388 PBINCIPLES OF CHEMISTRY 



acid carbonate, M'HCO 3 ; or if M" -be a bivalent metal (replacing H 2 ) its 

 normal carbonate will be M''CO 3 ; these metals do not usually form acid 

 salts, as we shall see further on. The bibasic character of carbonic acid 

 is akin to that of sulphuric acid, H 2 SO 4 , 13 but the latter, in distinction 

 from the former, is an example of the energetic or strong acids (such as 

 nitric or hydrochloric), whilst in carbonic acid we observe but feeble 

 development of the acid properties ; hence carbonic acid must be con- 

 sidered a weak acid. This conception must, however, be taken as only 

 comparative, as up to this time there is no definitely established rule for 

 measuring the energy 14 of acids. The feeble acid properties of carbonic 



13 Up to the year 1840, or thereabout, acids were not distinguished by their basicity*. 

 Graham, while studying phosphoric acid, H 3 P0 4 , and Liebig, while studying many organic 

 acids, distinguished mono-, bi-, and tri-basic acids. Gerhardt and Laurent generalised 

 these relations, showing that this distinction extends over many reactions (for instance, 

 to the faculty of bibasic acids of forming acid salts with alkalis, KHO or NaHO, or with 

 alcohols, RHO, &c.) ; but now, since a definite conception as to atoms and molecules 

 has been arrived at, the basicity of an acid is determined by the number of 

 hydrogen atoms, contained in a molecule of the acid, which can be exchanged for metals. 

 If carbonic acid forms acid salts, NaHCO 3 , and normal salts, Na 2 CO 3 , it is evident that 

 the hydrate is H 2 CO 3 , a bibasic acid. Otherwise it is at present impossible to account 

 for the composition of these salts. But when C = 6 and O = 8 were taken, then the formula 

 CO 2 expressed the composition, but not the molecular weight, of carbonic anhydride ; and 

 the composition of the normal salt would be Na^C-jOg or NaCO^, therefore carbonic acid 

 might have been considered as a monobasic acid. Then the acid salt would have been 

 represented by NaCO 3 ,HCO 3 . Such questions were the cause of much argument and dif- 

 ference of opinion among chemists about forty years ago. At present there cannot be two 

 opinions on the subject if the law of Avogadro-Gerhardt and its consequences be strictly 

 adhered to. It may, however, be observed here that the monobasic acids R(OH) were 

 for a long time considered to be incapable of being decomposed into water and anhydride, 

 and this property was ascribed to the bibasic acids R(OH) 2 as containing the elements 

 necessary for the separation of the molecule of water, H 2 0. Thus H 2 SO 4 or S0 2 (OH)2, 

 H 2 CO 3 , or CO(OH) 2 , and other bibasic acids decompose into an anhydride, RO, and water, 

 H 2 O. But as nitrous. HNOo, iodic, HIO 3 , hypochlorous, HC1O, and other monobasic 

 acids easily give their anhydrides N 2 O 3 , 1 2 O 5 , C1 2 O, &c., that method of distinguishing 

 the basicity of acids, although it fairly well satisfies the requirements of organic chemistry, 

 cannot be considered correct. It may also be remarked that up to the present time 

 not one of the bibasic acids has been found to have the faculty of being distilled without 

 being decomposed into anhydride and water (even H 2 SO 4 , on being evaporated and dis- 

 tilled, gives SO 3 + H 2 O), and the decomposition of acids into water and anhydride pro 

 ceeds particularly easily in dealing with feebly energetic acids, such as carbonic, nitrous, 

 boric, and hypochlorous. Let us add that carbonic acid, as a hydrate corresponding to 

 marsh gas, C(HO) 4 = CO 2 + 2H 2 O, ought to be tetrabasic. But in general it does not form 

 such salts. Basic salts, however, such as CuCOjCuO, may be regarded in this sense, 

 for CCu 2 O 4 corresponds with CH 4 O 4 , as Cu corresponds with H 2 . Amongst the 

 ethereal salts (alcoholic derivatives) of carbonic acid corresponding cases are, however, 

 observed; for instance, ethylic orthocarbonate, C(C 2 H 5 O) 4 (obtained by the action of 

 chloropicrin, C(NO.>)C1 3 , on sodium ethoxide, C 2 H 5 ONa; boiling point 158; specific 

 gravity, Q'92). The name orthocarbonic acid tor CH 4 O 4 is taken from orttiophosphorie 

 acid, PH 3 O 4 , which corresponds with PH 3 (see Chapter on Phosphorus). 



14 Long ago endeavours were made to find a measure of affinity of acids and bases, 

 because some of the acids, such as sulphuric or nitric, form comparatively stable salts, de- 



