Some large blocks of the glacier ice had fallen from above, and lay conveniently for examination. Whilst the walls of the ice-caves which have been cut into this and other glaciers present a perfectly smooth, continuous surface of clear ice, these fragments which had fallen from the surface exposed to the heat of the sun, were, as seen in the mass, white and opaque. When a stick was thrust into the mass, it broke into many-sided lumps of the size of a tennis-ball, which separated, and fell apart in a heap, like assorted coals thrown from a scuttle, though white instead of black. These were the curious glacier nodules, "grains du glacier," or "Gletcherkoerne," characteristic of glacier ice as contrasted with lake ice. This structure of the glacier ice is peculiar to it, and is only made evident where the sun's rays penetrate it and melt the less pure ice which holds together the crystalline nodules. According to Dr. J. Young Buchanan, these nodules are masses of ice crystals comparatively free from mineral matter, whilst the water around them, which freezes less readily, contains mineral impurities in solution. The presence of saline matter in solution lowers, in proportion to its amount, the freezing-point of the water. Accordingly, although frozen into one solid mass with the nodules, the cementing ice melts under the heat of the penetrating rays of the sun sooner--that is, at a lower temperature--than do the purer crystalline nodules, and allows them to separate. It is owing to this that the exposed surface of glacier ice is white and powdery, disintegrated by the superficial heat, and forming a rough surface, on which one can safely walk. Lake ice does not break up in this manner under the sun's rays, but as it melts retains its smooth, slippery surface. It is formed in water, and not from the cementing and regelation of the powdery crystalline snow, as is glacier ice.