ARE THE ELEMENTS ELEMENTARY ? 465 



The members of any one of these groups resemble each other chemi- 

 cally in the closest manner, forming compounds of strong similarity, 

 and often are very much alike in their physical properties also. The 

 thought at once arises, Can these elements be totally distinct from 

 each other have they nothing in common are these resemblances 

 only due to chance ? Such a supposition could scarcely be admitted, 

 since Science excludes chance from her list of natural agencies. These 

 relationships must mean something but what ? 



If we look beyond the points of similarity to the points of differ- 

 ence between related elements, we shall find that these too are sub- 

 ject to regularity. The members of a group vary from each other, 

 not in a meaningless, helter-skelter way, but systematically, so that 

 they may be arranged in regular series. Take, for example, the group 

 formed by the strikingly similar metals, calcium, strontium, and ba- 

 rium. If, now, we compare these with reference to any physical proj)- 

 erty, we shall find that strontium will always be between the other 

 two. It is heavier than calcium and lighter than barium ; and the 

 same thing holds true of strontium compounds when compared with 

 the corresponding compounds of its two associates. The integrity 

 of the series is perfect ; for in no case can the middle member be 

 placed either at the beginning or the end. The nitrogen group is even 

 more remarkable. Arranging its recognized members in the order of 

 their atomic weight, they are as follows : nitrogen, phosphorus, arse- 

 nic, antimony, and bismuth. The first of these elements is gaseous at 

 all known temperatures ; phosphorus is a solid, but easily convertible 

 into a gas by heat ; arsenic is a denser body still, and less readily va- 

 porized ; antimony follows in regular order; and finally, bismuth, 

 the heaviest of the series, can be distilled only with considerable dif- 

 ficulty. Here, then, is a gradation both in specific, gravity and in 

 boiling-point, the lowest member of the group, in each of these par- 

 ticulars, being that with the lowest atomic weight ; and the reverse. 

 If we ascend from these elements to their compounds, we shall also 

 notice some curious chemical regularities. Each member of the group 

 unites wdth oxygen to form a pentoxide, from which an acid may be 

 derived. Compare, now, these five acids : nitric is very strong, and 

 violently corrosive ; phosphoric is a little weaker, and acts much less 

 vigorously ; arsenic is feebler still ; antimonic is extremely weak ; 

 and the corresponding bismuth compound is just barely recognizable 

 as being an acid at all. Can these regular gradations be purely acci- 

 dental and meaningless ? 



Examples like these might be adduced almost indefinitely. Series 

 after series could be brought forward, all illustrating the same principle. 

 Exceptions occur now and then, but they are so few that for present 

 purposes they may be disregarded. Of course they mean something, 

 but they are neither sufficiently abundant nor important enough to 

 affect our arguments. The regularities are so numerous and so re- 

 VOL. vin. 30 



