402 PRINCIPLES OF CHEMISTEY 



whilst the amount of carbonic anhydride increases as the temperature 

 of the reaction decreases (generally it is more than 3 per cent.) 



Metals like iron and zinc which at a red heat are capable of 

 decomposing water with the formation of hydrogen, also decompose 

 carbonic anhydride with the formation of carbonic oxide ; so both 

 the ordinary products of complete combustion, water and carbonic 

 anhydride, are very similar in their reactions, and we shall therefore 

 presently compare hydrogen and carbonic oxide. The metallic oxides 

 of the above-mentioned metals, when reduced by charcoal, also give 

 carbonic oxide. Priestley obtained it by heating charcoal with zinc 

 oxide. As free carbonic anhydride may be transformed into carbonic 

 oxide, so, in precisely the same way, may that carbonic acid which is 

 in a state of combination ; hence, if magnesium or barium carbo- 

 nates (MgCO 3 or BaC0 3 ) be heated to redness with charcoal, or iron 

 or zinc, carbonic oxide will be produced for instance, it is obtained by 

 heating an intimate mixture of 9 parts of chalk and 1 part of charcoal 

 in a clay retort. 



Many organic substances 27 on being heated, or under the action of 

 various agents, yield carbonic oxide ; amongst these are many organic 

 or carboxylic acids. The simplest are formic and oxalic acids. Formic 

 acid, CH 2 O 2 , on being heated to 200, easily decomposes into carbonic 

 oxide and water, CH 2 O 2 = CO + H 2 O. 27 bis Usually, however, car- 

 bonic oxide is prepared in laboratories, not from formic but from oxalic 

 acid, C 2 H 2 O 4 , the more so as formic acid is itself prepared from oxalic 

 acid. The latter acid is easily obtained by the action of nitric acid on 

 starch, sugar, &c. ; it is also found in nature. Oxalic acid is easily 

 decomposed by heat , its crystals first lose water, then partly volatilise, 

 but the greater part is decomposed. The decomposition is of the 

 following nature it splits up into water, carbonic oxide, and carbonic 

 anhydride, 28 C 2 H 2 O 4 = H 2 O + C0 2 + CO. This decomposition is 

 generally practically effected by mixing oxalic acid with strong sul- 



87 The so-called yellow pruesiate, K 4 FeC 6 N 6 , on being heated with ten parts of strong 

 sulphuric acid forms a considerable quantity of very pure carbonic oxide quite free from 

 carbonic anhydride.. 



27 bls To perform this reaction, the formic acid is mixed with glycerine, because when 

 heated alone it volatilises much below its temperature of decomposition. When heated 

 with sulphuric acid the salts of formic acid yield carbonic oxide. 



29 The decomposition of formic and oxalic^acids, with the formation of carbonic oxide, 

 considering these acids as carboxyl derivatives, may be explained as follows : The firsfe 

 is H(COOH) and the second (COOH) 2 , or H 3 in which one or both halves of the hydrogen 

 are exchanged for carboxyl ; therefore they are equal to H 2 + CO 2 and H 2 + 2CO 2 ; but 

 H 2 reacts with COa, as has been stated above, forming CO and H 2 O. From this it is also 

 evident that oxalic acid on losing CO 2 forms formic acid, and also that the latter may 

 proceed from C0 + H 2 0, as we shall see further on. 



