Hart — Action of Light on Magnetism. 71 



the action of the light is to break or destroy a number of those 

 lines of force. On removal of the acting force, or source of 

 energy, there exists a tendency to return to the original form, 

 owing to action of magnetic lines in the immediate vicinity. 

 In the next two experiments, the instability is of the opposite 

 kind and there is no tendency to return to the. original form. 

 In both cases, where the iron was demagnetized by light, and 

 where the iron was magnetized by light, the action is similar to 

 that of a blow under the same circumstances. 



While this satisfactorily explains the mechanism of the 

 results of the action of light on a magnetized iron bar, that of 

 the direct action of the light is in doubt. Analogous effects 

 can be obtained from purely mechanical actions and I have 

 obtained them also from the action of electric waves. Whether 

 these quantitatively remarkable results are due to the magnetic 

 or purely mechanical action of light, is unknown. I deter- 

 mined therefore to make some experiment in regard to the 

 action of polarized light on the magnetism of iron. Bidwell 

 has shown that polarized light in the ordinary experiments has 

 the same effect as ordinary light. In order to make a more sensi- 

 tive action, I obtained from Professor A. W. Wright a num- 

 ber of films of iron deposited on thin glass plates by electrical 

 discharge in exhausted tubes. The method of production and 

 general properties of these films are given in this Journal for 

 January and September, 1877. The films of iron had a bril- 

 liant luster and a high degree of transparency and polarized 

 the light transmitted by them powerfully. The films were 

 approximately 2 cm long and 0*5 cm in width and the thickness 

 was very small, about a quarter of the wave length of red 

 light. The amount of iron in one of these magnets was 

 extremely small ; the magnetic moment of the largest one was 

 so small that when suspended by a single silk fiber, the finest 

 obtainable, the magnetic action of the earth's field was scarcely 

 noticeable. But the magnet possesses relatively strong per- 

 manent magnetism for the amount of iron involved, in fact, 

 the iron appears permanently saturated. A remarkable feature 

 of this magnetism is its strong resistance to a reversing force. 

 If we assume the reversal of any ordinary magnet by an oppo- 

 site magnetizing force to be due to the cumulative action 

 resulting when a few unstable molecular magnets are reversed 

 and these produce other unstable groups which are in their 

 turn reversed and so on, then this phenomenon can be explained 

 by the almost total absence of unstable groupings of mole- 

 cular magnets in the mass of the iron. In a magnetic field 

 of 400 C. G. S. units it required from three to five seconds 

 to reverse it, and in a field of 25 C. G. S. units it was im- 



