200 Mr. J. Parker on the Theory of Magnetism and 



position may, if the experiment is made steadily enough, be 

 exactly the same as if both bodies were at rest in that position 

 with their magnetizations invariable. 



In applying these assumptions to the second method of 

 making the experiment with the piece of bismuth, let us 

 suppose, merely for the sake of simplicity, that the magneti- 

 zation of the steel is " rigid," that is, not merely unstably 

 invariable, but invariable under all circumstances. Then let 

 the cycle be performed as follows : — 



(a) Let the bismuth be moved from X to Y so steadily 

 that its magnetization is unaltered. It is evident that no 

 thermal phenomenon will attend this operation, and that the 

 work done on the system will be < W. 



(b) The magnetization of the piece of bismuth now being 

 in unstable equilibrium, let a slight shake, by which no per- 

 ceptible amount of mechanical work is done, be given to the 

 bismuth, in consequence of which the strength of the magne- 

 tization will be affected, and a thermal phenomenon take 

 place. Then let the bismuth be held in the position Y until 

 the temperature is again 6, and the magnetization at its proper 

 value. 



(c) Next let the piece of bismuth be brought back from Y 

 to X by the first path reversed, and so steadily that the 

 magnetization does not change. The work restored by the 

 system during this operation is >W. 



(d) Lastly, let a slight shake be given and hold the piece of 

 bismuth in the position X until the system is in the original 

 state. 



We see therefore that if the two assumptions are made, the 

 hypothesis of diamagnetism enables us to obtain work from a 

 cycle in which heat can only be absorbed or given out at a 

 given constant temperature 6. We now contend that the 

 two assumptions are admissible. We shall establish the truth 

 of this assertion indirectly by showing what results the two 

 assumptions lead to when applied to several of the most 

 important propositions in magnetism and electricity. 



I. If in the modified Thomson's argument, we suppose the 

 piece of bismuth replaced by any substance S which is attracted 

 to the pole P, it will easily be seen that if the magnetization 

 of S increases as the distance from P decreases, our two 

 assumptions do not lead to a result in contradiction to the 

 principle of energy and to Carnot's principle. We there- 

 fore conclude that a substance may be attracted by a magnet 

 pole — a result which, of course, is in accordance with ex- 

 periment. 



It is necessary to notice another method of making the 



