Dr. Tyridail on t)iamdgnetism and MagnecrtjstaUic Action. 181 



A comparison of both tables shows us that the line which 

 stands equatorial in the magnetic field is most strongly repelled 

 upon the torsion-balance^ exactly as in the case of the crystal ; 

 the repulsion in the direction of this line and in a direction per- 

 pendicular to the same being nearly in the ratio of 3 : 2. Similar 

 experiments were made with cubes of powdered carbonate of 

 iron. The line of compression in the magnetic field stood axial ; 

 and when laid upon the torsion-balance, the mass in the direc- 

 tion of this line was attracted most strongly. 



At the last meeting of the British Association, an objection, 

 which will probably suggest itself to all who study the subject 

 as profoundly as he has done, was urged against this mode of 

 experiment by Professor William Thomson. " You have,^' he 

 said, ^^ reduced the mass to powder, but you have not thereby 

 destroyed the crystalline property ; your powder is a collection 

 of smaller crystals, and the pressing of the mass together gives 

 rise to a predominance of axes in a certain direction ; so that 

 the repulsion and attraction of the line of compression which you 

 refer to the mere closeness of aggregation is, after all, a product 

 of crystalhne action. ^^ 



I know that this objection, which was specially directed against 

 the experiment made with powdered bismuth and carbonate of 

 lime, floats in the minds of many both in Germany and England, 

 and I am therefore anxious to give it a full and fair reply. I might 

 urge, that in the case of the bismuth powder at least, the ten- 

 dency of compression would be to place the little component 

 ciystals in such a position, that a deportment precisely the re- 

 verse of that actually observed might be anticipated. If we 

 pound the crystal to the finest dust, the particles of this dust, to 

 render Mr. Thomson^s hypothesis intelligible, must have a cer- 

 tain predominant shape, otherwise there is no reason in the 

 world to suppose that pressure will always cause the axes of the 

 little crystals to take up the same predominant direction. Now 

 what shape is most likely here ? The crystal cleaves in one 

 direction more easily than in any other ; is it not then probable 

 that the powder will be chiefly composed of minute scales, whose 

 opposite flat surfaces are the surfaces of principal cleavage? 

 And what is the most probable effect of compression ? Will it 

 not be to place these little scales with their flat surfaces perpen- 

 dicular to the line in which the pressure is exerted ? In the 

 crystal, the line perpendicular to the principal cleavage sets axial, 

 and hence it might be expected that the line of compression in 

 the model would set axial also ; it does not, however, — it sets 

 equatorial. 



This, however, though a strong presumptive argument, is not 

 yet convincing ; and it is no easy matter to find one that shall 



Phil. Mag. S. 4. Vol. 2. No. 10. Sept. 1851. 



