26 Messrs. J. Tyndall and H. Knoblauch on the 



4 ; and along the short diagonal a row of molecules, the di- 

 stance between each two being represented by the figure 3. 

 In the magnetic field, therefore, the short diagonal will be 

 the line of elective polarity; and in magnetic crystals will 

 stand axial, in diamagnetic equatorial, which is precisely the 

 case exhibited by experiment. Thus the apparent anomaly 

 of carbonate of lime setting its long diagonal axial, and car- 

 bonate of iron its short diagonal axial, seems to be fully ex- 

 plained ; the position of the former line being due, not to any 

 endeavour on its part to stand parallel with the magnetic re- 

 sultant, but being the simple consequence of the repulsion of 

 the short diagonal. 



These conclusions are corroborated by experiment. Let a 

 rhombus be cut from pasteboard, of the same shape as a 

 rhombus of Iceland spar. Along the greater diagonal, fix 

 six or eight small magnetic pellets, and along the short 

 diagonal the same number. If this rhombus be suspended 

 horizontally in the magnetic field, on closing the circuit the 

 short diagonal will set itself from pole to pole. If pellets of 

 bismuth be used, the same diagonal will stand equatorial. 



There is no difficulty in extending the reasoning used above 

 to the case of full crystals. If this be done, it will be seen 

 that the line of closest proximity coincides with the optic axis, 

 which axis, in the magnetic field, will signalize itself accord- 

 ingly. A remarkable coincidence exists between this view 

 and that expressed by Mitscherlich in his beautiful investiga- 

 tions on the expansion of crystals by heat*. "If," says this 

 gifted philosopher, " we imagine the repulsive force of the 

 particles increased by the accession of heat, then we must 

 conclude that the line of greatest expansion will be that in 

 which the atoms lie most closely together." This line of 

 greatest expansion Mitscherlich found, in the case of Iceland 

 spar, to coincide with the optic axis. The same conclusion 

 has thus been arrived at by two modes of reasoning, as differ- 

 ent as can well be conceived. 



If, then, speculation and experiment concur in pronouncing 

 the line of closest proximity among the particles, to be that in 

 which the magnetic and diamagnetic forces will exhibit them- 

 selves with peculiar energy, thus determining the position of 

 the crystalline mass between the poles, we are furnished with 

 a valuable means of ascertaining the relative values of this 

 proximity in different directions through the mass. An order 

 of contact might, perhaps, by this means be established, of 

 great interest in a mineralogical point of view. In the case of a 

 right rhombic prism, for example, the long diagonal of the base 

 * PoggendorfPs Annalen, vol. x. p. 138. 



