240 



57. Dr. McCann writes (§ 25) in relation to potential energy, 

 " this sounds plausible enough while we use the mystic word 

 energy_, but as it is motion with which we are at present 

 concerned, we shall use that word instead .^^ Now, firstly_, there 

 is no mystery about energy if only it be properly understood, 

 and secondly, the gratuitous substitution of the term " motion 

 for '^energy,^^ would inevitably make nonsense of everything that 

 has been, or indeed can be, written on the subject. It appears, 

 moreover, from the contents of the same page, that the author^s 

 views of causation are as illogical and inconclusive as he holds 

 the sentiments of physicists to be. He puts the case of ^'^a 

 heavy book nicely balanced on the edge of the table; the 

 slightest touch of my finger causes it to fall to the ground.*^ 

 But the fall would not result from the slightest touch unless the 

 book were in a position of unstable equilibrium ; neither would it 

 result from the unstable position if the touch did not ensue; 

 the touch, therefore is no more entitled to be called the cause 

 of the fall, than the unstable position : both are conditions 

 precedent^ but the cause of the fall is the attraction of gravita- 

 tion. 



58. Again, he instances the explosion of a mine by a match 

 held between the finger and thumb, and then contrasts the 

 amount of energy expended in moving the finger and thumb, with 

 the amount developed by the explosion, as though there were any 

 conceivable connection between them, in relation to cause and 

 efiPect ; the match might just as well be supposed to be attached 

 to a steam hammer, and by its descent to explode a single grain 

 of gunpowder, when the balance of the employed and resulting 

 energies, which he pleases to call motions, that is_, of the 

 assumed cause and effect, would be all the other way. Dr. 

 McCann speaks of the applied match as the cause of the explo- 

 sion, — it may be so in a popular sense, but is the expression 

 logically accurate ? It is presumed not to be so. Two little 

 heaps of black granular powder are lying on the table, one 

 happens to be gunpowder and the other coal-dust ; a lighted 

 match is applied to each in succession, one explodes_, the other 

 remains unafi'ected : is the match a whit more the cause of the 

 explosion of one heap, than of the non-explosion of the other? 

 The application of heat is a necessary condition of the explosion, 

 but the cause " of both results is alike the chemical constitu- 

 tion of the respective kinds of matter : the potential energy of 

 chemical affinity, that exists in the former, but does not exist 

 in the latter substance. A similar discussion of all the views 

 set forth in these essays would lead to a wearisome dissertation, 

 far beyond the limits of a paper readable before this Society ; 

 but it is a grave question, whether if the amount of mental 



