40 Prof. L. Boltzmarm on the 



bodies at rest their particles are themselves in a state of 

 motion, which give rise to thermal phenomena, and whose 

 nature is especially sharply denned in the case of gases. 

 ( Clausius.) 



The theory of gases led to surprising prognoses ; thus, for 

 instance, that the coefficient of friction was independent of the 

 pressure, and certain relations between friction, diffusion, and 

 conductivity for heat, &c, &c. {Maxwell.) 



The aggregate of these methods was so successful that to 

 explain natural phenomena was defined as the aim of natural 

 science ; and what were formerly called the descriptive natural 

 sciences triumphed, when Darwin's hypothesis made it possible 

 not only to describe the various living forms and phenomena, 

 but also to explain them. Strangely enough Physics made 

 almost exactly at the same time a turn in the opposite 

 direction. 



To KirchhofF, more especially, it seemed doubtful whether 

 it was justifiable to assign to Forces that prominent position to 

 which they were raised by characterizing them as the causes 

 of the phenomena. 



Whether, with Kepler, the form of the orbit of a planet and 

 the velocity at each point is defined, or, with Newton, the force 

 at each point, both are really only different methods of de- 

 scribing the facts ; and Newton's merit is only the discovery 

 that the description of the motion of the celestial bodies is 

 especially simple if the second differential of their coordinates 

 in respect of time is given (Acceleration, Force) . In half a 

 page forces were defined away, and physics made a really 

 descriptive natural science. The framework of mechanics 

 was too firmly fixed for this change in the external aspect to 

 have any effect on the interior. The theories of elasticity 

 which did not involve the conception of molecules were of 

 older date (Stokes, Lame, Clebsch) . Yet in the development 

 of other branches of physics, Electrodynamics, theories of 

 pyro- and of piezoelectricity, the view gained ground that it 

 could not be the object of theory to penetrate the mechanism of 

 Nature, but that, merely starting from the simplest assump- 

 tions (that certain magnitudes are linear or other elementary 

 functions), to establish equations as elementary as possible 

 which enable the natural phenomena to be calculated with 

 the closest approximation ; as Hertz characteristically says, 

 only to express by bare equations the phenomena directly 

 observed without the variegated garments with which our 

 fantasy clothes them. 



Meanwhile several investigators had, from another side, 



