396 PRINCIPLES OF CHEMISTKY 



is a uuivalent 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 con- 

 tains one of the hydrogen atoms of carbonic acid, represents a group in 

 which the hydrogen is, exchangeable fo"f 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 ... w-basic acids. 

 Organic acids are the products of the carboxyl substitution in 

 hydrocarbons.^ bi3 If in the saturated hydrocarbons, C n H 2n+ 2 , ne part 

 of hydrogen is replaced by carboxyl, the monobasic saturated (or fatty) 

 acids, C n H 2n+l (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 C0 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, 

 <kc. To benzene, C 6 H b , correspond beuzoic acid, C 6 H. ) (CO2H), phthalic 

 acid (and its isomerides), C C H 4 (CO 2 H) 2 , up to inellitie 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 



is the anhydride of a bibasic acid, and carboxyl corresponds with it, re- 

 placing the hydrogen of hydrocarbons, and giving them the character of comparatively 

 feeble acids, then SO 5 is the anhydride of an energetic bibasic acid, and sulphoxyl, 

 SO 2 (OH), corresponds with it, being capable of replacing the hydrogen of hydrocarbons, 

 and forming comparatively energetic sulphur oxyacids (sulphonic acids) ; for instance, 

 CfiH (COOH), benzoic acid, and C 9 H i (SO i OH), benzenesulphonic acid, are derived from 

 C 6 H 6 . As the exchange of H for methyl, CH 5 , is equivalent to the addition of CH 2 , the 

 exchange of carboxyl, COOH, is equivalent to the addition of CO 2 ; so the exchange of H 

 'for sulphoxyl is equivalent to the addition of SOj. The latter proceeds directly, for 

 j instance : C 6 H 6 + SO 3 = C 6 H 6 (SO 2 OH). 



As, according to the determinations of Thomsen, the heat of combustion of the vapours 

 of acids RCOa is known where B is a hydrocarbon, and the heat of combustion of the 

 hydrocarbons R themselves, it may be soen that the formation of acids, BCO 2 , from 

 B + CO 2 is always accompanied by a small absorption or development of heat. We give 

 the heats of combustion in thousands of calories, referred to the molecular weights of 

 ,t\e substances: 



R = H.^ CH^ Coll Q CgHg 



68-4 212 870 777 



. RCO 2 = 69'4 225 387 766 



iThus HZ corresponds with formic acid, CH 2 0.j ; benzene, C 6 H , with benzoic acid, C 7 H 6 O 8 . 

 The data for the latter are taken from Stohmann, and refer to the. solid condition. For 

 formic acid Stohmann gives the heat of combustion as 69,000 calories in a liquid state, 

 but in a state of vapour, 64*6 thousand units, which is much less than according to 

 Thomsen. 



