CHEMICAL CHANGES IN THE COMPOSITION OF SEA-WATER. 497 



Part of the sulphur is thus fixed in the mud, and part, if there be not sufficient iron in 

 the mud, may escape into the water above, where, meeting oxygen, it will be converted 

 into sulphuric acid (H 2 S0 4 ), and return into KS0 4 . The products RC0 3 C0 2 in (2), and 

 EC0 3 in (3), or the bicarbonate and carbonate of the metal are found in the water 

 strained from the mud as above described. 



From these considerations it appears that the greater part of the oxygen for the 

 oxidation of the carbon and hydrogen of the organic substances in the Blue Muds is 

 derived from the sulphur salts of the alkaline and earthy alkaline metals in sea- water, 

 which, in the first instance, are reduced to the form of sulphides. These sulphides, owing 

 to their instability, especially in the presence of free or loosely-combined carbonic acid, 

 are decomposed as formed. The sulphur thus reduced from the sulphates may in part, 

 on passing as hydrosulphuric acid into the water immediately above the mud, become 

 oxidised back again into sulphuric acid, which in turn, decomposing the carbonate of lime 

 always present in the water (or in the deposit), would reform sulphates. 



This oxidation is effected but slowly, as the following laboratory experiments show : — 



Exp. I. A solution of hydrosulphuric acid (H 2 S) in pure water, which at first gave no 

 precipitate with barium chloride, after standing a month gave a distinct precipitate of 

 barium sulphate, showing that the hydrosulphuric acid had been oxidised into sulphuric 

 acid (S0 3 ). 



Exp. II. A solution of hydrosulphuric acid in sea-water was exposed to the air till 

 complete oxidation had taken place. On titration the sea-water had lost its alkalinity, 

 the sulphuric acid being proportionately increased. 



Exp. Ill Hydrosulphuric acid was passed into water holding carbonates of calcium 

 and magnesium in suspension, and resulted in a yellowish solution of the sulphides of cal- 

 cium and magnesium, carbonic acid being expelled. The sulphides in turn were decom- 

 posed by excess of carbonic acid, with evolution of hydrosulphuric acid, bicarbonates 

 being formed, the reaction apparently depending on which acid is in excess.* 



A certain part of the sulphides, or it may be of hydrosulphuric acid, derived from the 

 soluble sulphides by the action of the free carbonic acid present in the mud, reduces the 

 ferric oxides of the deposit, forming sulphide of iron, which so long as it is not exposed 

 to the action of oxygen, remains stable, being in this respect unlike sulphide of manganese, t 

 The sulphide of iron gives the characteristic blue-black colour to the great majority of the 

 Blue Muds, especially where there is abundance of organic matter. It is by this process 

 that sulphur is continually being abstracted from sea-water and locked up in marine 

 deposits, which may finally be converted into blue-coloured shales, schists, and marls.J In 



* See also Comptes Eendus, torn, lxxxiii. pp. 58 and 345 (1876). Note by Naudin and Montholon; also Sainte 

 Claire Deville, Lecons sur la Dissociation, 1864. t Irvine and Gibson, Proc. Roy. Soc. Edin., p. 37, 1891. 



X The black or dark blue colour of many stales and schists is due principally to the presence of iron, either 

 combined with silica as silicate, or more rarely in the condition of carbonate or oxide. These shales, schists, &c, contain 

 organic matter in a state of decomposition. In the older rocks its condition nearly approaches that of graphitic carbon ; 

 m a dark schist from Argyllshire only - 91 per cent, of graphitic organic matter was found. In the more recent forma- 

 tions the organic matter, if in sufficient quantity, may give rise to the formation of petroleum. See paper by Dr J. J. 

 Jahn, Jahrbuch der K. K. Geolog. Eeichsanstalt, 1892, Bd. 42, Heft 2. 



VOL. XXXVII. PART II. (NO. 23). 4 F 



