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phosphate of lime when hones are employed. Carbonate of potash isbetter for this purpose than phosphate of lime. The property possessedby minute particles of charcoal, of abstracting colouring matter fromliquids, depends, probably, on some chemical affinities existing betweencarbon and colouring matter. It is stated, in some works, that charcoalwhich has been once used cannot have its decolorizing propertyrestored by a fresh ignition, unless it be mixed with some inorganicsubstance. This, however, is an error. The animal charcoal which hasbeen used in sugar refining, is returned to the maker to be fresh ignited,and is then employed again, and this process of re-igniting, is, I aminformed, repeated many times, without any loss of decolorizing power.
Ac'idum Carbon'icum.—Carbonic Ac'id.
History and Synonymes. —Although the ancients were acquaintedwith the poisonous properties of carbonic acid gas, Dr. Black, in 1757,was the first who explained the nature of this substance. The spirituslethalis of the ancients is evidently this acid, as is also the spiritussylvestris or gas of Paracelsus and Van Helmont . Fixed air, acid vapour,and aerial acid, are other synonymes for it.
Natural History . —It is a constituent of both kingdoms of nature.
(a). In the unorganized kingdom. —Carbonic acid is a constituent of theatmosphere. In some parts of the world it is evolved from the earth inlarge quantities, particularly in old volcanic countries. Thus in thevicinity of the Lake of Laach, Bischof estimates the exhalation as equalto 600,0001bs. daily, or 219,000,0001bs., (equal to about 1,855,000,000cubic feet) annually ! (De la Beche , Theoret. Geology.) Some of theacid evolved in the Brohlthal, on the Rhine , is employed by him in themanufacture of chemical preparations on the large scale. Most personsare familiar, by report, with the Grotto del Cane, near Naples. It is acavity in a rock, through the fissures of which carbonic acid is evolved.It has received its name from the practice of putting dogs into it, whofall down suffocated. The Valley of Poison, in Java, which has beendescribed by Loudon, is another spot where this acid escapes from theearth. It is a cavity of an oval form, about three quarters of a mile in cir-cumference, and from thirty to thirty-five feet deep; filled to the heightof about eighteen feet with carbonic acid gas. The bottom of it iscovered with the skeletons of men and various other animals, who havefallen victims to its destructive operation. If a traveller should be sounfortunate as to enter it, he cannot be sensible of his danger until toolate to return. Mr. Loudon thrust a dog in ; he fell in fourteen seconds.A fowl thrown in appeared to be dead before it reached the ground !!
Carbonic acid gas is frequently met with in mines and wells ; and istermed by miners choke damp (from the German dampf, vapour).
Few mineral waters are without this acid, and in some it exists insuch quantity as to give them a sparkling or effervescent quality(see p. 146).
Lastly, carbonic acid is found (native) in combination with variousbases; as with soda, baryta, strontian, lime, magnesia, and the oxides ofmanganese, zinc, lead, iron, and copper. According to Mr. De la Beche (op. cit.) the average amount of carbonic acid locked up in every cubicyard of limestone, is about 16,000 cubic feet.