PHYSICS. 277 



himg so far apart as not to move toward one another. If the level of 

 the water be now quietly raised by letting a further quautity flow in 

 from below, as soon as the water covers the lower ends of the needles 

 they begin to approach, and when they are nearly immersed they rush 

 together. The effect appears to be due to the fact that when the grav- 

 itation force downward is partly counteracted by the upward hydro- 

 static force due to immersion, the magnetic force is able to assert itself, 

 being greater. — {N'ature, xxii, 133, June, 1880.) 



Mayer has contrived an ingenious magnetic experiment to illustrate 

 molecular groupings and molecular actions. Sewing needles are mag- 

 netized all in the same direction. They are then pushed through small 

 pieces of cork up to the eye. On placing them in water they float ver- 

 tically, with the point downward. Since the similar jioles are upward, 

 they repel each other. These needles represent molecules ; their mag- 

 netism the repulsive force. To obtain an attractive force, a bar magnet 

 is held vertically over the water, so that the pole of contrary name to the 

 tops of the needles shall be downward. The needles are attracted and 

 approach each other, producing groupings which depend on the inimber 

 of the needles, their initial position, and the impulses given. Twenty- 

 three of these groui>ings are represented in the paper, the most com- 

 plex figures being formed of twenty needles ; though the author has 

 formed much more complex ones, more than fifty needles having been 

 grouped in one of his figures. The author employs these groupings to 

 illustrate many of the laws of both molecular and atomic arrangement. 

 When projected by means of a vertical laiitern they are very striking, 

 — {Am. J. ScL, III, XV, 216; xvi, 247; J. Phys., viii, 32, January, 1879.) 



Barker has communicated to the ]!:>fational Academy the results of some 

 experiments made to test the alleged incorrectness of Arago's explana- 

 tion of the magnetism of a wire through which a current is passing, i. e., 

 that the wire is itself magnetic. Bache had explained the result by 

 supposing the iron filings, held by the wire in Arago's experiment, be- 

 came themselves magnets under the influence of the current, and so 

 became atta'ched to each other rather than to the wire. In proof of this 

 he says that if a card be scrai)ed along on the top of the wire the ring 

 of filings is broken and the whole mass falls. Barker used in his 

 experiments a quantity magneto-machine made by Wallace, the current 

 from which was capable of heating three feet of quarter-inch gas pipe 

 to bright redness in a minute. When this current was passed through 

 a copper wire, a 5-inch iron spike held close under the wire was mark- 

 edly attracted by it; and when i^laced in contact with the lower side of 

 the -wire, adhered readily magneticallj^, placing itself with its axis per- 

 pendicular to that of the wire. This result was the same Avhen the con- 

 ducting wire was red hot. In a second experiment the wire passed 

 through a hole in a large sheet of glass. When this glass was sprinkled 

 with iron filings concentric magnetic curves, complete and p;u'tial, were 

 formed round the wire for a distance of 20 inches. These experiments 



