108 SCIENTIFIC THOUGHT. 



It forms, therefore, no link in the actual development 

 of the energy-conception ; but it is a significant evidence of 

 the direction in which the ideas of natural philosophers 

 were then moving, and of the high degree of clearness 

 to which they rose in individual instances. When we 

 read the following words : " Besides the known fifty-four 

 chemical elements there exists in nature only one agent 

 more, and this is called ' Kraft ' ; it can under suitable 

 conditions appear as motion, cohesion, electricity, light, 

 heat, and magnetism," it seems difficult, even after the 

 lapse of two generations, to alter anything in this clear 

 and simple enunciation of the law of the conservation 

 of energy. It has indeed been stated that " unless 

 some still earlier author should be discovered, there 

 can be no doubt that Mohr is to be recognised as the 

 first to enunciate in its generality what we now call 

 ' conservation of energy.' " At the same time, the 

 case shows how little, at the beginning of a scientific 

 movement, purely abstract statements are capable of 

 really guiding research into fruitful channels. There is 

 with Mohr no attempt to establish or apply an actual 

 measure 2 of the amount of energy appearing in the 

 various instances which he mentioned. This further 

 12. step was taken five years later by J. E. Mayer, who 

 can claim to be the first 3 to have ventured on a 



tion ; his merit being variously Briefe,' ed. Wayrauch, p. 407, 



appraised according to the purely 

 scientific, the philosophical, or the 

 more practical standpoint taken up 

 by various critics. See, inter alia, 

 P. G. Tait's ' Recent Advances,' 3rd 

 ed., p. 60, &c. ; also the correspon- 

 dence of Mohr and Mayer in the 



&c. 



1 See the article on K. F. Mohr 

 in the 'Eucy. Brit.,' 9th ed. 



2 See on this point Weyrauch, in 

 Mayer's ' Kleinere Schriften,' p. 408. 



3 Helm (' Energetik,' p. 34) begins 

 the list of undoubted determina- 



latter's ' Kleinere Schriften und i tioiis of the heat-equivalent with 



