Prof. Bohn on the Conservation of Energy, 313 



John Bernoulli, however, gave the clearest exposition of the 

 principle of the conservation of vis viva, as will be admitted after 

 a perusal of his correspondence with Leibnitz ( Vivorum eel. G. 

 C. Leibnitii et Joh. Bernoulli commercium phil. et math.), of 

 his "Discours sur les lois de la communication du niouvement }i 

 [Opera omnia, torn. iii. p. 1), and especially of his memoir entitled 

 " De vera notione virium vivarum earumque usu in dynamicis w 

 [Opera omnia, torn. iii. p. 239). 



Allow me to draw your particular attention to the following 

 two passages of the last-named memoir : — 



"§ III. Hinc patet vim vivam [quae aptius vocaretur faculias 

 agencli, Gallice le pouvoif\ esse aliquid reale et substantial, quod 

 per se subsistit, et quantum in se est, non dependet ab alio. Unde 

 concludimus, quamlibet vim vivam habere suam determinatam 

 quantitatem, de qua nihil perire potest, quod non in effectu 

 edito reperiatur. Hinc sponte fluit, vim vivam semper conservari ; 

 adeo ut quse ante actionem residebat in uno pluribusve corpo- 

 ribus, nunc post actionem reperiatur necessario in alio, vel aliis 

 pluribus corporibus, nisi quid in prioribus remanserit. Atque 

 hoc est, quod vocamus conservationem virium vivarum. ,} 



One fact of peculiar interest is John Bernoulli's assertion that 

 the vis viva which apparently disappears — that is to say, the vis 

 viva which is not employed in external work such as the raising 

 of a weight — may be consumed in molecular work. The follow- 

 ing extract from § 9 of the above memoir will establish this 

 point. 



" Si corpora non sunt perfecte elastica, aliqua pars virium 

 vivarum, quae periisse videtur, consumitur in compressione cor- 

 porum, quando perfecte se non restituunt ; a quo autem nunc 

 abstrahimus, concipientes, compressionem illam esse similem. 

 compressioni elastri, quod post tensionem factam impediretur ab 

 aliquo retinaculo, quo minus se rursus dilatare posset, et sic non 

 redderet, sed in se retineret vim vivam, quam a corpore incur- 

 rente accepisset ; unde nihil virium periret, etsi periisse videretur." 

 The conversion of vis viva into heat, or at least the possibility of 

 such a conversion, was first asserted by Augustin Frcsnel. The 

 French translation of Thomson's l Chemistry ' contains an ap- 

 pendix " On Light " from Fresnel's pen, wherein he says : — 



" C'est un principe general du mouvement des lluides elas- 

 tiques, que, de quelque facon que Pebranlement s'etende ou se 

 subdivise, la somme totalc des forces vives reste constante. Et 

 voilk principalement pourquoi la force vivc doit etre consideree 

 comme la mesure de la lumiere, dont la quantite totale reste 

 toujours a tres peu pres la meme,tant qu'elle ne traverse du moins 

 que des milieux tres transparens. Les corps noirs et meme les 

 surfaces metalliques les plus brillantes ne reflcchissent pas h 



Phil. Mag. S. 4. Vol. 28. No. 189. Oct. 1864. Y 



