sulphuric acid, soda, potash, mag- 

 nesia, ammonia salts, organic matter, lime, carbon dioxide, oxide of iron, 

 and oxide of manganese. The solid matter amounts in France to about 

 147 £ to 156 kilograms per hectare. The importance of these minute 

 traces of gases and other substances in rain is enormous, especially in 

 relation to the nutrition of plants and the disintegration of rocks. But 

 no less important to mankind is the function of rain in clearing" the 

 atmosphere of these ingredients. Clouds and rain are at the same time 

 purifiers, filterers, and nourishers. In the words of the ancient declara- 

 tion, "the clouds drop fatness," and "the water returns not void." 

 The upper layers of earth have a remarkable power of purifying water, 

 so that what is useful to vegetation is retained near the surface and 

 the purified water passes down into deeper ground, where it may be 

 drawn from wells or emerge in springs. The process, first of wash- 

 ing the atmosphere and then of self- purification, is so complete that 

 though the mold swarms with organic life the water which has passed 

 through this upper earth may be described as practically pure and free 

 from organisms. 



