2 , or carboxyl. If the addition of CO 2 raises the basicity, the removal of CO 2 lowers it. Thus from the bibasic oxalic acid, C 2 H 2 O 4 , or phthalic acid, C 8 H 6 O 4 , by eliminating CO 2 (easily effected experimentally) we obtain the monobasic formic acid, CH 2 O 2 , or benzoic acid, C 7 H 6 O 2 , respectively. The nature of carboxyl is directly explained by the law of substitution. Judging from what has been stated in Chapters VI. and VIII. concerning this law, it is evident that CO 2 is CH 4 with the exchange of H 4 for O 2 , and that the hydrate of carbonic anhydride, H 2 CO 3 , is CO(OH) 2 , that is, methane, in which two parts of hydrogen are replaced by two parts of the water radical (OH, hydroxyl) and the other two by oxygen. Therefore the group CO(OH), or carboxyl, HCO 2 , is a part of carbonic acid, and is equivalent to (OH), and therefore also to H. That is, it is a univalent residue of carbonic acid capable of replacing one atom of hydrogen. Carbonic acid itself is a bibasic acid, both hydrogen atoms in it being replaceable by metals, therefore carboxyl, which contains one of the hydrogen atoms of carbonic acid, represents a group in which the hydrogen is exchangeable for metals. And therefore if 1, 2 ... n atoms of non-metallic hydrogen are exchanged 1, 2 ... n times for carboxyl, we ought to obtain 1, 2 ... n -basic acids. Organic acids are the products of the carboxyl substitution in hydrocarbons. If in the saturated hydrocarbons, C n H 2n + 2 , one part of hydrogen is replaced by carboxyl, the monobasic saturated (or fatty) acids, C n H 2n + 1 (CO 2 H), will be obtained, as, for instance, formic acid, HCO 2 H, acetic acid, CH 2 CO 2 H, ... stearic acid, C 17 H 35 CO 2 H, &c. The double substitution will give bibasic acids, C n H 2n (CO 2 H)(CO 2 H); for instance, oxalic acid n = 0, malonic acid n = 1, succinic acid n = 2, &c. To benzene, C 6 H 6 correspond benzoic acid, C 6 H 5 (CO 2 H), phthalic acid (and its isomerides), C 6 H 4 (CO 2 H) 2 , up to mellitic acid, C 6 (CO 2 H) 6 , in all of which the basicity is equal to the number of carboxyl groups. As many isomerides exist in hydrocarbons, it is readily understood not only that such can exist also in organic acids, but that their number and structure may be foreseen. This complex and most interesting branch of chemistry is treated separately in organic chemistry.
Carbonic Oxide.—This gas is formed whenever the combustion of organic substances takes place in the presence of a large excess of incandescent charcoal; the air first burns the carbon into carbonic anhydride, but this in penetrating through the red-hot charcoal is transformed into carbonic oxide, CO2 + C = 2CO. By this reaction carbonic oxide is prepared by passing carbonic anhydride through charcoal at a red heat. It may be separated from the excess of carbonic anhydride by passing it through a solution of alkali, which does not absorb carbonic oxide. This reduction of carbonic anhydride explains why carbonic oxide is formed in ordinary clear fires, where the incoming air passes over a large surface of heated coal. A blue flame is then observed burning above the coal; this is the burning carbonic oxide. When charcoal is burnt in stacks, or when a thick layer of coal is burning in a brazier, and under many similar circumstances, carbonic oxide is also formed. In metallurgical processes, for instance when iron is smelted from the ore, very often the same process of conversion of carbonic anhydride into carbonic oxide occurs, especially if the combustion of the coal be effected in high, so-called blast, furnaces and ovens, where the air enters at the lower part and is compelled to pass through a thick layer of incandescent coal. In this way, also, combustion with flame may be obtained from those kinds of fuel which under ordinary conditions burn without flame: for instance, anthracite, coke, charcoal. Heating by means of a gas-producer—that is, an apparatus producing combustible carbonic oxide from fuel—is carried on in the same manner.24 In transforming one part of charcoal into carbonic oxide 2,420 heat units are given out, and on burning to carbonic anhydride 8,080 heat units. It is evident that on transforming the charcoal first into carbonic oxide we obtain