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POPULAR SCIENCE REVIEW. 
amplitudes. This is true of mercury and glycerine, though the former is of 
high density, and the latter of great viscosity. 
Experiments with Floating Magnets, made by Alfred M. Mayer, show the 
motions and arrangements of freely moving bodies acted on by forces of 
attraction and repulsion. 
A dozen sewing needles," magnetized with their points of similar (N) 
polarity, are run into small corks so as to float upright in water, with the 
eyes above the surface. 
The north pole of a large cylindrical magnet being brought down over 
them, they arrange themselves as an equilateral triangle. Four needles 
form a square, or an equilateral triangle with a central needle. Five form 
a square with centre, or a pentagon ; six a pentagon with centre, or an 
equilateral triangle of six points arranged quincuncially ; seven form a regular 
hexagon with central needle, and so on. 
Floating needles may be used generally as delicate indicators of magnetic 
actions, such as the position of poles, and the displacement of lines of 
magnetic force during inductive action, or, for instance, in the telephone. 
ZOOLOGY. 
Aerial Respiration in Fishes. — Professor Jobert, of Dijon, who is now 
engaged in making some zoological investigations in Brazil, at the instance 
of the Emperor Don Pedro, has ascertained some exceedingly remarkable 
facts in connection with the respiration of certain fishes. A Siluroid fish, 
which inhabits the neighbourhood of Bio de Janeiro ( Callichthys asper'), and 
is noted for its power of living a long time out of the water, was found by 
him to swallow small portions of air, from which it partly absorbs the oxygen 
by the agency of the walls of the intestinal canal, the carbonic acid formed, and 
the unabsorbed nitrogen passing away by the anal aperture. On examining the 
structure of the intestine, Professor Jobert found its inner surface bearing a 
multitude of filiform appendages arranged in tufts, and composed essentially 
of blood-vessels. 
A somewhat analogous case was observed in several other fishes inhabit- 
ing the valley of the Amazon. They live in stagnant water, the temperature 
of which often exceeds 104° F. ; but this does not appear to be sufficient to 
support their respiration, and they are obliged to come frequently to the sur- 
face for a supply of air. Sometimes, also, the water in which they have been 
living is dried up, when they are seen making considerable journeys by land 
in search of more favourable localities, crawling on the ground by means of 
their pectoral fins. Some of these are species of Callichthys, and, like the 
C. asper of Rio de Janeiro, they possess a double respiration, — respiring the 
air contained in the water surrounding them by means of their gills, and also 
the atmospheric air which they swallow, and which passes through their in- 
testine. The escape of the exhausted air from the anal aperture in these 
fishes is said to produce a constant bubbling in the water they inhabit, and 
M. Jobert’s investigations, though imperfect, sufficed to convince him that the 
Air evacuated contained much carbonic acid, and less oxygen than atmospheric 
Air. The vascular tufts clothing the wall of the intestine originate from 
