68 THE PROGRESS OF SCIENCE n 



imaginable, particles were scattered through the 

 attenuated general substance of the plenum. A 

 rough analogy to the latter case would be afforded 

 by granules of ice diffused through water ; to the 

 former, such granules diffused through absolutely 

 empty space. 



In the latter part of the eighteenth century 

 the chemists had arrived at several very import- 

 ant generalisations respecting those properties o: 

 matter with which they were especially concerned 

 However plainly ponderable matter seemed to be 

 originated and destroyed in their operations, they 

 proved that, as mass or body, it remained in- 

 destructible and ingenerable ; and that, so far, it 

 varied only in its perceptibility by our senses 

 The course of investigation further proved that a 

 certain number of the chemically separable kinds 

 of matter were unalterable by any known means 

 (except in so far as they might be made to change 

 their state from solid to fluid, or vice versa), unless 

 they were brought into contact with other kinds 

 >f matter, and that the properties of tin 

 kinds of matter were always the same, wh 

 their origin. All other bodies were found to 

 -t of two or more of these, which thus took 

 the place of the four "elements" of the ancient 

 philosophers. Further, it was proved that, in 

 forming chemical compounds, bodies always unite 

 in a definite proportion by weight, or in simple 

 multiples of that proportion, and that, if any one 



