nary conditions of pressure and temperature,
carbon dioxide dissolves in water in accordance
with the law of Henry; that is to say, the
amount of gas absorbed is projiortional to the
pressure. This shows that the existence of the
Iivpothetical compound of water and carbon di-
oxide (H;0 + CO. = H.COs) in the free state is
highly improbable, though its derivatives, the
carbonates, are among the substances commonly
met with in nature. At 0° C. carbon dioxide
is liquefied under a pressure of 30 atmosjjheres.
The critical temperatiire was ascertained by
Andrews, and, more recently, by Dewar. The
latter investigator found that above 31.9° C. a
pressure of 77 atmospheres is necessary and
sullicient to liquefy it. Liquid carbonic acid is now
largely used in the arts. A mixture of liquid
carbonic acid and ether, if rapidly evaporated,
attains a temperature of about 100° C. below
the freezing-point of water. Chemically, car-
bonic acid is a dibasic acid capable of forming
salts in which metallic elements are substituted
either for one or for both of the hydrogen atoms.
These salts, called carbonates, are readily decom-
posed by most other acids known in chemistry,
carbonic acid being one of the weakest acids
known. Even the so-ealled acid carbonates of
sodium and potassium have a feebly alkaline re-
action, carbonic acid being incapable of neu-
tralizing the powerful alkaline properties of
sodium or potassium hydroxide. Carbonic acid
is the earliest known gas. Paracelsus and 'an
Helmont, who lived in the Sixteenth Century,
knew that the gas produced in burning charcoal
was identical with that evolved by limestone
heated to a high tem]ierature. About the middle
of the Eighteenth Century Joseph Black isolated
it in a perfectly pure state. In the latter part
of the Eighteenth Century Priestley discovered
it in the air, and Lavoisier showed that the same
gas was produced during the combustion and
decay of vegetable and animal matter, during
respiration, etc. Faraday was the first to liquefy
it. Sec Chemistry.
CARBONIC OXIDE, CO. A gaseous com- pound of carbon and oxygen. It does not occur naturally, but may be observed burning with a pale-blue flame in fireplaces and stoves. During the combustion of lower layers of the fuel, the oxygen of the air unites with the carbon of the fuel to form carbonic acid, C0~; and this gas, rising through red-hot coal, has part of its oxygen abstracted by the latter, and, as a re- sult, carbonic oxide is produced, which, taking fire on the top of the coals, burns, abstracting more o.xygcn from the air and re-forming car- bonic acid, CO..
Carbonic oxide may be prepared by heating either potassium ferrocyanide or o.xalie acid with strong sulidniric acid. It is a colorless gas somewhat lighter than air, in which it burns with a characteristic bluish flame. It is exceed- ingly poisonous, forming a chemical compound with the hemoglobin of the blood, and thus pre- venting the latter from carr ing the oxygen which is necessary for supporting the ])rocesses of life. The symptoms of carbonic-oxide poison- ing are headache, dizziness, and nausea, which, if the inhabit i(in of the gas is continued, ter- minate in death. Hence the great danger arising from checking the escape of the products of com- bustion in stoves and furnaces, or of allowing illuminating gas to escape into rooms. Ordinary coal-gas contains about 8 per cent., water-gas about 40 per cent., of carbonic cxide. The pres- ence of carbonic o.xide in the air may be best detected by means of a solution of palladium chloride ; if a cloth moistened with a strong solu- tion of this salt is exposed to air containing traces of the poisonous gas, a distinct brown col- oration is i)rodiu-ed. Among the compounds of car- bon, carbonic oxide is the only one in which that element occurs in the divalent state; in all other compounds carbon is quadrivalent. It has, however, been suggested that, on the contrary, the other element (oxygen) contained in it may be quadrivalent, though it is generally found combined as a divalent element. See Valency.
CARBONIF'EROTJS LIMESTONE ( Lat. cor6o, coal + ferre, to bear), or Mountain Limestone. A term used by Alurchison, Lyell, and others to include much of the middle and lower portion of the Carboniferous system (q.v.), as now defined.
CARBONIFEROUS SYSTEM. One of the main divisions of the Paleozoic group of rocks, comprising all strata that lie between the De- vonian and Triassic systems. The name was first used in England, because of the coal-seams contained in the strata, a characteristic now known to be of almost world-wide importance. The rocks of this system include a vast series of sandstones, shales, conglomerates, limestones, and beds of coal, which are of variable thickness and more or less interbedded. There is seldom any unconformity between the Carboniferous and underlying Devonian rocks in eitlier I'^.urope or America, and where isolated examiiles are known, as in parts of Nova Scotia, New" l?runs- wiek. Great Britain, and Bohemia, they indi- cate local disturbances. Coincident with this absence of any great stratigraphical break, there is also a gradual transition from the De- vonian to the Carboniferous faunas. The rocks often show a basin-shaped arrangement, and in some areas, as in northeastern Pennsylvania, thei'e is intense folding; but still there are thousands of square miles in China, western North America, and Russia which are under lain by nearly horizontal Carboniferous strata. The dillerent types of Carbonifennis rocks mentioned above are divisible into two geiicral groups — viz.: (1) Marine sediments, usually limestones, containing nianj- invertebrate re- mains, such as corals, mollusks, encrinites, etc. The abundance of coral remains in some of them lead us to suppose that they represent fossil coral reefs. These Carboniferous limestones sometimes assume great thickness, as in Great Britain and western North America. (2) Shal- low-water deposits, such as sandstones, con- glomerates, or shales, dejiositcd in shore waters or estuaries. These shallow estuaries or lagoons, by the formation of laud across their mouth, often became converted into seacoast swamps in which there was a profuse growth of tall plants. These swamps were often of enormous extent, but at times the long-i ontinued swamp growth became temporarily interrupted by the deposition of clay washed in either by Hooded rivers or by the muddy waters of the sea break- ing in. Or. again, the sinking of the area below the ocean level may have caused the accumulation of much sediment over the swamp growth.
