COMPOUNDS OF CARBON WITH' OXYGEN AND NITEOGEN 405 



oxygen, or in general with diatomic (bivalent) residues, Y for instance, 

 O, CO, CH 2 , S, Ca, &c. forming compounds 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 transi- 

 tion of CO into CO 2 and COC1 2 . This combining power of carbonic 

 oxide appears in many of its reactions. Thus it is very easily ab- 

 gorbed 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) rt 31 with platinum dichloride, PtCl 2 , with chlorine, C1 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 car* 

 bonic 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 po'tash 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 CHK0 2 is obtained by this absorption ; it cor- 

 responds with an acid found in nature namely, the simplest organic 

 (carboxyl'.c) a.cid, formic acid, CH 2 O 2 . 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 

 synjthesis of organic compounds, such as are now very numerous, and 

 are treated of in detail in works on organic chemistry. 



Formic acid, H(CHO 2 ), carbonic acid, HO(CHO 2 ), and oxalic acid, 

 (CH0 2 ) 2 , are the simple organic or carboxylic acids, R(CHO a ) cor- 



'* The molecule of metallic potassium (Scott, 1887), like that of mercury, contains only 

 one atom, and it is probably in virtue of this that the molecules CO and K combine together. 

 But as in the majority of cases potassium acts as a univalent radicle, the polymeride 

 K 2 C 2 2 is formed, and probably K^C^O^, -because products containing C 10 are formed 

 by the action of hydrochloric acid. The black mass formed by the combination of 

 carbonic oxide with potassium explodes with great ease, and oxidises in the air. Although 

 Brodie, Lerch, and Joannis (who obtained it in 1878 in a colourless form by means of 

 NH 3 K, described in Chapter VI., Note 14) have greatly extended our knowledge of thi 

 compound, much still remains unexplained. It probably exists in various polymeric and 

 isomeric forms, having the composition (KCO) and (NaCO). 



