542 Royal Society. 



not inconsistent with the notion of a magnetic medium, but follow 

 naturally from it ; and that his argument involves a tacit assumption 

 foreign to the theory under consideration. 



The first fact adduced is, that compression increases the attraction 

 of magnetic bodies, and the repulsion of diamagnetic bodies by the 

 magnet, in the direction of the line of compression. Now it is evident, 

 that a variation of pressure on a number of particles surrounded 

 by a magnetic medium may alter the attraction of the mass by a 

 magnet in two ways ; — first, by altering the density of the matter*; 

 secondly, by altering the density of the medium. 



In a cubical mass of carbonate of iron the material particles are 

 more magnetic than the medium which they displace, and the force 

 with which it is attracted is proportional to this excess. 



If it becomes more magnetic by compression, we must conclude 

 that the loss of magnetic medium from its interstices is more than 

 supplied by the magnetic matter which takes its place. 



Carbonate of lime is less magnetic than the quantity of medium 

 which its particles displace, and when these particles are brought 

 closer together by pressure, with diminution of the intervening spaces 

 occupied by the medium, the mass becomes more diamagnetic, be- 

 cause a certain quantity of the magnetic medium is thus replaced by 

 the less magnetic matter. 



Dr. Tyndall seems to have assumed, that on the compression 

 of an aggregate of particles of a diamagnetic substance, the medium 

 is not displaced by the particles in their change of position; — in which 

 case, his conclusion, that compression must increase the magnetic 

 functions of every substance, would no doubt follow from the notion 

 of a magnetic medium. 



The second fact adduced differs chiefly in form from the one just 

 considered. Crystals of carbonate of iron are attracted most strongly 

 by a magnet acting in the direction of the crystallographic axis. 

 Crystals of carbonate of lime, possessing the same form, are most 

 strongly repelled in the direction of the same axis. In this direction 

 the functions of the matter predominate more over those of the 

 medium than in other directions of the crystal ; so that with car- 

 bonate of iron, we have the strongest magnetism ; with carbonate of 

 lime, the strongest diamagnetism in this axis. One crystal consists 

 of magnetic medium with strongly magnetic matter; the other con- 

 sists of the medium with matter of very slight magnetic force. 



The crystallographic axis is in both crystals the direction in 

 which the function of matter predominates most strongly over that 

 of the medium ; so that in the iron salt it is the most magnetic, in 

 the lime salt the feeblest magnetic direction in the crystal. 



March 22. — The Lord Wrottesley, President, in the Chair. 



The following communication was read : — 



" Further observations on the Anatomy of Macgillivrctya, Chele- 

 tropis, and allied genera of pelagic Gasteropoda." By John Denis 

 Macdonald, Esq., R.N., Assistant-Surgeon H.M.S.V. ' Torch.' 



The author states, that in a late voyage from Sydney to Moreton 



* The word "matter" is here used for brevity to denote ponderable matter. 



