1851] WRITINGS OF JOSEPH HENRY. 303 



platina foil, an attraction in both cases will be exhibited. 

 If however the surface of a large circular metallic plate be 

 covered at intervals with short needles placed parallel to each 

 other, and a discharge of electricity be sent along the diameter 

 of the circle at right angles to the needles, on examination, 

 they will be found magnetized with different degrees of inten- 

 sity. Those in the direct line of the discharge will exhibit 

 a slight degree of polarity, while those at the circumference 

 of the plate will show a much greater amount of magnetic 

 force; proving that the electrical discharge, instead of pass- 

 ing in the shortest line between the two points, has divided 

 itself into two portions, each passing at as wide a distance 

 as possible from the other. This phenomenon is in strict 

 accordance with the hypothesis that the plate has been 

 traversed by an elastic fluid, the particles of which, being 

 self-repellant, have separated as far as possible from each 

 other; and it can therefore be referred to the action of a fluid 

 co-existing with, but independent of, ordinary matter; while 

 the phenomenon of the attraction of the two parallel con- 

 ductors before mentioned can only be explained by a change 

 in the condition of the gross matter itself combined perhaps 

 with the action of an elastic fluid. I ought to state in this 

 place that my friend Dr. Hare, from purely theoretical con- 

 siderations, independent of experiment, has arrived at a 

 similar conclusion. 



There is another phenomenon which I may mention as 

 producing a change in the properties of matter during the 

 instantaneous passage of an electrical discharge. At the 

 moment of the passage through the atmosphere of a discharge 

 of electricity, the particles of the air are suddenly endowed 

 with a surprisingly energetic repulsive tendency, to which is 

 mainly to be attributed the mechanical effects produced by 

 a discharge of lightning passing through a building. Also 

 in the development of magnetism in a bar of iron or steel 

 a change takes place ir^the ponderable molecules of the 

 metal; this is evident from the fact that at the moment of 

 magnetization a wave of undulation, capable of producing an 

 audible sound, is transmitted along the bar; and again. 



