312 A Sketch of the Laws of Chemical Combination. 



therefore to be considered as at that time;>osJ/iDe with regard to potassium. 

 Affain, mercury is oxidised by contact with the air near its boiling point ; 

 but at n. higher temperature it loses its affinity for oxygen. Besides this, 

 there seems to be a great difierence in the intensity of the electric polarity 

 in various bodies ; for we see that two bodies usually considered negative, 

 such as sulphur and oxygen, may combine with much greater force than 

 when one of them is positive. Berzehus gives a very ingenious explanation 

 of these facts, which is however too long to be inserted here (Op. cit. p. 56). 

 They show, however, that liis table can only be considered as an approxi- 

 mation lO the truth ; and it is given as such by its author. 



The opposite chemical i)roperties of the oxides of these elementary sub- 

 stances were recognised long before the study of their electrical states. 

 All those which are decidedly electro-uegative, form acids by combination 

 ■with oxygen ; and all the positive elements form alkaline bases ; but there 

 are many substances possessed of no distinct electric characters, whose 

 oxides exhibit acid or alkaline properties according to the quantity of 

 oxygen they contain : and in some cases, the same oxide will vary in its 

 relations according to the nature of the substance with which it combines ; 

 a feeble acid serving as a base to a stronger one, and a feeble base in like 

 manner acting as an acid to one of more decided alkaline i)roperties. Of 

 this nature are the sesquioxidcs of aluminium and of antimony, the deut- 

 oxide of tellurium, — and even water, as will presently a])pear. 



It may here be remarked that the term acid is now extended to all those 

 compounds which unite with potash or ammonia, and give rise to bodies 

 similar in their constitution and general character to the salts which the 

 sulphnric, or some admitted acid, forms with those alkalies. In like manner 

 the power of neutralising acids is now considered by chemists as the most 

 characteristic property of alkalies ; and therefore all those bodies are 

 classed among alkaline or salifiable bases, which unite definitely with ad- 

 mitted acids, and are capable of forming with them neutral saline com- 

 binations. In both these cases the more manifest properties of the body 

 may be concealed by its insolubility, which will prevent its nature from 

 being recognised by the taste, or by test paper. 



The compounds which are formed by the union of elementary bodies 

 may be indefinite or definite. The first division includes those whose 

 elements can ai)parently unite in any proportion in which they may be 

 mingled, — as alcohol and water : as well as those in which combination 

 occurs in every proportion within a certain limit, — as in the instance of 

 solutions of salts in water, alcohol, &c. Even in these cases, however, the 

 apparent variety of proportion may arise from the mixture or combination 

 of a few definite compounds with each other in various ways. 



The most interesting series of compounds is produced by substances 

 which unite in few proportions only ; and the laws which have been dis- 



