48 THE EARTH. 



Soda consists of twenty-three parts sodium with eight 

 oxygen, it however does not exist in this form in nature, 

 but in union with chlorine instead of oxygen, constituting 

 chloride of sodium or common salt, this substance forms 

 strata of considerable extent in some localities and being 

 soluble has no doubt been washed out by the rains from 

 many other places and has thus communicated saltness to 

 the waters of the ocean. The other metallic elements exist 

 chiefly in those forms of earth called " ores " being metals 

 in union with oxygen, sulphur, &c., or are found in the 

 metallic state, as gold, platinum, &c., it is from these 

 ores that most of our useful metals are procured by the 

 process called smelting, as in the case of iron, copper, lead, 

 tin, &c. All the substances known result from the combina- 

 tions of the elements, but these elements do not chemically 

 combine in all proportions, but in certain definite quantities 

 only; these quantities or proportions are signified by the 

 numbers attached to each element in the list given,* and the 

 union of these substances must be in the proportion of 

 these numbers (or multiples of them) only, all superfluity 

 of the substances combined remaining in a state of mere 

 mechanical mixture. For example, if six parts of carbon 

 (charcoal) made red-hot be plunged into a jar containing 

 twenty parts of oxygen, it will unite with sixteen parts 

 only of the oxygen (a multiple of eight), and form twenty- 

 two parts of carbonic acid, the extra four parts of oxygen 

 still remaining as oxygen mixed with the carbonic acid ; and 

 if into this carbonic acid twenty parts of lime be put, it will 

 unite with the twenty -two of carbonic acid, forming forty- 

 two of carbonate of lime, still leaving the four parts of 

 oxygen untouched. Had forty parts of lime been put into 

 the mixture, instead of twenty, still only forty-two of car- 

 bonate of lime would have been formed, and the other twenty 

 parts (like the four of oxygen) being superfluous, would 

 still have remained as lime, mixed with the carbonate of lime. 

 These combining quantities are called the " equivalents " of 

 the substances. 



* See the list of the elements at page 45 ; those which are distinguished by an 

 asterisk, are elements of which the " equivalents " are not known. 



