446 Royal Society : — 



hypothetical properties of the internal medium he discarded as arti- 

 ficial or inadmissible, then at present I see no way of escaping the 

 conclusion of Professor Tyndall's argument. 



With regard to the explanation given by Professor Williamson, it 

 will be observed that he pursues a path quite different from that of 

 Professor Tyndall, when he considers the effects produced by com- 

 pressing a number of particles surrounded by a magnetic medium. 

 This compression, he states, 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." By the 

 term 'density of matter' is usually understood the relation which 

 exists between the quantity of matter which a body contains, and 

 the volume of the space enclosed by its external surface. But in 

 the present case, where a comparison is instituted between the 

 matter of the body and the medium which is supposed to fill all its 

 pores, we must, I imagine, understand by the term ' density of 

 mutter,' the relation vi'hich exists between the sum of the masses of 

 the particles, and the sum of their volumes ; but if so, then, the par- 

 ticles themselves being incompressible, it is clear that compression 

 could not alter the ' density of matter.' 



As to the second effect of compression, viz. an alteration of the 

 density of the medium, it may be quite conceivable, although I do 

 not find that Professor Williamson has any where taken it into con- 

 sideration. The effects of compression may, therefore, be more cor- 

 rectly described as either — first, a diminution of tlie interstices of 

 a body, without altering the density of the medium which fills 

 them ; or secondly, a diminution of the interstices, accompanied by 

 an alteration of the density of the medium within them. 



Let us assume, as Professor Williamson has virtually done, that 

 the first of these effects takes place ; then, if we admit that " 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," we can by no 

 means admit that, because " it becomes more magnetic by compres- 

 sion, we must conclude that the loss of magnetic medium from its 

 interstices is more than supplied by the magnetic matter which takes 

 its place ;" for, .according to what has already been advanced, the 

 excess of the attraction of the material particles above that of the 

 medium they displace is the same after, as it was before compression ; 

 inasmuch as compression merely changes the relative situations of 

 the particles, by bringing them closer together, but does not in the 

 least alter their volume, and consequently does not in the least alter 

 the quantity of medium they displace. 



With respect to carbonate of lime. Professor Williamson's con- 

 clusion is, of course, untenable, because it is based u])on the fore- 

 going one. He says, "when these particles are brought closer 

 together by pressure, with diminution of the intervening spaces oc- 

 cupied by the medium, the mass becomes more diamagnetic, because 

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

 less magnetic matter." It is, however, manifest that exactly the 



