Prof. Tyndall on the Diamagnetic Force, i^c. 257 



in the perfectly gaseous state, the formula (3) refers to an ideal 

 ease, and I therefore refrain from enlarging upon it. 



The ratio =j-=^ corresponds to — ^ in Poisson's notation. 

 J^o^o y — 1 



I have the honour to be, Gentlemen, 



Your most obedient Servant, 

 Glasgow, September 21, 1855. W. J. Macquorn Rankine. 



59 St. Vincent Street. 



I 



XXXIV. On the Nature of the Force by ivhich Bodies are repelled 

 from the Poles of a Magnet; to ivhich is prefixed, an Account 

 of some Experiments on Molecular Influences. By John 

 Tyndall, Ph.D., F.R.S. Sfc. 



[Concluded from p. 1/9.] 

 3. Separate and joint action of a Magnet and a Voltaic Current 

 on Paramagnetic and Diamagnetic Bodies. 

 N operating upon bars of bismuth with the magnet, or the 

 current, or both combined, it vpas soon found that the gravest 

 mistakes might be committed if the question of structure was 

 not attended to ; that it is not more indefinite to speak of the 

 volume of a gas without giving its temperature, than to speak of 

 the deportment of bismuth without stating the relation of the 

 form of the mass to the planes of crystalhzation. Cut in one 

 direction, a bar of bismuth will set its length parallel to an elec- 

 tric current passing near it ; cut in another direction, it will set 

 Its length perpendicular to the same current. It was necessary 

 to study the deportment of both of these bars separately. 



A hehx was formed of covered copper wire one-twentieth of an 

 mch thick : the space within the helix was rectangular, and was 

 Fig. 6. 



1 inch long, 0*7 inch high, and 1 inch wide : the external dia- 

 meter of the helix was 3 inches. Within the rectangular space 



