4 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
about five times its velocity across the fibre: De la Rive, DeCandolle and myself 
have shown the influence of the same molecular grouping upon the propagation of 
heat. In the first section of the present paper, the influence of the molecular struc- 
ture of wood upon its magnetic deportment is described : De Senarmont has shown 
that the structure of crystals endows them with different powers of calorific conduction 
in different directions : Knoblauch has proved the same to be true, with regard to the 
transmission of radiant heat : Wiedeman finds the passage of frictional electricity 
along crystals to be affected by structure ; and some experiments, which I have not 
yet had time to follow out, seem to prove, that bismuth may, by the approximation of 
its particles, be caused to exhibit, in a greatly increased degree, those singular effects 
of induction which are so strikingly exhibited by copper, and other metals of high 
conducting power. 
Indeed the mere a priori consideration of the subject must render all the effects 
here referred to extremely probable. Supposing the propagation of the forces to 
depend upon a subtle agent, distinct from matter, it is evident that the progress of such 
an agent from particle to particle must be influenced by the manner in which these 
particles are arranged. If the particles be twice as near each other in one direction as 
in another, it is certain that the agent of which we speak will not pass with the same 
facility in both directions. Or supposing the effects to which we have alluded to be 
produced by motion of some kind, it is just as certain that the propagation of this 
motion must be affected by the manner in which the particles which transmit it are 
grouped together. Whether, therefore, we take the old hypothesis of imponderables, 
or the new, and more philosophic one, of modes of motion, the result is still the 
same. 
If this reasoning be correct, it would follow, that, if the molecular arrangement of 
a body be changed, such a change will manifest itself by an alteration of deportment 
towards any force operating upon the body: the action of compressed glass upon 
light, which Wertheim in his recent researches* has so beautifully turned to account 
in the estimation of pressures, is an illustration in point ; and the inference also receives 
the fullest corroboration from experiments, some of which are recorded in the papers 
alluded to, and which show that all the phenomena of magnecrystaliic action may be 
produced by simple mechanical agency. What the crystalline forces do in one case, 
mechanical force, under the control of the human will, accomplishes in the other. A 
crystal of carbonate of iron, for example, suspended in the magnetic field, exhibits a 
certain deportment : the crystal may be removed, pounded into the finest dust, and 
the particles so put together that the mass shall exhibit the same deportment as 
before. A bismuth crystal suspended in the magnetic field, with its planes of prin- 
cipal cleavage vertical, will set those planes equatorial ; but if the crystalline planes 
be squeezed sufficiently together by a suitable mechanical force, this deportment is 
quite changed, and the line which formerly set equatorial now sets axial'f'. 
* Phil. Mag. October and November 1854. t Phil. Mag. vol. ii. Ser. 4. p. 183. 
