584 POPULAR SCIENCE MONTHLY. 



regularity is to be noticed, as is to be seen in the accompanying dia- 

 gram. Tbere, it will be noticed, the elements, sodium, potassium, 

 rubidium and cesium, occur at the summits of somewhat irregular 

 curves. Such relations, among others, led him to formulate the prop- 

 osition that the properties of the elements are periodic functions of 

 their atomic weights; that is, they vary in a systematic manner, 

 either positively or negatively, as the scale of the atomic weights is 

 ascended. 



The division of the elements into metals and non-metals corresponds 

 broadly with another well-marked division — that into basic and acidic. 

 Generally speaking, it is the oxides of the metallic elements which 

 react with water to form bases ; and those of the non-metals which form 

 acids with water. This distinction was recognized by Lavoisier, when 

 he named oxygen the acid- forming element. But what is a base ? And 

 what is an acid? The old definition was — two classes of substances, 

 which, when brought together, react to form a salt. But the definition 

 may now be made with greater definiteness. It was known to Cavendish 

 and to Priestley, that when a current of electricity was passed through 

 the solution of a salt in water, one portion of the salt, namely the basic 

 portion, came towards the negative pole; and it was believed that this 

 was due to the basic portion possessing a positive charge. Similarly, 

 the acid portion, possessing a negative charge, traveled towards the 

 positive pole. Hence, bases were said to be electro-negative, and acids, 

 electro-positive. And Sir Humphry Davy arranged the elements m a 

 series, of which one was supposed to be electro-positive to its neighbor 

 on the right, and electro-negative to its left-hand neighbor. 



According to modern ideas, bases, by the mere act of solution in 

 water, are supposed to be split up into two portions, for which the term 

 ion, invented by Faraday, has been retained ; one ion is charged by the 

 process of solution with a positive charge, and that portion is usually 

 a metal; the other portion, which consists of one or more groups 

 of hydrogen and oxygen in combination, termed ''hydroxy 1' — OH — 

 has a negative charge. A base, indeed, is a compound which splits in 

 this manner. On the other hand, an acid, when dissolved in water, 

 undergoes an analogous split; but in this case the electro-positive ion 

 is always hydrogen, while the electro-negative ion may either be an ele- 

 ment such as chlorine, or a group of elements such as exist in nitric 

 acid (NO3). 



The order of the various elements in the electric series has been 

 determined ; and not merely determined, but to each has been attached 

 a numerical value. This value is identical with what is termed 'chem- 

 ical affinity'; and it represents the electric potential of the element 

 with reference to an arbitrary starting-point, which does not differ 

 much from that of nickel, an element closely related to iron. Only 



