^{page 481 note *} Dittmar, “Challenger Report on the Composition of Ocean Water,” Phys. Chem. Chall. Exp., pt. i. p. 43.

^{page 482 note *} See Murray, and Renard, , Challenger Report on Deep-Sea Deposits, London, 1891, p. 229Google Scholar.

^{page 482 note †} See Murray and Renard, op. cit., p. 185.

^{page 483 note *} During the winter of 1891–92, water was collected in a similar way from the deep basins in the Clyde Sea-Area, The great distance through which the mud had to be hauled to the surface (50 to 70 fathoms) rendered the operation much more difficult than when dealing with harbour mud lying in 1 or 2 fathoms, it being too much washed and mixed with the superincumbent water to give satisfactory results. In order to conduct this work successfully, it would be necessary to construct a special apparatus to bring up the soft mud with the water directly associated with it. The results so far agree with those obtained on the Forth.

^{page 484 note *} From a quantity of mud taken from the bed of the Clyde opposite the Gareloch, mud-waters were obtained having a mean density of 1035; these waters were not chemically changed, and should this observation be verified it may point to a process of concentration in sea-water muds, even when covered by water of normal density.

^{page 484 note †} The method adopted for the determination of the saline ammonia was as follows:—Pure potash solution was added to the sea-water, and the precipitate formed removed by filtration through filter-paper, from which any trace of ammonia (generally present in filter-paper) had been removed by washing with pure potash solution, the clear filtrate being nesslerised in the usual manner. The saline ammonia is more abundant in tropical oceanic waters than in water of temperate zones (see Murray and Irvine on Coral-Reefs, &c, Proc. Roy. Soc. Edin., 1889–90, p. 89).

^{page 485 note *} We have seen that the accidental mixture with the overlying water, and also the increased amount of sulphates produced by oxidation of sulphide of iron (FeS) in the mud, caused variations which tended to conceal the true nature of the mud-water, and to alter the density of the various portions. If the mud had been thoroughly mixed up and filtered in an atmosphere free from oxygen (e.g., nitrogen), the water in all the portions obtained by dripping would probably have been alike in composition.

^{page 485 note †} See Scottish Fishery Board Report, 1887.

^{page 485 note ‡} In the 1st, and perhaps also in the 2nd, portions of the filtrates, where, as we have seen, the oxidation of the sulphide of iron had increased both the sulphuric acid and the lime, the lime was evidently derived from the carbonate of lime shells, or precipitated carbonate of lime, present in the deposit.

^{page 486 note *} Reaction shown on page 496.

^{page 488 note *} See Dittmar, op. cit., part i. p. 56.

^{page 490 note *} This reaction may be stated thus:—The sulphide of iron in the mud is oxidised by the atmosphere into sulphate, which reacts on carbonate of lime in the mud or in solution, forming sulphate of lime and momentarily carbonate of iron, changing to oxide of iron, liberates carbonic acid, which, along with the sulphate of lime, remains in solution and increases the density of these 1st portions.

^{page 490 note *} See Dittmar, op. cit., part i. pp. 137–138 and 203.

^{page 491 note *} See top of page 489.

^{page 492 note *} We are especially indebted to Dr W. S. Bruce, surgeon of the Antarctic whaler “Balæna,” who brought us samples in the spring of 1893, and to Captains Thomas S. Knox and George Reid of the Anchor Line for samples from the Mediterranean, Red Sea, Indian Ocean, and Atlantic. Mr Murray had still in his possession several samples collected by the “Challenger.”

^{page 495 note *} See Gibson, Proc. Roy. Soc. Edin., 1893, and Dickson, Journal Scottish Geographical Magazine, Jan. 1893, p. 17.

^{page 495 note †} Chemische Untersuchungen im östlichen Mittelmeer, Wien, 1892Google Scholar.

^{page 495 note ‡} Various constants are used by authors in expressing the relations which occur in sea-water analyses.

1. The D value may be taken at any temperature, and may be expressed by D_t. The D_t (on the assumption that there is no appreciable difference in the chemical composition of sea-water in different regions) is a constant for any temperature at which the observations are made as well as at 0° C.; but when the temperature is not mentioned, it is understood to be D at 0° C. It may be represented by .

2. The D_A value, as stated in the text, is the density at 0° C. minus 1000, divided by the alkalinity (in milligrammes of carbonic acid per kilogramme of water), and expresses the relation of alkalinity to density, in the same way as D expresses the relation of chlorine to density, and, like D, is unchanged by dilution or concentration of the water.

3. The relation of chlorine per kilogramme to total salts per kilogramme is also expressed by a constant which Dittmar gives as 1·8058. The chlorine (χ), multiplied by this figure and divided by 10, gives the percentage salinity of the sea-water. As has been pointed out by various writers, such as Pettersson, Ekman, Krummel, and Gibson, and confirmed by ourselves, this constant may rise much higher in brackish water, such as the Baltic, varying from 1·801 to 2·15. (See Pettersson, Grunddragen af Skageracks och Kattegats Hydrografi, Stockholm, 1891, and Krümmel, , Geophysikalische Beobachtungen der Plankton Expedition, Keil, Leipzig, 1893.Google Scholar)

4. A constant used by Krümmel is that of the density to the total salts. Its value is given as 1312, and is found by dividing the total salts per kilogramme by the density at 17°·5 C. after deducting 1 (pure water at 17°·5 C. = l).

^{page 496 note *} Dittmar, op. cit., p. 136.

^{page 496 note †} Dittmar, op. cit., p. 43.

^{page 496 note ‡} Dittmar, op. cit., pp. 126 and 129.

^{page 496 note §} Of course, the same reaction happens when sulphates of alkalies are reduced.

^{page 497 note *} See also Comptes Rendus, tom, lxxxiii. pp. 58 and 345 (1876). Note by Naudin and Montholon; also Sainte Claire Deville, Leçons sur la Dissociation, 1864.

^{page 497 note †} Irvine and Gibson, Proc. Roy. Soc. Edin., p. 37, 1891.

^{page 497 note ‡} The black or dark blue colour of many shales 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; in a dark schist from Argyllshire only 0·91 per cent, of graphitic organic matter was found. In the more recent formations 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. Reichsanstalt, 1892, Bd. 42, Heft 42.

^{page 498 note *} Ferrous sulphide, to which Blue Muds mainly owe their deep black colour, gradually becomes converted into ferric sulphide as these muds harden into shales and schists. During this action or change of condition part of the iron of the ferrous sulphide probably is oxidised, the sulphur set free either combining with the ferrous sulphide to form ferric sulphide, or a portion of the ferrous sulphide in the Blue Muds may be changed by oxidation into ferric sulphate, which, in the presence of organic matter, may become reduced to ferrous sulphide and free sulphur, thus providing material for the formation of the iron pyrites (FeS₂) so commonly associated with shales, schists, slates, &c.

^{page 499 note *} “On Deep-Sea Research in the Black Sea,” giving the results of an expedition (under the superintendence of Colonel J. B. Spindler) sent out by the Russian Government in 1890 and 1891. These results have already been partly published in the preliminary transactions of the Russian Geographical Society (in Russian), the physical results in German by Prof. Woicjkoff in Petermann's Mitteilungen. An abstract of Andrussow's paper has been published in the Royal Geographical Society's Journal, January 1893, giving a very fair epitome of the various points dealt with.

^{page 500 note *} See Murray, , “The Maltese Islands with reference to their Geological Structure, Jour. Scott. Geog. Mag., vol. vi. p. 481Google Scholar.