PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
43 
as the weight, the times of oscillation of two masses of the same length, but possess- 
ing different numbers of attracting particles, must be the same. Coulomb indeed 
mixed iron filings with wax, so as to remove the particles out of the sphere of their 
mutual inductive action, and proved that when needles of equal lengths, but of 
different diameters, were formed from the same mixture, the duration of an oscilla- 
tion was the same for all. From this he inferred that the attractive force is simply 
proportional to the number of ferruginous particles ; but this could not be the case 
if these particles exerted any sensible reciprocal action, either tending to augment or 
diminish the induction due to the direct action of the magnet. On account of such a 
mutual action, two bars of soft iron, of the same length, and of different diameters, 
have not the same time of oscillation. 
In examining the question whether the particles of diamagnetic bodies exert a 
similar reciprocal action, M. Matteucci fills quills of the same length, and of different 
diameters, with powdered bismuth, and finds that there is no difference between 
the duration of an oscillation of the thick ones and the slender ones ; from this he 
infers that there can be no reciprocal action among the particles of the bismuth. 
Now it is not to be imagined that even in Coulomb’s experiments with the iron 
filings the molecular induction was absolutely nothing, but simply that it was so 
enfeebled by the separation of the particles that it was insensible in the experiments. 
This rem.ark applies with still greater force to M. Matteucci’s experiments with the 
bism.uth powder ; for the enfeeblement of a force already so weak, by the division of 
the diamagnetic mass into powder, must of course practically extinguish all reciprocal 
action of the particles, even supposing a weak action of the kind to exist when the 
mass is compact. 
1 will not here refer to my own experiments on compressed bismuth, but will take 
a result arrived at by M. Matteucci himself while repeating and corroborating these 
experiments. made,” says M. Matteucci, “two cylinders of bismuth precisely of 
the same dimensions, the one compressed, the other in its natural state, and found that 
the compressed mass had a diamagnetic power distinctly superior to that of natural 
bismuth*.” Now M. Matteucci, in his ‘ Cours Special,’ has made his own choice of a 
test of reciprocal molecular action ; he assumes that if cylinders of the same length, 
but of different masses, have equal times of oscillation, it is a conclusive proof that 
there is no aetion of the kind referred to. This necessarily implies the assumption, 
that were the times of oscillation different, a reciprocal action would be demon- 
strated. Now, according to his experiments described in the Association Report, the 
times of oscillation are different ; the diamagnetism of the compressed cylinder is 
“ distinctly superior’’’’ to that of the uncompressed one : the diamagnetic effect increases 
in a greater proportion than the quantity of matter ; and hence, on M. Matteucci’s 
own principles, the result negatived by his experiments on powdered bismuth is fairly 
established by those which he has made with the compressed substance. 
* Report of British Association for 1852, Transactions of Sections, p. 7. 
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