of a Rectilinear Electrical Current on itself. 367 



The fact that a current is passing along a chain of molecules is 

 a proof that these molecules are not statically in the same condi- 

 tion. This fact has been proved experimentally by Kohlrausch, 

 in his investigations into the laws of Ohm. The differences in 

 their statical condition are the immediate cause of the current ; 

 for the current is simply the discharge of the electricities of the 

 contiguous molecules upon each other, so as to bring them under 

 the same conditions. What, then, are the mutual tendencies of 

 molecules thus differently charged ? Is it to approach or to re- 

 cede from each other ? Prof, van Breda and others reply, facts 

 show that they do recede from each other. Be it so ; still we 

 must allow that the electricities with which the separate mole- 

 cules are charged do not recede. If the molecules themselves 

 recede, the electricities approach ; for it is the tendency of the 

 electricities on the contiguous molecules to combine which pro- 

 duces the current. The contiguous sections of the current must 

 therefore attract, not repel each other. It may be difficult to 

 conceive how a mutual attraction of the contiguous sections of 

 the current should result in a mutual repulsion in the contiguous 

 molecules of the conductor carrying the current ; but if such 

 be the fact, this is enough. 



The difficulty will, however, in a great measure disappear 

 when we reflect that the molecules offer resistance to the trans- 

 ference which constitutes the current, the amount of the resist- 

 ance depending upon the nature of the molecules, the conduc- 

 tibility of bodies for electricity being inversely as the amount of 

 resistance offered by their molecules. Now if the current is 

 produced by a tendency in the electricities on the contiguous 

 molecules to combine, then it is quite natural to suppose that 

 the molecules themselves would recede from each other in order 

 to prevent combination. Their very endeavours to resist the 

 transference of their electricities would cause them to recede 

 from each other. If we suppose that the molecules are not 

 simply passive, but active in their resistance (and we have every 

 reason to believe that they are), then recession from each other, 

 when the current is passing, is what might be anticipated. 



The same conclusion will follow should we suppose that the 

 passing of the current tends to produce a disturbance in the equi- 

 librium of the molecules; for each molecule, in its endeavour to 

 avoid disturbance in its statical condition caused by the presence 

 of its neighbour, will recede from it. In fact recession is implied 

 in the very idea of molecular resistance. 

 I am, Gentlemen, 



Your most obedient Servant, 



James Croll. 



Glasgow, March 4, 1862. 



