Page:Popular Science Monthly Volume 12.djvu/394

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380
THE POPULAR SCIENCE MONTHLY.

not in these cases the preserving agent, in Experiment VI. a stone-ware vessel was charged underneath the spongy, iron with pyrolusite and sand, so as to abstract the iron from the water before it came in contact with the meat: again the meat was fresh after four weeks' filtration. Experiment VII.: by a separate experiment it had been ascertained by Dr. Bischof that the oxygen is completely abstracted from water during its passage through spongy iron. To determine whether the absence of oxygen is the cause of the preservation of the meat, and whether the bacteria or their germs are killed or can be revived when supplied with oxygen, an evaporating basin was inverted over the meat. Though this must have retained a quantity of air in its cavity, the meat still was found fresh after four weeks. In the final experiment, fresh meat was placed at the bottom of a glass vessel and left standing covered, with about four inches of spongy iron and water. After three weeks the meat was very bad, thus showing that the action of the bacteria of putrefaction adhering to the meat was not prevented by the spongy iron above; and if, during the previous experiments with spongy iron, agencies capable of causing putrefaction had at any time come in contact with the meat—in other words, if the bacteria had not been killed in their passage through the spongy iron—the meat must have shown marks of their action.

The author accounts as follows for the action of this material: "I believe that the action of spongy iron on organic matter largely consists in a reduction of ferric hydrate by organic impurities in water. . . . Ferric hydrate is always found in the upper part of a layer of spongy iron, when water is passed through that material. The ferrous hydrate resulting from the reduction by organic matter may be reoxidized by oxygen dissolved in the water, and thus the two reactions repeat themselves. This would explain why the action of the spongy iron continues so long."

 

Marine Fishes in Lake Nicaragua.—The fish fauna of Lake Nicaragua has long been known to include a few species elsewhere found only in salt-water, as a Megalops, a shark, and a sawfish. How did these marine forms first enter the lake? Dr. Theodore Gill and Dr. J. F. Bransford, in a "synopsis" of the fishes of this lake, communicated to the Academy of Natural Sciences of Philadelphia, remark that this combination of species may have resulted—1. From the intrusion of the salt-water types into the fresh waters; or, 2. From the detention and survival of the salt-water fishes in inlets of the sea that have become isolated, and so, in course of time, fresh-water lakes. The latter hypothesis is declared the more probable one. By the uplift of the land an inlet of the Pacific might have been shut off from communication with the ocean, and the character of the water would be soon changed by the copious showers of that tropical country. The shark, sawfish, Megalops, and other species, mostly found in the sea, would have time to accommodate themselves to the altered conditions. At the same time, it must be remembered that most of the marine types in question are wont to ascend high up streams, and even into fresh water. Still, the numerous rapids of the river discharging from the lake discourage the idea that the species enumerated have voluntarily ascended the river and entered the lake. Of these fresh-water sharks of Lake Nicaragua, Squier says that "they are called tigrones from their rapacity," and that "instances are known of their having attacked and killed bathers within a stone's-throw of the beach at Granada."

 

Individual Hygiene.—Among the subjects discussed at a recent Educational Conference held in London was the importance of a knowledge of the laws of health. Mr. Thomas Bond, assistant surgeon to Westminster Hospital, asserted that, on an average, one-half of the number of out-patients treated by a hospital-surgeon suffer from diseases due primarily to a want of knowledge of the laws of health and cleanliness, chiefly in regard to dress, ablution, and ventilation. Varicose ulcers are most frequently caused by the use of elastic garters: these should never be worn by children, as the stocking can be perfectly well kept up by attachment of elastic straps to the waistband. If elastic garters are worn at all, they should be applied above the knee, and not below, where they obstruct all the super-