331. =Why the Above Exception to Expansion by Heat Exists.=--That we may see the reasons in part for the grand exception to the general law of expansion by heat which I have illustrated, let us see what would be some of the results if the exception did not exist. In that case the process of freezing would be as follows: The water would communicate its heat from the surface to the air, as before described, and there would be a constant downward current of the cooled water. When any portion of the water became cooled by the air down to 32°, it would become ice, and would sink to the bottom. And after the process of freezing had once begun, there would be a continual accumulation of ice at the bottom so long as the air remained cold enough to cool the water with which it comes in contact down to 32°. The result may be stated in the general thus: Freezing would not begin so quickly as it now does; but when once begun it would prove very destructive. It would not begin as soon, because the whole of any body of water must be cooled down to just this side of 32° before it could begin. This would not take long where the water is shallow, but it would where it is deep. All shallow bodies of water, then, would be frozen up quite early in the winter; and as water is a poor conductor, and thawing must go from above downward, some of them would not be thawed out again fully till quite into the next summer, if even then. And where the water is quite deep ice would at length begin to form, and when formed it would be exceedingly slow in thawing. In some cases it would never be thawed with such a body of non-conducting water to guard it against the warmth above. It is easy to see that the heat of spring and summer would not thaw out any thing like the quantity of ice that it now does. The reign of ice and snow on our earth would therefore be vastly more extensive than now, and what is worse, it would be extended more and more every year. Under such circumstances there would be great destruction of both animal and vegetable life. I will mention, however, but a single item, as it would occupy too much space to go into this subject with any fullness. In the water under the ice, which is always above 39°, except that which is close to the ice when freezing is going on, there is a vast amount of busy life which would be destroyed if ice were formed at the bottom, chilling all the water above.