H. A. Rowland and L. Bell — Action of a Magnet, etc. 39 



served among all the craters, and invariably the longest diame- 

 ter is north and sonth." It would be more correct to say that 

 the major axes of the great craters are usually at right an- 

 gles to the general axis of the group, i. e., about X. 30 deg. E. 

 Haleakala and the ancient Kipahulu crater appear to take the 

 other direction, but the statement is certainly true of the great 

 craters of Kilauea and Mokuaweoweo, which have other points 

 of resemblance. 



Thus in both the highest walls are on the western side, and 

 in both the action is working toward the southwest, as is indi- 

 cated by the fact that the northeast craters are nearly filled up, 

 while the deepest and active craters are in the southwest end 

 of the depression. 



Art. IV. — On an Explanation of the action of a Magnet 

 on Chemical Action ; by Henry A. Eowlaxd and Louis 

 Bell.* 



In the year 1881 Prof. Eemsen discovered that magnetism 

 had a very remarkable action on the deposition of copper from 

 one of its solutions on an iron plate, and he published an account 

 in the American Chemical Journal for the year 1881. There 

 were two distinct phenomena then described, the deposit of 

 the copper in lines approximating to the equipotential lines 

 of the magnet, and the protection of the iron from chemical 

 action in lines around the edge of the poles. , It seemed proba- 

 ble that the first effect was due to currents in the liquid pro- 

 duced by the action of the magnet on the electric currents set 

 up in the liquid by the deposited copper in contact with the 

 iron plate. The theory of the second kind of action was 

 given by one of us, the action being ascribed to the actual 

 attraction of the magnet for the iron and not to the magnetic 

 state of the latter. It is well known since the time of Faraday 

 that a particle of magnetic material in a magnetic field tends 

 to pass from the weaker to tlie stronger portions of the field, 

 and this is expressed mathematically by stating that the force 

 acting on the particle in any direction is proportional to the 

 rate of variation of the square of the magnetic force in that 

 direction. This rate of variation is greatest near the edges 

 and points of a magnetic pole, and more work will be required 

 to tear away a particle of iron or steel from such an edge or 

 point than from a hollow. This follows whether the tearing 

 away is done mechanically or chemically. Hence the points 

 and edges of a magnetic pole, either of a permanent or induced 

 magnet, are protected from chemical action. 



* Read at the Manchester meeting of the British Association, September, 1887. 



