Magnetic Medium in Space. 



291 



may enhance, in the direction of the strain, that quality of the 

 substance by which it lessens the magnetizability of the space 

 from which it displaces air or "aether;" just as a similar state 

 may enhance, in the direction of compression, the augmenting 

 power of a paramagnetic substance. 



By the bye, a long time ago (rather more than a year after 

 the Edinburgh meeting of the British Association) I repeated 

 with much pleasure some of your compression experiments, and 

 found a piece of fresh bread instantly affected by pressure, so as 

 always to turn the compressed line perpendicular to the lines of 

 force, to whatever form the fragment was reduced. A very slight 

 squeeze between the fingers was quite enough to produce this 

 property, or again to alter it so as to make a new line of com- 

 pression set equatorially. I repeated it a few days ago with the 

 same results, and got a ball of bismuth, too, to act similarly. I 

 remember formerly finding the bread attracted as a whole, instead 

 of being repelled, as I expected from your results. I suppose, 

 however, this must have resulted from some ferruginous impu- 

 rities, which it may readily have got either in the course of the 

 experiments with it, or in the baking. I mean to try this again*. 



I do not quite admit the argument you draw from your com- 

 pression experiments regarding the effect of contiguity of par- 

 ticles, because in fact we know nothing of the actual state of the 

 molecules of a strained solid. You have made out a most in- 

 teresting fact regarding their magnetic bearings; but experi- 

 ments are neither wanted, nor can be made, to show any sensible 

 effect whatever of the mutual influence of a row of small pieces 

 of bismuth placed near one another, or touching one another. 

 It is perfectly easy to demonstrate that it must be such as to 

 impair the " diamagnetization " of each piece when the line of 

 the row is parallel to the lines of force, and to enhance it when 

 that line is perpendicular to the lines of force, but in each case 

 to so infinitesimally minute a degree, as to be wholly inappre- 

 ciable to the most refined tests that have ever been applied. For 

 let the lines of force be parallel to the line shown in the figure, 



->7l 



and act on a steel needle in the manner there represented. Then, 

 whatever hypothesis be true for diauiagneti.sm, there is not a 



* Prof . Thomson's supposition ie correct; pure bread is repelled by a 

 <■ |i ill*. I may remark that I am :ii present engaged in the further 

 examination of the influence of compression, ami have already obtained 

 numerous instructive results. — J. T. 



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