532 LECTURE LV. 



forces have been more particularly investigated by Coulomb* and others. 

 There are the same objections to these hypotheses as to those which con- 

 stitute the theory of electricity, if considered as original and fundamental 

 properties of matter : and it is additionally difficult to imagine, why iron, 

 and iron only, whether apparently magnetic or not, should repel 'similar 

 particles of iron with a peculiar force, which happens to be precisely a 

 balance to the attraction of the magnetic fluid for iron. This is obviously 

 improbable ; but the hypotheses are still of great utility in assisting us to 

 generalise, and to retain in memory, a number of particular facts which 

 would otherwise be insulated. The doctrine of the circulation of streams 

 of the magnetic fluid has been justly and universally abandoned, and 

 some other theories, much more ingenious and more probable, for instance 

 that of Mr. Prevost,t appear to be too complicated, and too little supported 

 by facts, to require much of our attention. 



The distinction between conductors and nonconductors is, with respect 

 to the electric fluid, irregular and intricate : but in magnetism, the softness 

 or hardness of the iron or steel constitutes the only difference. Heat, as 

 softening iron, must consequently render ,it a conductor : even the heat of 

 boiling water affects it in a certain degree, although it can scarcely be sup- 

 posed to alter its temper ; but the effect of a moderate heat is not so con- 

 siderable in magnetism as in electricity. A strong degree of heat appears, 

 from the experiments of Gilbert, J and of Mr. Cavallo, to destroy com- 

 pletely all magnetic action. 



It is perfectly certain that magnetic effects are produced by quantities 

 of iron incapable of being detected either by their weight or by any 

 chemical tests. Mr. Cavallo || found that a few particles of steel, adhering 

 to a hone, on which the point of a needle was slightly rubbed, imparted to 

 it magnetic properties ; and Mr. Coulomb^f has observed that there are 

 scarcely any bodies in nature which do not exhibit some marks of being 

 subjected to the influence of magnetism, although its force is always pro- 

 portional to the quantity of iron which they contain, as far as that quan- 

 tity can be ascertained ; a single grain being sufficient to make 20 pounds 

 of another metal sensibly magnetic. A combination with a large propor- 

 tion of oxygen deprives iron of the whole or the greater part of its 

 magnetic properties ; finery cinder is still considerably magnetic, but the 

 more perfect oxids and the salts of iron only in a slight degree ; it is also 

 said that antimony renders iron incapable of being attracted by the magnet. 

 Nickel, when freed from arsenic and from cobalt, is decidedly magnetic, 

 and the more so as it contains less iron. Some of the older chemists sup- 

 posed nickel to be a compound metal containing iron, and we may still 

 venture to assume this opinion as a magnetical hypothesis. There is in- 

 deed no way of demonstrating that it is impossible for two substances to 

 be so united as to be incapable of separation by the art of the chemist ; 



* Hist, et Mem. 1785, pp. 569, 578 ; 1789, p. 455. Mem. de 1'Instit. iii. 176. 

 f De 1'Origine des Forces Magnetiques, Geneve, 1788. 

 t De Magnete, fol. Lond. 1600. 



Ph. Tr. 1787, p. 6. || Ibid. 1786, p. 62. 



H Bulletin de la Soc. Philom. No. 61, 63. Jour, de Phy. liv. 240, 267, 454; See 

 Young, Jour, of the Roy. Inst. i. 134, 217. 



