42 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
one so put ; it is simply an opinion, and I can scarcely say more than that mine does 
not coincide with it. I would gladly enter upon the subject and endeavour to give 
the objection a scientific form were the necessary time at my disposal, but this, I 
regret to say, is not the case at present. I shall moreover be better pleased to deal 
with the objection after it has assumed a more definite form in the hands of its pro- 
poser, for I entertain no doubt that it is capable of a sufficient answer. The second 
objection M. Matteucci considers to be a more grave one. The facts are as follows : — 
the repulsion of a mass of crystallized bismuth depends upon the direction in which 
the mass is magnetized. When the magnetizing force acts in a certain direction, the 
intensity of magnetization, and the consequent repulsion of the mass, is a maximum. 
This is proved by placing the mass upon the end of a torsion beam and bringing its 
several directions successively into the line of the magnetic force. Poisson would 
have called such a direction through the mass a principal axis of magnetic induction, 
and I have elsewhere called it a line of elective polarity. When a sphere or cube of 
bismuth is freely suspended in the magnetic field, with the direction referred to hori- 
zontal, in all positions except two the forces acting on the mass tend to turn it ; those 
positions are, when the line of maximum magnetization is axial and when it is equa- 
torial, the former being a position of unstable, and the latter a position of stable 
equilibrium. When the above line is oblique to the direction magnetization, the 
sphere or cube will turn round its axis of suspension until the direction referred to 
has set itself at right angles to the line joining the poles. Now if the direction of 
maximum magnetization be transverse to an elongated mass of bismuth, such a mass 
must, when the said direction recedes to the equator, set its length from pole to pole. 
The facts observed by M. Matteucci seem to me to be a simple corroboration of this 
deduction*. 
The third objection is directed against an imaginary case, “ si Ton consid^re les 
plans de clivage et la ligne de compression comme jouissant les meme propri6t6s.” 
It must be evident that a crystal like bismuth, possessing a number of cleavages of 
unequal values, cannot be compared in all respects with a body which has suffered 
pressure in one direction only. I have no doubt whatever that by a proper applica- 
tion of force, in different directions, a compressed mass might be caused to imitate to 
perfection everyone of the actions exhibited by crystallized bismuth. Indeed I would 
go farther, and say, that I shall be happy to undertake to reproduce, with bismuth 
powder, the deportment of any diamagnetic crystal whatever that M. Matteucci 
may think proper to name. 
In looking further over M. Matteucci’s instructive book, I find another point 
alluded to in a manner which tempts me to make a few remarks in anticipation of a 
fuller examination of the subject. The point refers to the reciprocal action of the 
particles of magnetic and diamagnetic bodies. It is easy to see, that if the attraction 
of a bar of iron varies simply as the number of the particles attracted, then, inas- 
much as the weight of the body varies in the same ratio, and the moment of inertia 
* For a more complete examination of this subject see the “Appendix” to this paper. — J. T., May 1855. 
