376 PRINCIPLES OF CHEMISTRY 



formed from the composition of the salts of carbonic acid, because a 

 hydrate is nothing but a salt in which the metal is replaced by 

 hydrogen. As carbonic anhydride forms salts of the composition 

 K 2 CO 3 , Na 2 CO 3 , HNaCO : j, &c., therefore carbonic acid ought to have 

 the composition H 2 CO 3 that is, it ought to be formed from CO 2 -f H.,0 

 Whenever this substance is formed, it decomposes into its component 

 parts that is, into water and carbonic anhydride. The acid properties 

 of carbonic anhydride are demonstrated by its being directly absorbed 

 by alkaline solutions and forming salts with them. In distinction from 

 nitric, HNO 3 , and similar monobasic acids which with univalent metals 

 (exchanging one atom for one atom of hydrogen) give salts such as 

 those of potassium, sodium, and silver containing only one atom of the 

 metal (NaNO 3 , AgNO 3 ), and with bivalent 12 metals (such as calcium, 

 barium, lead) salts containing two acid groups for example Ca(NO 3 ) 2 , 

 Pb(NO 3 ) 2 carbonic acid, H 2 C0 3 , is bibasic, that is, contains two atoms 

 of hydrogen in the hydrate or two atoms of univalent metals in their 

 salts : for example Na 2 C0 3 is washing soda, a normal salt ; XaHCO 3 is 

 the bicarbonate or an acid salt. Therefore, if M' be a univalent metal, 

 its carbonates in general are the normal carbonate M'.,C0 3 and the 

 acid carbonate M'HC0 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 



12 The reasons for distinguishing the uni-, bi-, tri-, and quudri-valent metals will bo 

 examined hereafter on passing from the univalent metals (Na, K, Li) to the divalent ( .M;_. 

 Ca, Ba). 



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

 Graham, while studying phosphoric acid, H^PO.^ 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 KHO, &c.); but now, from the determination of a hard-and-fast conception as 

 to atoms and molecules, the basicity of an acid is determined by the nmnhcr of 

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

 If carbonic acid forms acid salts, NaHCO 5 , and normal salts, Na,,CO 5 , it is evident that 

 the hydrate is H 2 CO 5 , 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 2 C 2 O 6 or NaCO 5 , therefore carbonic acid 

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

 represented by NaCO.^HCO.v Such questions were the cause of much argument and 

 difference 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 sequences be strictly 

 adhered to. Let us, however, remark 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 B(OH) 2 as containing the elements neces- 

 sary for the separation of the molecule of water H 2 O. This H 2 SO 4 or SO 2 (OH) 2 , H 2 CO 3 , 



