When the ice had lain a short time, cracks appeared on the surface exposed to the sun, and spread like a network from the edges towards the centre of the surface.  At first there was no regularity in the connection of these lines, and the several meshes were of very different sizes.  After a time, the larger meshes split up into smaller, and the system of network was found to penetrate below the surface, the cracks deepening into furrows, which descended perpendicularly from the surface, and divided the ice into long thin rhomboidal pillars.  The surface-end of some of these pillars was strongly marked with right lines parallel to one of the sides of the mesh, and it was found that there was a tendency in the ice to split down planes through these lines and parallel to the corresponding side-plane.  Parallel to the original surface of the mass of ice, the pillars broke off evenly.  The side-planes had a rounded, wrinkled appearance; and their mutual inclinations—­as far as could be determined—­were from 105 deg. to 115 deg., and from 66 deg. to 75 deg..  When these ice-pillars were examined by means of polarised light, they were found to possess a feeble double-refracting power.

The writer of the article in Poggendorff suggests a question which he was not sure how to answer:—­Is this appearance in correspondence with the original formation of the ice, or does it only appear under slow thaw?

It is worthy of remark, that from the 1st to the 11th of February the thermometer was never higher than 22 deg..8 F., and during that time fell as low as 21 deg. below zero, i.e. 43 deg. below the freezing point.

Professor Tyndall has informed me that in the winters of 1849, 1850, 1851, he found the banks of a river in Germany loaded with massive layers of drift-ice, in a state of thaw, and was struck by the fact that every layer displayed the prismatic structure described above, the axes of the prisms being at right angles to the surfaces of freezing.  It may be, he adds, that this structure is in the first place determined by the act of freezing, but it does not develop itself until the ice thaws.

M. Hassenfratz observed an appearance in ice on the Danube at Vienna[208] corresponding to that described at Jena.  He gives no information as to the state of the weather or the temperature at the time, nor any of the circumstances under which the ice came under his notice.  One of the masses of ice which he describes was crystallised in prisms of various numbers of sides:  of these prisms the greater part were hexahedral and irregular.  Another mass was composed of prisms in the form of truncated pyramids; and in another he found quadrilateral and octahedral prisms, the former splitting parallel to the faces, and also truncated pyramids with five and six sides.  He adds, that he had frequently seen in the upper valleys tufts of ice growing, as it were, out of the ground, and striated externally, but had never succeeded in discovering any internal organisation, until one evening in a time of thaw, when he found by means of a microscope that the striated tufts of ice had assumed the same structure on a small scale as that which he had observed on the Danube.