117 



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 follow^ing 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 



