246 PROGRESS IN CHEMISTRY. 



strongest, was thus seen to contain no oxygen, and Davy expressed 

 the view, founded on his observation, that iodic "'acid,'' I2O5, was 

 devoid of acid properties until dissolved in water, and that the essen- 

 tial constituent of all acids was hydrogen, not oxygen. The bearing 

 of this theory on the dualistic theory is, that while, e. g., sulphuric acid 

 was regarded by Berzelius as SO3, containing no hydrogen, and was sup- 

 posed to be separated as such at the positive pole of a batter}^ Davy's 

 suggestion led to the opposite conclusion, that the formula of sulphuric 

 acid is HgSO^, and that by the current it is resolved into H^ and SO^. 

 Faraday's electrolytic law, that when a current is passed through elec- 

 trolytes in solution the elements are liberated in quantities propor- 

 tional to their equivalents, led to the abandonment of the dualistic 

 theory. For when a current is passed in succession through acidified 

 water, fused lead chloride, and a solution of potassium sulphate, the 

 quantities of h^^drogen and oxygen from the water, of lead and chlorin 

 from the lead chloride, and the potassium of the sulphate are in accord- 

 ance with Faraday's law. But in addition to the potassium there is 

 liberated at the same pole an equivalent of hydrogen. Now, if Eer- 

 zelius's theory be true, the products should be SO3 and K^O, but if the 

 opposite view be correct, then Kg is liberated first and by its subse- 

 quent action on water it yields potash, and its equivalent of hydrogen. 

 This was pointed out first by Daniell, professor at King's College, 

 London, and it was regarded as a powerful argument against Berze- 

 lius's system. In 1833, too, Graham investigated the phosphoric acids, 

 and prepared the salts of three, to which he gave the names ortho-, 

 pyro-, and meta- phosphoric acids. To understand the bearing of this 

 on the doctrine of dualism it must be remembered that P2O5. pent- 

 oxide of phosphorus, was at that date named phosphoric acid. When 

 dissolved in water, it reacts with bases, forming salts — the phosphates. 

 But the quantity of water necessary was not then considered essential; 

 Graham, however, showed that there exist three series of salts; one 

 set derived from PgOg, SH^O; one from PgOg, 2H2O, and a third from 

 P2O5, HoO. His way of stating the fact was that water could play the 

 part of a base; for example, the ordinary phosphate of commerce pos- 

 sessed, according to him, the formula P2O5, QNagO, HgO; two-thirds 

 of the "water of constitution" being replaced by oxide of sodium. 

 Liebig, then professor at Giessen (1803-1873), founded on these and on 

 similar observations of his own the doctrine of pol3^basic acids — acids 

 in which one, two, three, or more atoms of hydrogen were replaceable 

 by metals. Thus, instead of writing, as Graham did, P.O.r,, ^^NagO, HgO, 

 he wrote PO^NajH; and for orthophosphoric acid, PO^Hg. The group 

 of atoms (POJ therefore existed throughout the whole series of ortho- 

 phosphates, and could exist in combination with hydrogen, with hydro- 

 gen and metals, or with metals alone. Similarly the group (P^O^) was 

 characteristic of pvrophosphates and (PC,) of metaphosphatcs, for 

 P2O,, 2H2O (P2O,) H,; and F,0,, H„0=^^(P03)H. 



