Fig. 206.
Fig. 207.
Fig. 208.
290. Rain, Snow, and Hail.—When it rains the vesicles or minute bubbles of which the clouds are composed are broken up, and each drop of rain contains the water which came from a multitude of these vesicles. But let us see exactly how this result is produced. Rain comes from the contraction of the clouds by cold. A cold current of air coming in contact with a cloud will condense its bubbles into drops, and these of course will fall. The same result occurs if a cloud passes into a cold stratum of air. But let us look at the process more minutely. Let us see what the effect of cold is upon the bubbles. The first effect may be made clear by Fig. 208. If a bubble be contracted by the influence of cold, the water of its wall being made thicker, there will be a gathering of it from gravitation at the lower part, as represented by the dotted line. You often see a similar effect in the soap-bubble. It rises filled with the warm air from your lungs, and as it goes up it is contracted by the colder air which is around it. This contraction makes the water hang downward from the bottom of it. And as the soap-bubble at length perhaps bursts in the air from the weight of this water, so it is with the vesicles in the cloud. And many of these, united together by attraction, form a drop. When the cold is sufficiently severe it makes the water of the ruptured vesicles of the cloud arrange itself in snow-crystals instead of drops. And when the cold acts with great rapidity upon a cloud it presses the particles of the water together so suddenly that there is not time for the crystalline arrangement, and hail is formed.
291. Vaporization.—The production of vapor by boiling differs in some respects from quiet evaporation. Here the liquid is raised in temperature to its boiling point, and the formation of vapor is not confined to the surface. In water the boiling point is 212°, but it varies more or less from this in other liquids. Thus the boiling point of alcohol is 173°, of ether 95°, oil of turpentine 568°, and mercury 652°.
