i3? CORRELATION OF PHYSICAL FORCES. 



some antecedent force equal in amount to that produced, and 

 therefore the word initiation cannot in strictness apply, but 

 must only be taken as signifying the force selected as the 

 first : this is another reason why the idea of abstract causa- 

 tion is inapplicable to physical production. To this point I 

 shall again advert. 



Electricity may thus be produced directly by magnetism, 

 either when the magnet as a mass is in motion, or when its 

 magnetism is commencing, increasing, decreasing, or ceasing ; 

 and heat may seemingly be directly produced by magnetism. 

 Mr. Joule and Mr. Van Breda showed that iron was heated 

 when magnetised or demagnetised, and I, not knowing of their 

 experiments, took some pains to prove this, to eliminate 

 courses of error and to show that the heating effect took place 

 with other magnetic metals than iron. This was done first 

 by subjecting a bar of iron, nickel, or cobalt to the influence 

 of a powerful electro-magnet, which was rapidly magnetised 

 and demagnetised in reverse directions, the electro-magnet 

 itself being kept cool by cisterns of water, so ^that the mag- 

 netic metal subjected to the influence of magnetism was raised 

 to a higher temperature than the electro-magnet itself, and 

 could not, therefore, have acquired its increased temperature 

 by conduction or radiation of heat from the electro-magnet ; 

 and secondly, by rotating a permanent steel magnet with 

 its poles opposite to a bar of iron, a thermo-electric pile being 

 placed opposite the latter. 



It may, however, be fairly contended that in all these 

 experiments the heating effect was the result of electrical 

 currents developed by the magnet, and not directly by the 

 magnetism. 



Dr. Maggi covered a plate of homogeneous soft iron with 

 a thin coating of wax mixed with oil ; a tube traversed the 

 centre, through which the vapour of boiling water was passed. 

 The plate was made to rest on the poles of an electro-magnet, 

 with card interposed. When the iron is not magnetised, the 

 melted wax assumes a circular form, the tube occupying the 

 centre, but when the electro-magnet is put in action, the curve 

 marking the boundary of the melted substance changes its 

 form and becomes elongated in a direction transverse to the 



