On a Permanent Magnetic Field. 307 



6. We shall make no attempt to recognize the true law of 

 force acting between the molecules ; but it is obvious that b 

 does not depend upon V any further, so that, with p to denote 

 the density of the medium, our equation becomes 



p/p=nt(l + T lP ), ..... (23) 



Ti being a function of the absolute temperature. 



Now by similar considerations which, I think, it is un- 

 necessary to reproduce at length, we may generalize our 

 deduction by taking higher systems of molecules into account. 

 Suppose we stop at /-molecular systems and neglect the 

 number of those formed by i + 1 molecules ; then the equation 

 will be 



i Vp = B*(l + T 1 p + V+... +HV 1 ) . . (24) 



(the T's being functions of t), and is thus seen to be of the 

 general type suggested by Maxwell (' Scientific Papers,' 

 vol. ii. p. 407) and recently supported by Lord Rayleigh 

 (' Nature,' vol. xlv. p. 81). Equations of that kind are care- 

 fully discussed in Thiesen's elaborate paper in Wiedemann's 

 Annalen, vol. xxiv. p. 467 ; see also ibidem, vol. xxxiii. 

 p. 701. It would seem therefore that, in the proximity of 

 the critical state, double and triple molecules are prevalent ; 

 while (if we adopt a recent suggestion from Blimicke, Zeitsch. 

 fur phys. Chemie, vol. viii. p. 562) in the proximity of the 

 solid state even the effect of systems consisting of eight 

 molecules each may become traceable. 



XXXII. On a Permanent Magnetic Field. 

 By W. Hibbekt, A.I.E.E., F.I.C* 



XN the electrical laboratory of the Polytechnic Institute, 

 Regent Street, the earth's magnetic field varies so much 

 that it cannot be assumed as a basis for reasonably accurate 

 measurement. For this reason it was formerly my habit to 

 give students a certain bar-magnet as a temporary standard, 

 the number of lines passing out of the bar being determined 

 afresh from time to time. This magnet was an old one, and 

 its varied experience (now described under the term " ageing ") 

 had given it an approximate constancy. The constancy was, 

 in fact, good enough to suggest the possibility of getting a 

 really permanent magnet. I therefore made some tentative 

 efforts to ascertain the effect of slight variations in the hard- 

 ness and temper of steel on its magnetic properties, with the 

 result that I found the subject too great for my resources. 



* Communicated by the Physical Society : read December 4, 1891. 



