When water that contains solid matter in solution or in suspension is frozen, the solid matter is mostly eliminated, so that the ice is much purer than the water from which it is produced. Some of the solids are almost in variably entangled among the interlacing crys tals of the ice, however, so that numerous little particles of foreign matter often remain in the ice, imprisoned in tiny cavities. Bacteria and other germs that may have been present in the original water appear to be largely excluded from natural ice by the freezing process, though some of them are undoubtedly caught among the crystals and retained. In artificial ice, where a mass of water is frozen simultaneously on all sides, so that the solidification•proceeds from the outside toward the centre of the cake in all directions, purification from this cause is hardly possible, and the middle part of the ice cake is likely to be rich in whatever germs the original water may have contained. Fortunately the recent experiments of Sedgwick and others indicate that freezing and protracted storage of the ice is much more fatal to typhoid bacilli than was formerly supposed. Artificial ice, if
prepared from distilled water, or from water that is certainly known to be free from disease germs, is undoubtedly safer than natural ice that is taken from streams or ponds of un known purity; but in choosing between natural and artificial ice from the same identical water, the preference should be given to the natural product.
Water expands upon freezing, one volume of water at 32° F. becoming transformed, by freez ing, into 1.0908 volumes of ice at the same tem perature; which is equivalent to saying that water expands by one-eleventh of its own bulk freezing. The quantity of heat required to melt one pound of ice, from the state of ice at 32° F. to that of water at 32° F., is 142 times as great as the quantity of heat required to raise the temperature of a pound of water from 32° F. to 33° F. The specific heat of ice, near the temperature 32° F., is approximately 0.50. The production of ice by artificial means has increased rapidly in recent years. See RE