January 24, 1895] 



NATURE 



305 



relatively so few, these differences so slight, so mixed up with 

 observational errors, and so irregular in their geographical and 

 bathymetrical distribution, that they cannot be said to indicate 

 any general law other than a greater quantity of lime in deep 

 water. 



But there is abundant evidence that great changes in chemical 

 composition take place in the substances deposited on the floor 

 of the ocean, and, with the view of throwing some light on the 

 manner in which these changes are brought about, it occurred 

 to us to examine the compo-i'ion of the sea-water associated or 

 mixed up with marine deposits on the floor of the ocean, and 

 especially with that variety of marine deposits known as Blue 

 Mud. 



The depth at which a fine blue mud may form in the sea, 

 depends entirely on the dejjlh of water and the extent of the 

 basin ; or, in ether words, on the height and length of the 

 waves.' In harbours it may be deposited not deeper than i 

 or 2 fathoms, while along the western coasts of Scotland and 

 Ireland, which are exposed to the waves of the wide and deep 

 Atlantic, the true mud-line may be situated at a depth of about 

 150 or 200 fathoms. 



In thi-i paper we state the results of our investigation into the 

 composition of the sea salts in samples of water enclosed in the 

 blue muds -from Granton Harbour and Quarry, near Edinburgh, 

 at Queensferry, N.B., and other places. The water was 

 obtained by filling a canvas bag with the mud and collecting the 

 water which filt»?red through, the first portions being rejected. 



The Spscific Gravity of the filtrate was about normal for in- 

 shore waer. ranging from 1024 to 1026. 



Chemical C.'mposilion of the Sea-water Salts in Mud-water. — 

 On comparison with normil sea-water salts, the sulphates were 

 greatly reduced, while the alkalinity (combined carbonic acid) 

 was correspondingly increased, sometimes rising to ten times 

 above the normal. 



When a portion of the clear water filtered from the harbour 

 muds was boiled, a precipitate was thrown down in a crystal- 

 line form, consisting of carbonates of lime and magnesia in 

 the following proportions : — 



CaCOj 

 MgCO, 



73 '3° 

 26-70 



Before boiling, the water had an alkalinity of o-776ogrms. 

 per litre, while after boiling it showed an alkalinity of only 

 0-2200 grms., thus proving that the alkalinity was mainly due 

 to the formation and presence of these carbonates rendered 

 soluble by free carbonic acid. 



Saline and alliumenoid ammonia ranged from 4 to &o parts, 

 and I to 5 parts per million, respectively. Lime was much 

 less, and magiresia slightly less than the normal. The chlorine 

 and the total baes were higher than normal water of equal 

 density. Bicarbonate of manganese was present in the water 

 up to I part in 16,000. Normal sea-water contains no 

 manganese. 



The following table give^ a comparison of the composition 

 of normal and mud-waters: — 



Sodium chloride, XaCI 

 Magnesium ,, MgCU .. 

 M.ignesium bromide, MgBr;, 

 Magne-ium sulphate, MiSO) 

 Potassium ,, K.^S04 



.Vmmonium ,, (NHj)„S04 

 Magnesium carbonate, MjCO, .. 

 Calcium ,, CaCO, .. 



Calcium sulphate, CaSOj 

 Manganous carbonate, iMnCO, ., 



' See Miirrav and Renard, " Challtngcr Report on Deep-Sea Deposits,' 

 p. 1S5. (t. .nion, 1891.) 

 - Sec Dittmar, op. cit. pp. 137-138 and 203. 



NO. 131 7, VOL. 51] 



It will be seen that the total salts of the mudwater are low 

 in proportion to the chlorine, consequently the D value — that 

 is, th?.;density minus looo divided by the chlorine — will be lower 

 than normal water. In normal water the D value is 1-457, in 

 mud water i -430. 



The reactions that take flacc in Blue Muds seem to be the 

 following, and may be distinctly proved from the analyses of 

 mud-water, as well as from a consideration of the whole sub- 

 jec During the process of decomposition 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 oxi- 

 dised back again into sulphuric acid, which in turn, decom- 

 posing the carbonate of lime always present in the water (or in 

 the deposit), would re-form sulphates. 



This oxidation is effected but slowly, as the following labora- 

 tory experiments show : — 



Exp. I. — .\ solution of hydrosulphuric acid'(H„S) in pure 

 water, which at first gave no precipitate with barium chloride, 

 alter standing a month, gave a distinct precipitate of barium 

 sulphate, showing that the hydrosulphuric acid had been oxidised 

 into sulphuric acid (SO3). 



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. III. — Hydrosulphuric acid was passed into water hold- 

 ing carbonates of calcium and magnesium in suspension, and 

 resulted in a yel'owish solution of the sulphides of calcium and 

 magnesium, carbonic acid being expelled. The sulphides so 

 formed were in turn decomposed 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.-' 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 continu- 

 ally being abstracted from sea-water and locked up in marine 

 deposits, which may finally be converted into blue-coloured 

 shales, schists, and marls.' In these rocks the crystalline 

 pyrites (FeS.-) has evidently its origin in the processes of death 

 and decay going on at the time of their deposition at the sea- 

 bottom, the sulphur of the sulphide of iron being derived from 

 the sulphates of the sea-water, and not from the sulphur of the 

 organisms, as generally supposed. This decomposition seems 

 to be due to the action of bacteria in causing putrefactive 

 changes in dead organic mailer. We have found that if sea- 

 water containing putrescitile organic matter be sterilised by 

 boiling, and thereafter care be taken to prevent the ingress of 

 bacteiia to this cooled liquid, the changes above indicated do 

 not take place. .Vpparently the organic matter must be broken 

 down by bacteria into its component elements, which in the 

 nascent condition are capable of reducing the sulphates to a 

 lower form of combination. The bisulphide of iron in the coal 

 measures has without doubt a similar origin. 



I See also C,'w//« rrndus, torn. Ix.v.xiii. pp. 58 and 345 (1S76). Note by 

 Nau.iin and IMoniholon ; also .Sainic Cl.->ire Deville, Lefons snr la Disao- 

 ciatioUt i86.|. 



■-' lr\ine and Gibson, rroc. Roy. Soc. F.din. p. 37, 1S91. 



=* Ttie black or dark blue colour of many shales and schists is due princi- 

 pally to ihe presence of iron, eiltier combuied with silica as silicate, 01 more 

 rarely in the condition of carbonate or oxide. 'I tic^e shales, schists. &c.. 

 contain org.imc matter in a state of decomposition. In llie older rocks its 

 conditioii nearly approaches that of graphitic carbon ; in a d.ark schist from 

 Argyllshire only o'ot jwr cent, of i;raplntic organic m.-ittcr was found. In 

 the more recent foruiations the organic matter, "if in siiflficicnt quantity*, may 

 Rive rise to the formation of petroleum. See paper by Ur. J.J. lahn, /d/o-- 

 tiuchdcr K.K. Geotog. Keichsanstatl, iSjj, lid. 42, Heft j. 



