DynamicaUdeas in Chemistry. 399 



sceudant importance in the economy of intellectual effort, an 

 answer is imperatively required. Moreover, as in every science 

 the same demand springs up, how vast must be the interests 

 concerned, how great the value of a right response ! 



The question stated presupposes that a deficiency or a require- 

 ment does actually exist — in other words, that the science from 

 which the question comes in some respect differs from the general 

 scientific level. Now in Part I. of these papers*, I pointed out 

 what is the universal criterion of progress — namely, " the most 

 general idea existing at a given time as a factor in every branch 

 of science." My position may be illustrated by an experimental 

 usage. Suppose a phenomenon has to be observed by reading 

 off a number ; this is done, not once, but many times, and the 

 average result is calculated. The average is certainly more va- 

 luable than either of the observations taken singly ; and, if these 

 are regardetl as combining towards an end, it is contained to a 

 greater or less extent in each of them. The idea of pure motion 

 is asserted to be the mean or general idea sought. I now pro- 

 pose to explain more fully in what sense this assertion is made, 

 and to discuss more at large than was possible in Parts II. and 

 III. the nature of the criterion itself. 



The general nature of the criterion establishes its universal 

 applicability. But I address myself more especially to chemists ; 

 and their science is at the present time, and at this instant to 

 the judicial reader, in the very crisis when the adoption of a real 

 criterion is of the supremest consequence. Sad in the poverty 

 of symbolic resources, miserably unreasoned and deficient in 

 power, it urgently requires an entire reform of its prevailing 

 theory. The student more especially, and the teacher (who is 

 in each instruction a necessary artificer of prejudices), have need 

 to pause, and find or verify that (< word/' which, like the /cpi- 

 tikos \6yos of St. Paulf, is to divide and penetrate all the 

 problems of knowledge. 



The history of the idea of pure motion is comparatively brief 

 and simple. Herakleitos of Ephesus (460 B.C.), who first an- 

 nounced it, had clearly seen its vast importance and universal 

 applicability ; and, though termed by his successors The Dark 

 (S/eoretvo?), he was undoubtedly understood by many of them. 

 The Sokrates of Plato J seems on the whole, to accept Hera- 

 kleitos's doctrine § and frequently alludes to it; where he treats 



* Phil. Mag. S. 4. vol. xxxvii. p. 461. 



t Hebrews, chap, iv. verses 12, 13. 



X In the Thecetetus. 



§ Ferrier (Lectures on Greek Philosophy, vol. i. pp. 145, 146) writes of 

 Herakleitos, " here, if anywhere, is the embryo of the solution of the enigma 

 of the universe. 1 am convinced that the unity of contraries is the law of 



