CONTENTS. IX 



PAGE 



48. So far we have followed a more or less hypothetical develop- 

 ment of mechanics, because we do not know precisely the 

 route by which Newton reached his concept of force. 

 With the concept of vis viva it is otherwise 107 



49. Huygens openly derives it from the "instinctive perception" 

 that the centre of gravity of a system left to itself cannot 

 rise. This was developed into the Principle of Vis Viva 

 by the Bernoullis 108 



50. Upon this principle Helmholtz based his wider Principle of 

 the Conservation of Energy which first brought the facts 

 of Heat into a line with those of Mechanics 109 



51. But this doctrine, though it connects temperature changes 

 with mechanical facts, does not effect a reduction of the 

 former to the latter 110 



52. Nor is this reduction effected by Lord Kelvin's absolute 



therm ody namic scale ... ... ... ... ... ... Ill 



53. For the latter makes no attempt to deduce from dynamical 

 data the experiences to which the notion of temperature 

 refers 112 



54. Even the theory of Helmholtz (which is the most complete 

 of such attempts) only achieves very partial success in this 

 direction 114 



55. Meanwhile dynamical formulae may be extended to include 

 primary facts in different physical sciences, without any 

 attempt at a mechanical reduction of these facts 117 



56. It is doubtful, however, whether temperature changes can 



properly be dealt with in this way 120 



57. Thus the attempted reduction has not, up to the present, 

 been effected. The modern science of Energetics contents 

 , itself with attempting to bring physical facts into a form 

 of unity without reducing them to one type. The 

 Principle of the Thermodynamic Potential plays here the 

 part which 'the Principle of Virtual Velocities plays in 

 dynamics 120 



58. A summary of the results of Chapters III and IV ... ... 123 



CHAPTER V. 



59. The question of the actual achievements of Science now 

 arises. How far is the pre-critical view of the real 

 validity of hypotheses to be accepted ? 126 



>5 60. There are three distinct forms of hypotheses. The first class 

 includes attempts to supplement the data so as to produce 

 a complete spatio-temporal chain 128 



61. An important sub-class contains hypotheses of an " ejective " 



type ... 129 



