Royal Society. 387 



dl ' dm 



mr , -I . 



A 2 F(B r a , -B r_1 Ba , .. B]"aoj = F m ,„- r 

 A T F/da , -d s ~'da , .. B*aoWF mi „_ s 



A^F(Va , ^""Sao, .. S>o) = F m , 



of which the last is a simple invariant, since, omitting the factor 

 w(n-l)..2.1, 



O a =a , -3" S a =a,, .. B a =an. 



With the exact relation between these functions and covariants I 

 am not at present acquainted. 



Jan. 25, 1855. — The Lord Wrottesley, President, in the Chair. 



The following communications were read : — 



" On the Nature of the Force by which Bodies are repelled from 

 the Poles of a Magnet ; preceded by an Account of some Experi- 

 ments on Molecular Influences." — The Bakerian Lecture. By John 

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



The paper commences with an introduction, in which the present 

 aspect of the portions of science to which it refers is briefly sketched. 

 A section is devoted to the examination of the magnetic properties 

 of wood, which substance, the author finds, except where extraneous 

 impurities are present, to be always diamagnetic, and to set in the 

 magnetic field with its fibre equatorial. The influence of the shape 

 of pointed and flat poles is studied, and those curious phenomena of 

 rotation, first observed by M. Pliicker, and attributed by him to the 

 action of two conflicting forces, are referred to molecular structure 

 as a cause. Between flat poles, it is proved that the line joining the 

 centres of the two poles is the line of minimum force ; that is to say, 

 the force increases more quickly from the central point of the mag- 

 netic field in an equatorial direction than in an axial one. Reflect- 

 ing on the great diversity of opinion at present existing with regard 

 to the real nature of the diamagnetic force, the author deemed it 

 necessary to commence at the foundation of the inquiry. A funda- 

 mental question in the present case is the following : — Are dia- 

 magnetic bodies repelled by a magnet in virtue of any constant pro- 

 perty possessed by the mass ? Is the force in question a mere repul- 

 sion of ordinary matter, or is the repulsion exercised in virtue of a 

 state of magnetization into which the body is first thrown ? This 

 question is answered in a manner which admits of no doubt. 



It is proved that the repulsion of diamagnetic bodies increases in 

 a quicker ratio than the strength of the magnet which produces the 

 repulsion. Within wide limits, indeed, the repulsion, instead of 

 2 C 2 



