18 GUSTAV MAGNUS. 



and constituted; only with such can we make our 

 observations and experiments. Their actions are made 

 up of the actions which each of their parts contributes 

 to the sum of the whole; and hence, if we wish to 

 know the simplest and most general law of the action 

 of masses and substances found in nature upon one 

 another, and if we wish to divest these laws of the 

 accidents of form, magnitude, and position of the 

 bodies concerned, we must go back to the laws of 

 action of the smallest particles, or, as mathematicians 

 designate it, the elementary volume. But these are 

 not, like the atoms, disparate and heterogeneous, but 

 continuous and homogeneous. 



The characteristic properties of the elementary 

 volumes of different bodies are to be found experi- 

 mentally, either directly, where the knowledge of 

 the sum is sufficient to discover the constituents, 

 or hypothetically, where the calculated sum of effects 

 in the greatest possible number of different cases 

 must be compared with actual fact by observation 

 and by experiment. It is thus admitted that mathe- 

 matical physics only investigates the laws of action 

 of the elements of a body independently of the acci- 

 dents of form, in a purely empirical manner, and is there- 

 fore just as much under the control of experience as 

 what are called experimental physics. In principle 

 they are not at all different, and the former only con- 



