STEEL, NICKEL, AND COBALT TUBES IN THE MAGNETIC FIELD. 479 



of the magnet itself be observed " has no real relevancy ; for the change of volume 

 of the magnet itself was not the subject of inquiry. 



Nagaoka and Honda's experimental determination of the very slight effect of 

 hydrostatic pressure on the magnetization is of high interest. It emphasises the view 

 expressed above that the strain accompanying magnetization is mainly a shear. 

 Whether we accept Kirchhoff's theory or not, we should expect to find small cubical 

 dilatation under magnetization to be associated with small magnetic change under 

 increased hydrostatic pressure. This result, consequently, can hardly be regarded as a 

 verification of Kirchhoff's theory. A similar remark may be made in regard to other 

 reciprocal relations ; and before such a theory as Kirchhoff's can be accepted as 

 established, there should be approximate numerical identity between theory and 

 experiment. Mere qualitative agreement in a few particulars between theory and 

 experiment may be largely a matter of chance, and cannot be put in the same category 

 with one serious discrepancy. Nagaoka and Honda admit that " Kirchhoff's theory 

 is a rough approximation," but conclude " that, excepting the theoretical deduction as 

 to the effect of hydrostatic pressure on the magnetization of iron, there are no serious 

 discrepancies between theory and experiment." Their high class experimental work, 

 by bringing to light one serious discrepancy and other discrepancies of a less serious 

 character, seems to me to demonstrate the insufficiency of Kirchhoff's theory. This, 

 of itself, is important enough ; but, in the present state of our knowledge, the new 

 experimental facts discovered by Nagaoka and Honda are of much greater importance. 



