Page:The Geologist, volume 5.djvu/177

Rh alluvium." The glaciers still occupied for a long time the deepest valleys, and prevented their being filled with alluvial deposits; then they melted, and the Lakes Maggiore, Como, and Leeco, Orta and Iseo were formed. This was the fourth and last part of the glacial epoch, which gradually merged into the present period.

.—Gustav Rose has been making new experiments on the deportment of carbonate of lime at high temperatures, both with and without fluxes; and, from their results, he has arrived at the conclusion that rhombohedral carbonate of lime is never a direct product.

According to the experiments of Sir James Hall, made in 1804, this has been directly produced when chalk and compact limestone were exposed to a high temperature under great pressure.

Hall's experiment has therefore been repeated by MM. Rose and Siemans. A gun-barrel was charged with dry elutriated chalk, rammed into a compact mass, and the gun-barrel then hermetically sealed at both ends, and exposed to the heat of one of M. Siemans' gas-furnaces. During the experiment the gun-barrel sprung, and in the crack there appeared a faint blue flame, evidently of carbonic oxide. The gun-barrel was then removed from the furnace, and on opening it the chalk was found converted into a light bluish-white coherent mass, slightly lustrous on the fracture, and with cracks running through the whole. The surface was covered with a snow-white, earthy, well-defined crust, and the cracks were lined with white earthy particles; these, as well as the crust, were composed of caustic lime. The compact mass, however, proved, on examination, to be unchanged in chemical properties; and in physical properties, though seemingly changed, when examined under the microscope, it showed the same small globules, and identically the same properties, as the unignited amorphous chalk. Although somewhat more coherent, the chalk was not materially altered, and in nowise converted into crystalline calcite. Another experiment was made with fragments of rhombohedral calc-spar, but was also interrupted by the rupture of the gun-barrel.

M. Rose considers, from these experiments, that chalk or compact limestone cannot be converted into crystalline limestone or calc-spar by expo- sure to a high temperature in closed vessels; and, as a general fact, that rhombohedral carbonate of lime is not formed in the dry way. He also observes that, on comparing accurately the description of Hall's experiments and Bucholz's observations incidentally made in the production of caustic lime from chalk, probably they obtained results similar to his own, and that the slightly coherent, but otherwise unaltered mass, was erroneously considered to be crystalline marble. But what is most singular is, that notwithstanding Hall's experiment has been quoted and use made of it, not only in explaining geological phenomena, but in serving as the foundation of theories, it was never repeated or confirmed; and the experiments of M. Rose show it at least to have been hasty, although we do not think M. Rose's have been as complete nor as long continued as