io Elementary Biology. 



possible, however, that that number may be increased as 

 new methods of investigation are discovered. The method 

 of spectrum analysis, for instance, already alluded to above, 

 has added many to the list. Since not a few of the elements 

 have been discovered by the decomposition of substances 

 which were believed, in a less advanced condition of the 

 science, to be themselves elements, it is by no means im- 

 probable that continued research in the same direction may 

 prove productive of further results of a similar nature. 

 Indeed, the suggestion has been thrown out by some bolder 

 speculators that all the elements are simply different modi- 

 fications or conditions of one primary elemental form of 

 matter. 



We have hitherto purposely omitted to mention one 

 property of matter,, namely inertia, which is defined by 

 Newton as * the tendency of every body to persevere in a 

 state of rest, or of uniform motion in a straight line> 

 unless in so far as it is acted upon by an impressed force/ 

 We are thus introduced to two new ideas * motion ' and 

 * force. 5 The conception of ' motion ' is one so familiar to 

 us, that it does not require further definition. Like 'matter' 

 and ' energy,' the essential nature of force is unknown. It 

 may be defined, however, from a study of its effects, as that 

 which produces, arrests, accelerates, retards, or changes the 

 direction of motion. 



We have already seen that the same kind of matter 

 may exist in the different states known as solid, liquid, and 

 gaseous, and in intermediate states. We have also seen that 

 these states of matter are transformable, the one into the 

 other. The transformation is possible only in consequence 

 of the application of a certain amount of force. 



When left alone, the solid is bounded by definite free sur- 

 faces, often, as in crystals, natural planes. Liquids have one 

 free surface, namely, that exposed to the external atmosphere, 

 and other surfaces which are in contact with the containing 



