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regard to it, that, while the doctrine of the conservation of 
energy demands the doctrine of conversion, the doctrine of 
conversion does not necessarily entail that of conservation. 
The justly-celebrated experiments of Dr. Joule on the “ Me- 
chanical Equivalent of Heat” are usually quoted as demonstrating 
this conversion in the clearest manner. They are recorded in 
Philosophical Transactions for 1850. It is manifestly impos- 
sible for me to detail here the experiments there described ; but 
he feels himself justified in stating the following conclusions : — 
“ 1st. That the quantity of heat produced by the friction of 
bodies, whether solid or liquid, is always proportional to the 
quantity of force expended; and 2nd. That the quantity of heat 
capable of increasing the temperature of a pound of water 
(weighed in vacuo and taken at between 55° and 60°) by 1° 
Fahr., requires for its evolution the expenditure of a mechanical 
force represented by the force of 772 lb., through the space 
of 1 foot/'’ The experiments, from a scientific point of view, are 
very beautiful; but the inferences, from a philosophical point 
of view, are not so conclusive. I cannot, however, state my 
own conceptions better than Mr. Moore has done for me in 
his own words : — 
36. “ The question how much mechanical work can be done by a 
given quantity of heat is far from settled. Now, to the physicist 
the downward motion of the weight is so much ‘ mechanical 
energy/ the heat produced so much ‘ work done/ To the 
philosopher, on the other hand, the motion of the weight is not 
energy or force at all, but simply an effect determined by the 
earth^s force of gravity, while the action of the heat is another 
effect. The whole series of effects, beginning with the descent 
of the weight, and terminating with the heat generated, the 
philosopher refers to a specific action of the force of gravity. 
This force he views as distributed, each effect expending a por- 
tion of the force. The physicist regards the heat produced as 
transformed mechanical energy or motion, while the philosopher 
sees in this not the conversion, but the correlation of two 
physical forces, the action of gravity supplying the condition of 
the action of the heat previously existent, though latent, in the 
water. To the physicist the descent of the weight viewed in 
relation to the heat is a cause . To the philosopher this motion, 
viewed in the same relation, is not a cause, but a condition.” 
37. Mr. Grove, in his well-known work on the “ Correlation of 
Physical Forces,” seems somewhat contradictory in his utter- 
ances, and appears to confuse correlation with conversion. His 
definition of correlation is sound ; he says it is “ a necessary 
mutual or reciprocal dependence of two ideas, inseparable even 
in mental conception ; thus, the idea of height cannot exist 
