HELMHOLTZ ON "T AND T'" 195 



faults of logical presentation. For example, very soon after the publication 

 of the work Maxwell in a letter to Tait made the following criticism of 

 some of the statements in paragraphs 207 and 208. 



"207. Matter is never perceived by the senses. According to Torricelli quoted 

 by Berkeley ' Matter is nothing but an enchanted vase of Circe, fitted to receive 

 Impulse and Energy, essences so subtle that nothing but the inmost nature of 

 material bodies is able to contain them.'... 



" 208. Newton's statement is meant to distinguish matter from space or volume, 

 not to explain either matter or density. 



" Def. The mass of a body is that factor by which we must multiply the velocity 

 to get the momentum, and by which we must multiply the half square of the velocity 

 to get its energy. 



" Hence if we take the exchequer pound as unit of mass (which is made of 

 platinum) and if we find a piece of copper such that when it and the exchequer 

 pound move with equal velocity they have the same momentum (describe experiment) 

 then the copper has a mass of one pound. 



"You may place the two masses in a common balance (which proves their 

 weights equal), you may then cause the whole machine to move up or down. If 

 the arm of the balance moves parallel to itself the masses must also be equal. 



"Some illustration of this sort (what you please) is good against heresy in the 

 doctrine of the mass. Next show examples of things which are not matter, though 

 they may be moved and acted on by forces, (i) The path of a body, (2) Its axis 

 of rotation, (3) The form of a steady motion, (4) An undulation (sound or light), 

 (5) Boscovich's centres of force. Next things which are matter such as the luminiferous 

 aether, and if there be anything capable of momentum and kinetic energy." 



But faults of the kind indicated were like spots in the sun. The greatness 

 of the book became more evident the closer it was studied. Since the days 

 of Newton's Principia, no work on Natural Philosophy of anything like the 

 same originality had been produced in England. Thomson and Tail's 

 Treatise must ever rank with the classical works of Lagrange and Laplace. 



It was not long before Helmholtz took steps for the preparation of a 

 German translation; and in 1871, after some delay on account of the 

 Franco-German War, this translation was finally published under the combined 

 authority of Helmholtz and Wertheim. A few sentences from Helmholtz's 

 Preface will indicate his own view of the value of the Treatise. 



"The present volume will introduce to the physical and mathematical German 

 public the beginning of a work of high scientific significance, which will, in the most 

 excellent fashion, fill in a very perceptible gap in the literature of the subject... 



"One of the authors, Sir William Thomson, has long been known in Germany 

 as one of the most penetrating and ingenious of thinkers who have applied them- 

 selves to our Science. When such a one undertakes to lead us, as it were, into the 



