18. When ordinary lake-ice is intersected by a strong sunbeam it liquefies so as to form flower-shaped figures within the mass; each flower consists of six petals with a vacuous space at the centre; the flowers are always formed parallel to the planes of freezing, and depend on the crystallization of the substance.
19. Innumerable liquid disks, with vacuous spots, are also formed by the solar beams in glacier-ice. These empty spaces have been hitherto mistaken for air-bubbles, the flat form of the disks being erroneously regarded as the result of pressure.
20. These disks are indicators of the intimate constitution of glacier-ice, and they teach us that it is composed of an aggregate of parts with surfaces of crystallization in all possible planes.
21. There are also innumerable small cells in glacier-ice holding air and water; such cells also occur in lake-ice; and here they are due to the melting of the ice in contact with the bubble of air. Experiments are needed on glacier-ice in reference to this point.
22. At a free surface within or without, ice melts with more ease than in the centre of a compact mass. The motion which we call heat is less controlled at a free surface, and it liberates the molecules from the solid condition sooner than when the atoms are surrounded on all sides by other atoms which impede the molecular motion. Regelation is the complementary effect to the above; for here the superficial portions of a mass of ice are made virtually central by the contact of a second mass.
23. The dirt-bands have their origin in the ice-cascades. The glacier, in passing the brow, is transversely fractured; ridges are formed with hollows between them; these transverse hollows are the principal receptacles of the fine débris scattered over the glacier; and after the ridges have been melted away, the dirt remains in successive stripes upon the glacier.
24. The ice of many glaciers is laminated, and when weathered may be cloven into thin plates. In the sound ice the lamination manifests itself in blue stripes drawn through the general whitish mass of the glacier; these blue veins representing portions of ice from which the air-bubbles have been more completely expelled. This is the veined structure of the ice. It is divided into marginal, transverse, and longitudinal structure; which may be regarded as complementary to marginal, longitudinal, and transverse crevasses. The latter are produced by tension, the former by pressure, which acts in two different ways: firstly, the pressure acts upon the ice as it has acted upon rocks which exhibit the lamination technically called cleavage; secondly, it produces partial liquefaction of the ice. The liquid spaces thus formed help the escape of the air from the glacier; and the water produced, being refrozen when the pressure is relieved, helps to form the blue veins.