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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 
effect ; 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 unaffected : 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 
