To trace the steps of the process from its commencement, let us imagine a vessel of water placed over the fire, and receiving constantly a supply of heat from that source; the effect is, that its temperature gradually rises from about 50° or 60°, the usual warmth of ordinary water, to 212°, the point at which boiling takes place; but before it reaches that height, a number of bubbles may be observed forming on the sides of the vessel; these gradually increase in size, and when they become sufficiently buoyant, quit their position, rise to the surface, and escape; they consist of air previously dissolved in the water, and which is expelled by the increased heat. Water which has been boiled and allowed to become cold, without much exposure to the air, fails to re-absorb the quantity it previously contained, and consequently has its character somewhat altered. Thus, it freezes more readily than water which has not been boiled, in consequence of the air not having to be expelled in the act of solidifying, as is usually the case: hence, the ice from boiled water is free from those numerous air bubbles which are always to be observed in common ice. It possesses also a mawkish unpleasant taste, and is totally unable to preserve the life of any aquatic animal. The presence of this minute quantity of air in ordinary water, is very essential to its utility. Faraday found that water, _totally_ destitute of air, does not boil in the usual mode, but when heated to the boiling point, it at once, with an instantaneous and violent explosion, passes into the form of steam. This strange fact, which shows upon what small, and, apparently, trivial circumstances, the comfort—nay, we may truly say—the existence of man depends, is strikingly shown by a very ingenious experiment, devised by that most celebrated chemist. He took a piece of Wenham Lake ice (which, from peculiar local causes, such as being formed from spring water, is totally destitute of air), and melted it under a covering of sweet oil; this prevented the absorption of any air during the liquefaction; on continuing the heat, the water rose in temperature, and on reaching the boiling point, suddenly burst into steam, with an explosive power, sufficiently great to scatter the glass vessel in which the experiment was made into fragments; and had it not been for a protecting covering of wire gauze, very serious effects might have ensued. From the precipitation of the dissolved chalk present in most kinds of water, a cloudiness or slight turbidity is always to be observed in boiled water.