166 PROGRESS OF SCIENCE IN THE CENTURY. 



solid; our instinctive desire to visualise what may 

 be going on beyond the limits of the visible, naturally 

 leads us to imagine matter as having a " grained 

 structure," as being made up of minute particles 

 separated by minute intervals which change with the 

 state of the substance, with conditions of temper- 

 ature and pressure. 



The general idea is simple; the details of the 

 theory are profoundly difficult. " Imagine matter 

 to consist of a crowd of separate particles with in- 

 terspaces. Contraction and expansion are then 

 merely a drawing in and a widening out of the 

 crowd. Solution is merely a mingling of two crowds, 

 and evaporation merely a dispersal from the out- 

 skirts. The most evident properties of matter are 

 then similar to what may be observed in any public 

 meeting." * 



Among the many theories of matter, the following 

 stand out prominently. 



Perfectly Hard Atoms. (1) The idea which was 

 expressed by Democritus and Lucretius, which re- 

 ceived some measure of approbation from Newton, 

 was that matter consists of perfectly hard atoms 

 with void spaces between these. Newton used this 

 theory in his interpretation of the propagation of 

 sound. 



Centres of Force. (2) A second view, which is 

 associated with the name of Boscovich, replaces the 

 perfectly hard atom by a centre of repulsive and at- 

 tractive forces. " According to Boscovich an atom 

 is an indivisible point, having position in space, 

 capable of motion, and possessing mass. ... It has 

 no parts or dimensions; it is a mere geometrical 



* J. J. Poynting. Address Section A, Rep. Brit. Ass. for 

 1899, p. 619. 



