All the various phenomena of air currents are dependent upon one unchanging law: that gaseous bodies--and all but two others--always greatly expand under the influence of heat. There are two noted partial exceptions: one of these prevents our globe from becoming a complete iceberg, and is as important as the law itself. Iron expands, till its melting point; but in its liquid state it occupies less space than when solid. Water contracts under the influence of cold, until the temperature of 39° is reached; after that it expands: and when frozen occupies about one-eighth more space than before. This wise provision of the Creator is second to none in importance, as regards its influence upon the climate of the earth at large. Had it been otherwise--did ice sink instead of float, our rivers and seas would in time become solid masses of ice; for water is so poor a conductor of heat, that its under-currents warm very slowly. Any one who plunges into a lake in mid-summer may often find the water warm at the surface, and of almost icy coldness a short distance beneath. The great Polar current comes down from Baffin’s Bay, and off the coast of Newfoundland it plunges beneath the warm, lighter current of the Gulf Stream; but it is not warmed by it. Registering thermometers detect its icy coldness almost unchanged in the realms of the tropics, far beneath the surface. Note some simple illustrations of the expansive force of freezing water. Every housewife knows that a bottle left full of water will burst when the water freezes. The same power is shown in the gradual disintegration of rocks by alternate freezing and thawing. Water freezing in the crevices bursts off small particles, or even large fragments; so that rocks long exposed to the weather, crumble more or less. Every one is familiar with the appearance presented by steep clay-banks, in late winter and early spring, of ragged masses and fragments ready to fall at any time. Still another instance of this destructive power is shown in the killing of vegetation by freezing. Plants are built of myriads of tiny cells. The moisture within freezes and bursts the cell-walls, destroying the plant life. Certain plants have cells more elastic than others, which in consequence are not destroyed by freezing. But as an expanded cell does not readily shrink to its former size, subsequent freezings, when the cell contains more water than before, may finally destroy it. So wheat is “winter-killed,” by too frequent freezing. So globes of steel may be burst by this force. To show the poor qualities of water as a conductor of heat, take a long glass tube and fill with water. Then put a piece of ice in one end. The water at the other end may now be brought to the boiling point by means of the flame of a lamp, ere the ice at the other end is melted.