ARMY SURGEON AT POTSDAM 49 



well-known law of the conservation of vis viva, and delayed 

 the strict mathematical expression of the law which he divined. 

 Helmholtz, on the other hand (by analogy with the name 

 1 quantity of vis viva ', which was used by Leibniz to express 

 the work-equivalent of the velocity of the moving masses), 

 gave the name 'quantity of tensional force* to this product, 

 and in thus expressing the work-value of those forces which 

 are actually engaged in the stress of producing motion, he 

 established a connexion between actual and potential energy, 

 the sum of which is constant for all transformations. 



This conception of a definite store of energy in the universe 

 was quite new and due to Helmholtz alone; it was defined 

 'as a quantity which can no more be destroyed nor added to 

 than a substance, which acts in space and yet cannot be sub- 

 divided with space like a material substance, because each 

 division of space would not involve the portion of " tensional 

 energy " which exists between the particles of matter on either 

 side of the dividing surface/ a conception familiar to modern 

 science, and founded solely upon the mighty work of Helm- 

 holtz and the splendid pioneering achievements of Lord 

 Kelvin. 



Twenty years later Helmholtz again took occasion to ascribe 

 priority in the conception of the conservation of energy to 

 Robert Mayer, as against the claims of Joule. In a letter 

 to Tait on the occasion of a dispute about priority in the 

 question of absorption and radiation, he puts Kirchhoff for- 

 ward, since he was the first to formulate the law, and thus 

 made the great discoveries that are involved in it possible. 

 Helmholtz maintained that Kirchhoff 's work in this field 

 represented one of the most instructive cases in the history 

 of science. Many investigators had been on the verge of 

 the same discoveries, but the development of spectral analysis 

 as a whole became possible only after Kirchhoff had theoreti- 

 cally determined those general properties of heat which are 

 its fundamental basis. He clearly enunciates the relation of 

 Mayer to Joule, and thus indirectly to himself also : 



1 Robert Mayer was not in a position to carry out experi- 

 ments. He was repulsed by the physicists with whom he 

 was acquainted, and could hardly find acceptance for his 

 first condensed statement. While no one can deny that Joule 



