i 5 4 THE POPULAR SCIENCE MONTHLY. 



A very great number of smooth, elastic spheres, equal in every respect, are 

 in motion within a region of space of a given volume, and therefore occasionally 

 impinge upon each other with various degrees of relative velocity, and in various 

 directions. 



The italics in this passage, as well as in all past and future quota- 

 tions, are mine. 



In justice to Professor Newcomb, however, we must look at his 

 entire sentence, which is this : " No elasticity is assigned to the mole- 

 cules in the kinetic theory, but only an insuperable, repulsive force, 

 which causes the molecules to repel each other when they are brought 

 sufficiently near together.'''' This information, Professor Newcomb 

 hopes, will "relieve me." I am indeed relieved ! AVhat the learned 

 jDrofessor tells me in the last part of his sentence certainly simplifies 

 matters to the last degree. All that needs be assigned to the mole- 

 cules is an "insuperable repulsive force." Such a force is the greatest 

 convenience for the physicist that can possibly be devised ; it not only 

 effects a simple and satisfactory solution of the difficulties set forth in 

 my fourth and eighth chapters, but it enables us at once to get over 

 every other difficulty that may be suggested. It is singular that Sir 

 Isaac Newton did not understand this when he was distressed about 

 the mechanism of gravitation ; for, obviously, all that is required to 

 explain it is to assign to the molecules an attractive force. Sir Isaac's 

 ignorance is all the more remarkable because, coming to think of it, I 

 now recollect that the philosophy of which Professor Newcomb is the 

 able exponent was very clearly set forth, just fourteen years before 

 the appearance of Newton's "Principia," in a profound metaphysical 

 treatise published by one Jean-Baptiste Poquelin (otherwise called 

 Moliere) under the somew T hat whimsical title " Le Malade Imaginaire." 

 Toward the close of that great work (w r hich is in the form of dia- 

 logues), one of the interlocutors, Bachelierus, philosophizes as follows : 



"Mihi a docto doctore 

 Domandatur causam et rationem quare 

 Opium facit dormire. 

 A quoi respondeo 

 Quia est in eo 

 Virtus dormitiva 

 Cujus est natura 

 Sensus assoupire." 



Of course, we are not to be embarrassed by anything John Ber- 

 noulli has written about " insuperable forces " as mathematical or phys- 

 ical, functions ; nor is it worth while to be disturbed by considerations 

 respecting the effect of their assumption upon the doctrine of the con- 

 servation of energy. 



Professor Newcomb's indignation at my treatment of the kinetic 

 theory of gases is very great indeed. " There is no theory of modern 

 physics," he says, " the processes supposed by which are invisible to 



