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 combinations 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,[1] 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 carbonate 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. [Footnote 1: 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.]