in the Description of Physical Phenomena. 421 



the resistance to evaporation, on the analogy of Ohm's law, is 

 the ratio of the canse, or heat expended, to the effect, or 

 amount of liquid evaporated, no matter how these are 

 measured. 



We may add a further illustration. Suppose that any 

 substance is heated. Different substances will rise to different 

 temperatures in virtue of their different specific heats. Here 

 the resistance to heating up, on the analogy of Ohm's law, is 

 the ratio of the cause, or heat expended, to the effect, or 

 rise of temperature produced, no matter how these are 

 measured. 



In all these cases we have a perfectly intelligible physical 

 conception of resistance, strictly in accordance with the 

 natural meaning of the word in the English language. 



But I think we should not apply by way of objection in 

 such cases arguments such as the following : — i. e. (1) That 

 the resistance so measured is not of the same dimensions as 

 electrical resistance ; or (2) that the resistance so measured 

 is not a constant, but liable to variation depending on the 

 conditions, as well as to complication by other factors. 



The ground that underlies the employment which has been 

 made of these two arguments in opposition to the use of the 

 term resistance in connexion with magnetism, is really the 

 assumption that the same word ought to imply identity in the 

 two cases of electricity and magnetism, and not analogy. If 

 it be admitted that analogy is sufficient ground for the use of 

 a word, the objections disappear. 



But if used in connexion with two different subjects, it is 

 quite impossible that the meanings of the word can be 

 identical. And that there is no objection in practice to the use 

 of the same word in meanings justified by analogy only may be 

 shown by many illustrations. Take the case of the word 

 Potential. It originated I suppose in connexion with 

 mechanical theory. It has a definite mechanical meaning, 

 and definite dimensions regarded as a mechanical quantity. 

 But we extend the term without difficulty to both the mag- 

 netic and electrical analogies ; and in each of these cases it 

 has another definite meaning, and other definite dimensions. 

 Yet nobody thinks of objecting to either of these uses of the 

 term potential, because they involve definitions of the dimen- 

 sions of potential, different both from that used in mechanics 

 and from each other. 



Similarly with the word Force. This is a purely mechanical 

 term. But nobody objects to its use in the terms electro- 

 motive force, or magnetizing force because these uses involve 

 Phil. Mag. S, 5. Vol. 25. No. 156. May 1888. 2 F 



