Page:Popular Science Monthly Volume 32.djvu/699

Rh But let us return to our ice. He who is familiar with the researches of Tyndall and other physicists on the structure of ice, knows how little we can be aware, from the simple inspection of a lump of clear ice, beautiful as it is, how marvelously it is built up crystal by crystal into the solid form we know so well. But if we turn a beam of sunlight upon it, concentrated by a lens, the exquisite and varied steltate figures which flash out within the solid mass as the magic touch of the sunbeam releases the molecules of water from their crystal bonds, give us enchanting glimpses of the still but half-won secrets of beauty and of order with which Nature so fondly sports and still so cleverly conceals.

But the resources of the physicist do not suffice to conjure all its secrets from a block of ice. It is left for the student of that phase of Nature which we call life to discover that this very type of cold impassive lifelessness may be fairly teeming, absolutely transparent though it be, with whole families and races of living things—dormant from chill it is true, but ready at the touch of warmth, and in the presence of their food, to start on a career of growth and multiplication to which the increase in the world's populousness since the old Ice age faded is but a poor and halting comparison.

We can not follow the student of these lowly forms of life, which have become entangled among the ice-crystals, as he calls them back from their torpor, separates them one by one, and patiently studies their life-history. It is not enough to melt the ice and look at the resulting water through the microscope. But he mingles the melted ice with a transparent compound of gelatin and beef-tea, and puts the whole in a warm place, and after a few hours or days, wherever in this semi-solid gelatin a living germ from the ice had lain, a tiny speck or rounded mass appears—a "colony" he calls it—which is made up of thousands of the descendants of the old rescued and thawed bacterial ancestor. And so the biologist can separate the species one from another, cultivate them in various receptacles, and learn whether they belong among man's friends or foes.

A great deal of careful experiment has shown that water in freezing largely expels its coarser visible contaminations, and also that a large proportion of the invisible bacteria which it contains may be destroyed, even as many as ninety per cent. But still large numbers may remain alive, for many species are quite invulnerable to the action of cold. It has been found that in ice formed from water containing many bacteria, such as water with sewage contamination, the snow-ice almost invariably contains many more living bacteria than the more solid, transparent part; so that the snow layer should be especially avoided in ice obtained from questionable sources.

Unfortunately, the bacteria which cause typhoid fever are not readily killed by cold, and may remain alive for months, fast frozen in