Carbonates, the combinations of carbonic acid with bases. These are very numerous, notwithstanding that the acid itself is one of the most feeble, and is easily expelled with effervescence by nearly all the other acids. Merely a red heat alone expels it from all the carbonates, except those of potassa, soda, lithia, ba-rytes, and strontia; and the last two are decomposed by an intense white heat. In contact with charcoal heated to redness, all are decomposed, and a metal or an oxide is produced. The carbonates of ammonia, soda, and potassa alone are readily dissolved in water; the others are insoluble, or nearly so; but if free carbonic acid is present, their solubility is increased. The numerous and important class of salts formed by carbonic acid have till lately been regarded as monobasic; in which case they would contain one equivalent of the anhydride to one of the base. But in the case of the alkalies a second equivalent of the anhydride may be combined with the metallic oxide; thus with potash there is also a bicarbonate or acid carbonate.

Owing to the existence of these salts, conjoined with a consideration of the properties of many of the compounds which carbonic anhydride forms with certain organic substances, the acid is now very generally regarded as dibasic, in which case its formula would be the double of that formerly adopted; carbonate of calcium would then be represented as CaCO3; carbonate of potassium as K2CO3; and bicarbonate or the acid carbonate of potassium as KHC03. The formulas of all the carbonates hitherto regarded as neutral would, upon this view, be doubled, while those of the acid carbonates and some of the double carbonates would retain their former value unchanged. 'The carbonates have considerable tendency to combine with each other and form double salts, like dolomite, which is a double carbonate of calcium and magnesium (MgCa, 2C03). - Among the most important carbonates are those of lime, potassa, soda, and ammonia, the last three of which will be found treated of under the names of their bases. Carbonate of lime, in its purest natural form, is the mineral calcareous spar. Chalk is also composed of it, and it is the principal ingredient in the limestones and marbles.

So abundantly diffused is limestone among the strata that form the crust of the earth, tha,t it has by some been supposed to constitute about one half of their substance. It is recognized by its moderate degree of hardness, being easily scratched with a knife to a white powder, whatever the color of the stone may be, and by its effervescing with acids. Heated to redness, its carbonic acid gas escapes, and quicklime remains. But it may be subjected to intense heat in strong close vessels, so that it may even be melted and still retain its original composition. Larny has invented a pyrometer for the measurement of intense heat, founded upon this property of carbonate of lime to dissociate its carbonic acid at high temperatures, to be again absorbed on cooling, the volume of the dissociated gas being used as the measure of the heat. - If M and M' represent the atom of any two metallic monads, such as potassium and sodium, the general formulas of the carbonates will be thus indicated: normal salt, M2CO3; acid salt, MHCO3; double salt, MM'C03. The following table exhibits the composition of important carbonates, some of which will be described under their respective bases: