390 PRINCIPLES OF CHEMISTRY 



many cases themselves acting as alkalis. " The acid salts of these alkalis, 

 NaHCO 3 and KHCO 3 , have a neutral reaction on litmus, although they, 

 like acids, contain hydrogen, which may be exchanged for metals, The 

 acid salts of such acids as, for instance, of sulphuric acid, NaHS0 4 

 have a c'early defined acid reaction, and therefore carbonic acid is un- 

 able to neutralise the powerful basic properties of suph alkalis as potash 

 or soda. Carbonic acid does not even combine at all with feeble bases, 

 such as alumina, A1 2 3 , and therefore if a strong solution of sodium 

 carbonate, Na 2 00 3 , be added to a strong solution of aluminium sulphate, 

 A1 2 (S0 4 ) 3 , although according to double saline decompositions alu- 

 minium carbonate, A1 2 (CO 3 ) 3 , ought to be formed, the carbonic acid 

 separates, for this salt splits up in the presence of water into aluminium 

 hydroxide and carbonic anhydride : A] 2 (C0 3 ) 3 + 3H 2 O = A1 2 (OH) 6 

 -f 3C0 2 . Thus feeble bases are unable to retain carbonic acid even at 

 ordinary temperatures. For^the same reason, in the case of bases of 

 medium energy, although they form carbonates, the latter are compara- 

 tively easily decomposed by heating, as is shown by the decomposition of 

 copper carbonate, CuC0 3 (see Introduction), and even of calcium carbo- 



the annexed figures, in which the energy of hydrochloric acid is taken as equal to 

 100: 



I II III 



Hydrochloric acid, HC1 100 100 100 



Hydrobromic acid, HBr 

 Nitric acid, HNO 3 

 Sulphuric acid, H 2 S0 4 

 Formic acid, CH 2 O 2 

 Acetic acid, CoH^O* 

 Oxalic acid, C 3 H 2 O 4 



101 98 105 



100 100 96 



65 49 74 



2 4 1 



121 



20 24 18 



Phosphoric acid, PH 3 O 4 7 6 



The coincidence of these figures, obtained by so many various methods, presents a 

 most**importaiit and instructive relation between phenomena of different kinds, but in my 

 opinion it does not permit us to assert that the degree of affinity existing between bases 

 and various acids is determined by all these various methods, because the influence of the 

 water must be taken iiito consideration. On this account, until the theory of solution 

 is mtore thoroughly worked out, this subject (which for the present ought to be treated 

 of in special treatises on chemical mechanics) must be treated with great caution. 

 But now we may hope to decide this question guided by a study of the rate of reaction, 

 the influence of acids and bases upon indicators, &c., all of which are treated fully in 

 works on physical and theoretical chemistry. 



15 Thus, for instance, in the washing of fabrics the caustic alkalis, such as sodium 

 hydroxide, in weak solutions, act in removing the fatty matter just in the same way as 

 carbonate solutions ; for instance, a solution of soda crystals, Na 2 CO 3 . Soap acts in the 

 same way, being composed of feeble acids, either fatty or resinous, combined with alkali. 

 On this account all such substances are applied in manufacturing processes, and answer 

 equally well in practice for bleaching and washing fabrics. Soda crystals or soap are 

 preferred to caustic alkali, because an excess of the latter may have a destructive effect 

 on the fabrics. It may be supposed that in aqueous solutions of soap or soda 

 crystals, part of the base will form caustic alkali ; that is to say, the water will compete 

 with the weak acids, and the alkali will be distributed between them and the water. 



