Prof. E. Bouty's Studies on Magnetism. 203 



is variable in different bars : sometimes it forms a sort of net- 

 work with closed meshes, filled up with the more attackable 

 ferruginous mass ; sometimes the two substances are more inti- 

 mately mixed, and the carbonaceous steel is in fine isolated 

 grains. Physical heterogeneity involves magnetic heteroge- 

 neity ; Holtz, in fact, having measured the magnetic moment 

 of magnetized bars before and after the more or less prolonged 

 action of chlorhydric acid, believes it to be deducible from the 

 whole of his experiments that the carbonaceous steel is the real 

 seat of the coercive force, and that the rest of the mass is almost 

 destitute of it. 



It is not for us to pass judgment on the degree of confi- 

 dence deserved by A. Holtz's conclusions from the chemical 

 point of view ; but the hypothesis of the magnetic heterogeneity 

 of steel does appear to merit serious examination. In the first 

 place, it has the advantage of affording a very simple explana- 

 tion of the phenomena connected with the superposition of tem- 

 porary and permanent magnetism, for which it was imagined. 

 If, moreover, we consider (1) that the laws of the temporary 

 magnetism of steel differ in nothing essential from those of in- 

 duced magnetism in soft iron, and (2) that the development of 

 permanent magnetism is eminently variable from one species of 

 iron or steel to another, and for one and the same species, 

 according to physical conditions sometimes insignificant, we 

 shall be induced to attribute to this hypothesis a certain degree 

 of probability. 



In any case the study of mixed magnets would be truly in- 

 teresting, apart from the hypothesis which led us to undertake 

 it. On this we shall only say a few words here, reserving to 

 ourselves to return to a subject which can be fully elucidated 

 only by long series of experiments. 



Theory of a Mixed Magnet. — Let us imagine a cylinder of 

 elementary dimensions, but of very great length compared with 

 its diameter. Suppose it to be formed by the combination of 

 two sorts of magnetic elements, scattered at random, but in a 

 determinate proportion in every part of the cylinder. The ele- 

 ments A, endowed with coercive power, are such that they con- 

 serve all the magnetism which they acquire ; while the elements 

 B are absolutely destitute of coercive power. J 



Let a magnetic force F act in the direction of the axis of the 

 cylinder, and let k¥Av be the magnetic moment which the 

 cylinder would receive if the molecules B existed alone. Av 

 represents the volume of the cylinder, and k a coefficient which 

 depends on the density* of the elements in question. In like 



* That is to say, on the number of elements contained in the unit of 

 volume. 



