22 PHYSICAL SCIENCE 



With metaphysical theories of matter, Physical 

 Science has no direct concern ; and mechanics, at 

 any rate, deals only with matter as that concep- 

 tion, which, in our mental image of phenomena, 

 is always associated with another and more 

 definite conception, that of mass. We need not 

 ask whether matter has any objective existence, 

 or whether our conception of mass corresponds 

 with any actual property possessed by a real 

 thing- in- itself. Such inquiries are of great 

 interest and importance ; but they are meta- 

 physical inquiries, not those which the physicist, 

 as physicist, must answer. 



The conception of mass, as distinct from that 

 of weight, may arise from the results of our daily 

 experience. Let us suppose, for instance, that 

 two fly-wheels of the same size, one of wood and 

 the other of iron, were mounted on axles, and 

 were free to revolve. When the wheels are set 

 spinning, the weights do not come into play, for 

 neither wheel is raised or lowered as a whole. 

 Nevertheless, a great difference will be felt if we 

 try to set the two wheels in motion suddenly. 

 It takes either a much harder push or a much 

 longer time to produce a certain velocity of 

 rotation in the iron wheel than in the one made 

 of wood, and, on the other hand, once moving, 

 the iron wheel is much more difficult to stop. It 

 is these results which lead us to say that the 

 mass of the iron wheel is the greater. 



The idea of mass first arises from the sense- 

 perception of force ; but, to examine mass quanti- 

 tatively, more definite observation is necessary. 

 The mutual action of two bodies, as examined 

 by experiment, is such that our description of 



