224 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



Not only is the raindrop a composition of solids, liquids, and gases, 

 but it is of unequal consistency if the inner be compared with the outer 

 part. Every drop surrounded by air is compressed into the spherical 

 shape by an outer film of water which partakes of the character of an 

 elastic skin. In the free air cloud globules and small rain can not easily 

 coalesce on account of this elastic film enveloping them. They may 

 impinge against each other, but uidess the concussion be forcible they 

 rebound. Similarly the drops falling from a fountain may be seen to 

 run along the surface of the water like pearls before they unite with it. 

 So also small drops of water falling from an artificial jet rebound and 

 do not unite on collision. But let a stick of sealing wax be rubbed on 

 flannel and held at a distance of several feet from the thickly falling 

 drops; they at once cease to rebound, they unite into large drops, or 

 else the jet keeps falling as a continuous stream and does not separate 

 into drops as before. Again, let the drops be strongly electrified, they 

 do not unite but repel each other. 



Large drops attract small drops similarly electrified, and drops 

 unequally electrified attract each other. The weak charge of similar 

 electricity, which causes the globules to approach each other forcibly, 

 is sufficient to break the enveloping film, but a stronger charge pro- 

 duces repulsion of the drops. In these observed facts we have what 

 seems a very satisfactory explanation of some of the phenomena of 

 thunderstorms; for example, the sudden heavy downpour and sudden 

 cessation, and the apparent effect of flashes of lightning on the rain or 

 hail. Finely divided water exhibits another property which is of great 

 importance in the formation of rain, hail, and snow. Down to a very 

 low temperature, 10° to 20° or more below the freezing point, according 

 to the size of the particles, it resists congelation. This property is of 

 immense effect throughout nature, and the life of plant and animals to 

 a great extent depends upon it. When globules of water below the 

 freezing point are touched by a frozen drop or by a snowflake they are 

 instantly frozen. A crystal of ice is the most powerful of all sub- 

 stances in congealing water below the freezing point. Yery many falls 

 of rain, hail, and snow are due to this cause. The minute crystal as it 

 descends through dense cloud gathers particles on its way until it has 

 grown to be a large snowflake; and whenever the lower air is warm 

 enough, snowflakes thus formed melt and fall as rain. Bain is much 

 more often than we suppose melted snow. The minute flakes which 

 would melt and evaporate if they did not meet with the water cloud, 

 grow rapidly in the cloud, which would of itself be incompetent to 

 precipitate. 



When a flake of snow or kernel of ice falls through dense cloud, such 

 as the towering cumulus which stacks itself to a great elevation in a 

 thunderstorm, it electrically attracts the particles of unfrozen water, 

 below the freezing point, through whicli it passes, and every particle 

 attached and instantly frozen adds to the electric charge, so that more 



