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THE POPULAR SCIENCE MONTHLY. 



not in these cases the preserving agent, in Ex- 

 periment 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 complete- 

 ly 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 evap- 

 orating 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 experi- 

 ment, 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 experi- 

 ments 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 hy- 

 drate 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 fer- 

 rous 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 ma- 

 rine forms first enter the lake ? Dr. Theo- 



dore Gill and Dr. J. F. Bransford, in a " sy- 

 nopsis" of the fishes of this lake, communi- 

 cated to the Academy of Natural Sciences 

 of Philadelphia, remark that this combina- 

 tion 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 dis- 

 courage the idea that the species enumer- 

 ated 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 ra- 

 pacity," and that " instances are known 

 of their having attacked and killed bathers 

 within a stone's-throw of the beach at Gra- 

 nada." 



Individnal Hygiene. Among the sub- 

 jects discussed at a recent Educational Con- 

 ference held in London was the importance 

 of a knowledge of the laws of health. Mr. 

 Thomas Bond, assistant surgeon to West- 

 minster Hospital, asserted that, on an aver- 

 age, one-half of the number of out-patients 

 treated by a hospital-surgeon suffer from 

 diseases due primarily to a want of knowl- 

 edge of the laws of health and cleanliness, 

 chiefly in regard to dress, ablution, and ven- 

 tilation. Varicose ulcers are most frequent- 

 ly 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 waist- 

 band. If elastic garters are worn at all, 

 they should be applied above the knee, and 

 not below, where they obstruct all the super- 



