"It appears, then," says Professor Guthrie, "that under these conditions the freezing of sea-water is little more than the freezing of ice, and that the almost undiminished saltness of the unpressed ice is due, as suggested by Dr. Rae, to the entanglement amidst the ice-crystals of a brine richer in solid constituents than the original water itself. Such brine, which is here squeezed out in the press, drains in nature down from the upper surface of the ice-floe by gravitation, and also is replaced by osmic action by new sea-water which again yields up fresh ice; so that while new floes are porous and salt, old ones are more compact and much fresher, as the traveller observed. . The degree of saltness of a floe depends not only upon its age, but also upon the rapidity with which it was at first formed, and upon the lowest temperature to which it has subsequently been exposed." Mr. Guthrie justly supposed that the ice of the sea is mainly formed at or near the surface by radiation from the surface into space, and by contact with the colder air. He imitated this by hanging a blackened tin pan, containing a freezing-mixture, within one-eighth of an inch of the surface of the sea-water and thus obtained the ice which, on being pressed, contained a minimum solid residue. CONCLUSION. It had been my intention to enter into practical details concerning refrigerators, refrigerator-cars, and the methods of making pure ice economically. The time and space occupied preclude this now, but the opportunity for giving a connected history of the numerous efforts made of late years to extend the benefits of refrigeration for man's wants will not be neglected, if only in the fishing interests. WASHINGTON, October, 1879. ALPHABETICAL INDEX NOTE.-A separate index to the report of the Commissioner will be found at its close, and another Page. Acetic acid, crystallized.........920, 921 Acknowledgments of the receipt of Act regulating salmon fisheries .... Artificial hatching of cod........578, 585. Blue-Light, U. S. steamer.. 887 |