PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
3 
Through the admirable lectures of Professor Bunsen on Electro-chemistry in 1848, 
I was first made acquainted with the existence of the diamagnetic force ; and in the 
month of November 1849 my friend Professor Knoblauch, then of Marburg, now of 
the University of Halle, suggested to me the idea of repeating the experiments of 
M. Plucker and Mr. Faraday. He had procured the necessary apparatus with the 
view of prosecuting the subject himself, but the pressure of other duties prevented 
him from carrying out his intention. I adopted the suggestion and entered upon the 
inquiry in M. Knoblauch’s cabinet. Our frequent conversations upon the subject 
led to the idea of our making a joint publication of the results : this we accordingly 
did, in two papers, the first of which, containing a brief account of some of the 
earliest experiments, appeared in the Philosophical Magazine for March 1850, and 
some time afterwards in Poggendorff’s Annalen ; while the second and principal 
memoir appeared in the Philosophical Magazine for July 1850, and in Poggendorff’s 
Annalen about January 1851*. I afterwards continued my researches in the private 
laboratory of Professor Magnus of Berlin, who with prompt kindness and a lively 
interest in the furtherance of the inquiry, placed all necessary apparatus at my dis- 
posal. The results of this investigation are described in a paper published in the Philo- 
sophical Magazine for September 1851, and in Poggendorff’s Annalen, vol. Ixxxiii. 
In these memoirs it was shown that the law according to which the axes of posi- 
tive crystals are attracted and those of negative crystals repelled, was contradicted 
by the deportment of numerous crystals both positive and negative. It was also 
proved that the force which determined the position of the optic axes in the mag- 
netic field was not independent of the magnetism or diamagnetism of the mass of the 
crystal ; inasmuch as two crystals, of the same form and structure, exhibited altogether 
different effects, when one of them was magnetic and the other diamagnetic. It was 
shown, for example, that pure carbonate of lime was diamagnetic, and always set its 
optic axis equatorial ; but that when a portion of the calcium was replaced by an iso- 
morphous magnetic constituent, which neither altered the structure nor affected the 
perfect transparency of the crystal, the optic axis set itself from pole to pole. The 
various complex phenomena exhibited by crystals in the magnetic field were finally 
referred to the modification of the magnetic and diamagnetic forces by the pecu- 
liarities of molecular arrangement. 
This result is in perfect conformity with all that we know experimentally regarding 
the connexion of matter and force. Indeed it may be safely asserted that every force 
which makes matter its vehicle of transmission must be influenced by the manner in 
which the material particles are grouped together. The phenomena of double refrac- 
tion and polarization illustrate the influence of molecular aggregation upon light. 
Wertheim has shown that the velocity of sound through wood, along the fibre, is 
* The memoirs in the Philosophical Magazine were written by me, and the second one has, I believe, been 
translated into German by Dr. Kronig : the papers in Poggendorff’s Annalen were edited by my colleague. 
— J. T. 
