XX′Y; but diatomic residues, Y, sometimes capable of existing separately may combine together, forming YY′ and with X 2 or XX′, as we see from the transition of CO into CO 2 and COCl 2 . This combining power of carbonic oxide appears in many of its reactions. Thus it is very easily absorbed by cuprous chloride, CuCl, dissolved in fuming hydrochloric acid, forming a crystalline compound, COCu 2 Cl 2 ,2H 2 O, decomposable by water; it combines directly with potassium (at 90°), forming (KCO) n with platinum dichloride, PtCl 2 , with chlorine, Cl 2 , &c.
But the most remarkable compounds are (1) the compound of CO with metallic nickel, a colourless volatile liquid, Ni(CO)4, obtained by L. Mond (described in Chapter XXII.) and (2) the compounds of carbonic oxide with the alkalis, for instance with potassium or barium hydroxide, &c.—although it is not directly absorbed by them, as it has no acid properties. Berthelot (1861) showed that potash in the presence of water is capable of absorbing carbonic oxide, but the absorption takes place slowly, little by little, and it is only after being heated for many hours that the whole of the carbonic oxide is absorbed by the potash. The salt CHKO2 is obtained by this absorption; it corresponds with an acid found in nature—namely, the simplest organic (carboxylic) acid, formic acid, CH2O2. It can be extracted from the potassium salt by means of distillation with dilute sulphuric acid, just as nitric acid is prepared from sodium nitrate. The same acid is found in ants and in nettles (when the stings of the nettles puncture the skin they break, and the corrosive formic acid enters into the body); it is also obtained during the action of oxidising agents on many organic substances; it is formed from oxalic acid, and under many conditions splits up into carbonic oxide and water. In the formation of formic acid from carbonic oxide we observe an example of the synthesis of organic compounds, such as are now very numerous, and are treated of in detail in works on organic chemistry.