290 Prof. Challis on Newton's "Foundation of all Philosophy." 



son and Tait consider that " the simple experiment of melting 

 two pieces of ice by rubbing them together" justifies the general 

 conclusion that " sensible heat is motion." And again, the law 

 of the dynamical equivalence of heat, established by experiment, 

 is made the foundation of a dynamical theory of heat, and of one 

 more comprehensive, termed the "Conservation of Energy." 

 This mode of philosophizing avowedly belongs to the age of 

 Bacon. It certainly is not the philosophy of the Newtonian 

 epoch. A philosopher of Newton's school would regard the 

 above-mentioned experiment, or the still more significant one of 

 making a bar of iron red-hot by repeated blows, as simply a fact 

 which he is required to explain by a course of reasoning founded 

 on ultimate principles. So the law of the dynamical equivalence 

 of heat would only be looked upon as a law which must admit 

 of explanation by the proper course of reasoning starting from 

 the same fundamental principles. If these explanations be 

 given, all the demands of science are satisfied. It would be 

 contrary to the principle of this method to theorize on any other 

 foundation than that of ultimate ideas respecting matter and 

 force, and their necessary concomitant the aether. 



But let us look a little more closely into the above generaliza- 

 tions. Heat is affirmed to be motion. But heat, whatever else 

 it be, is force, for it does what only force can do. The very law 

 of the dynamical equivalence of heat, of which Professor Thomson 

 has made so much use, proves that heat is force ; for what but 

 force can have a dynamical equivalent ? Now force is not mo- 

 tion, but something extraneous which causes a change of motion. 

 How then can heat be defined or described as motion ? I ob- 

 serve that Professor Tyndall calls heat "a mode of motion;" 

 but as this designation only implies that there are other modes 

 of motion besides heat, and does not admit the idea of force, it 

 is open to all the foregoing objections. Another objection may 

 be raised on the ground that to assert heat to be motion is to 

 contradict common understanding. Let us suppose an unscien- 

 tific person, after witnessing the experiment of striking a bar of 

 iron till it was red-hot, to ask a philosopher what was the cause 

 of the heat, and to get the answer, "heat is motion;" he might 

 reasonably reply, " I know what heat is, and what motion is, 

 and I know that heat is not motion." The logic of this answer, 

 which amounts to saying that the language of the philosopher is 

 inconsistent with common experience, is irresistible, because 

 scientific language is not exempt from the necessity of fulfilling 

 the condition of informing common understandings. r ^ 



For the sake of contrasting this mode of philosophy with that 

 which I am advocating, I will now give the answer to the above 

 question which is supplied by the principles of the latter. The 



