42 THE ADVANCE OF SCIENCE 



known as Mechanics, Hydrostatics, and 

 Pneumatics. These laws had been shown 

 to hold good, so far as they could be 

 checked by observation and experiment, 

 throughout the universe, on the assump- 

 tion that all such masses of matter pos- 

 sessed inertia and were susceptible of ac- 

 quiring motion, in two ways, firstly by 

 impact, or impulse from without ; and, 

 secondly, by the operation of certain 

 hypothetical causes of motion termed 

 'forces,' which were usually supposed to 

 be resident in the particles of the masses 

 themselves, and to operate at a distance, 

 in such a way as to tend to draw any two 

 such masses together, or to separate them 

 more widely. 

 The two With respect to the ultimate constitu- 



theories 



as to tion of these masses, the same two antago- 



ma or ' nistic ojDinions which had existed since 



the time of Democritus and of Aristotle 



were still face to face. According to the 



one, matter was discontinuous and con- 



