THE HISTORY OF A DOCTRINE. 213 



the army which obeys an impulse from one head has more error 

 than truth in it; and, though all similes are more or less mis- 

 leading, I would almost prefer to ask you to think rather of a 

 moving crowd, where the direction of the whole comes somehow 

 from the independent impulses of its individual members, not 

 wholly unlike a pack of hounds, which, in the long run, perhaps 

 catches its game, but where, nevertheless, when at fault, each 

 individual goes his own way by scent, not by sight, some running 

 back and some forward ; where the louder- voiced bring many to 

 follow them, nearly as often in a wrong path as in a right one ; 

 where the entire pack even has been known to move off bodily on 

 a false scent ; for this, if a less dignified illustration, would be one 

 which had the merit of having a considerable truth in it, but one 

 left out of sight by the writers of books. 



At any rate, the actual movement has been tortuous, or often 

 even retrograde, to a degree of which you will get no idea from 

 the account in the text-book or encyclopaedia, where, in the 

 main, only the resultant of all these vacillating motions is given. 

 With rare exceptions, the backward steps — that is, the errors and 

 mistakes, which count in reality for nearly half, and sometimes 

 for more than half the whole — are left out of scientific history ; 

 and the reader, while he knows that mistakes have been made, 

 has no just idea how intimately error and truth are mingled in a 

 sort of chemical union, even in the work of the great discoverers, 

 and how it is the test of time chiefly which enables us to say 

 which is progress when the man himself could not. If this be a 

 truism, it is one which is often forgotten, and which we shall do 

 well to here keep before us. 



This is not the occasion to review the vague speculations of 

 the ancient natural philosophers from Aristotle to Zeno, or to 

 give the opinion of the school-men on our subject. We take it up 

 with the immediate predecessors of Newton, among whom we 

 may have been prepared to expect some obscure recognition of 

 heat as a mode of motion, but where it has been, to me at least, 

 surprising, on consulting their original works, to find how general 

 and how clear an anticipation of our modern doctrine may be 

 fairly said to exist. Whether this early recognition of the atomic 

 and vibratory theories be a legacy from the Lucretian philosophy, 

 it is not necessary to here consider. The interesting fact, how- 

 ever it came about, is the extent to which seventeenth-century 

 thought is found to be occupied with views which we are apt to 

 think very recent. 



Descartes, in 1664, commences his " Le Monde " by a treatise 

 on the propagation of light, and what we should now call radiant 

 heat, by vibrations, and further associates this view of heat as 

 imotion with the distinct additional conception that in the cause 



