138 LAWS OF COMBINATION. 



or fusion which it accomplishes. Sulphur and iron, for 

 example, require the aid of heat to bring about their 

 union. The sulphur melts, and then combines with 

 the iron. 



326. Further heating, has often just the 



Mention an- ~, , , 



other effect of contrary enect. it causes substances al- 

 heat. ready combined, to separate from each other 



again. This is especially the case, when one of 

 them is a gas. Thus, if oxide of silver or gold is 

 heated, the oxygen passes off in the gaseous form, and 

 leaves the metal behind. 



327. Heat owes its decomposing effect, 



Why does heat . , . _ . .. 



have this ef- m this and similar cases, to the tendency 



which it imparts to certain substances, to 

 assume the gaseous form. And as all bodies would, 

 probably, be gaseous, at a sufficiently high tempera- 

 ture, sufficient heat would probably decompose all 

 chemical compounds. 



328. EFFECT OF SOLUTION. The solu- 



What is the 



effect of solu- tion of one or both of two substances to be 

 combined, has, in a multitude of cases, 

 the same effect, in promoting chemical combination, 

 as that produced by heat. The reason is also the 

 same. It brings them into more general and thorough 

 contact. This is illustrated in the case of ordinary soda 

 powders, the two constituents of which, will not act 

 on each other, unless one, at least, is dissolved. 



329. ELECTRICAL RELATIONS OF ELE- 



What are the 



electrical rela- MENTs. 1 he metals are sometimes spoken 

 ^ as e l ectro ~P s i t i ve an( l tne metalloids as 

 electro-negative, for reasons given in the 



