“Could our vision penetrate into the body of the glacier, we should find that the change from white to blue essentially consists in the gradual expulsion of the air which was originally entangled in the meshes of the fallen snow. Whiteness always results from the intimate and irregular mixture of air and a transparent solid; a crushed diamond would resemble snow. If we pound the more transparent rock-salt into powder, we have a substance as white as the whitest culinary salt; and the colorless glass vessel which holds the salt would also, if pounded, give a powder as white as the salt itself. It is a law of light that, in passing from one substance to another possessing a different power of refraction, a portion of it is always reflected. Hence, when light falls upon a transparent solid mixed with air, at each passage of light from the air to the solid and from the solid to the air, a portion of it is reflected; and in the case of a powder, this reflection occurs so frequently that the passage of the light is practically cut off. Thus, from the mixture of two perfectly transparent substances we obtain an opaque one; from the intimate mixture of air and water we obtain foam. Clouds owe their opacity to the same principle; and the condensed steam of a locomotive casts a shadow upon the fields adjacent to the line, because the sunlight is wasted in echoes at the innumerable limiting surfaces of water and air. “The snow which falls upon high mountain-eminences has often a temperature far below the freezing-point of water. Such snow is _dry_, and if it always continued so the formation of a glacier from it would be impossible. The first action of the summer’s sun is to raise the temperature of the superficial snow to 32°, and afterwards to melt it. The water thus formed percolates through the colder mass underneath, and this I take to be the first active agency in expelling the air entangled in the snow. But as the liquid trickles over the surfaces of granules colder than itself, it is partially deposited in a solid form on the surfaces, thus augmenting the size of the granules, and cementing them together. When the mass thus formed is examined, the air within it is found as _round bubbles_. Now it is manifest that the air caught in the irregular interstices of the snow can have no tendency to assume this form so long as the snow remains solid; but the process to which I have referred—the saturation of the lower portions of the snow by the water produced by the melting of the superficial portions—enables the air to form itself into globules, and to give the ice of the _névé_ its peculiar character. Thus we see that, though the sun can not get directly at the deeper portions of the snow, by liquefying the upper layer he charges it with heat, and makes it a messenger to the cold subjacent mass.