1-:.vvELori-:s or THE E.-r.1'n.] whole atmosphere probably exceeds what would be disen- gaged if all the vegetable and animal matter on the earth’s surface were burnt. The other substances present in nmch more minute quantities are gases, vapours, and solid particles. Of these by much the most important is the vapour of water, which is always present, but in very variable amount according to temperature, ranging from about 4 to a maximum of 10 grains in l00() grains of air.‘ It is this vapour which con- denses into dew, rain, hail, and snov. In assuming a visible form, and descending through the atmosphere, it takes up a minute quantity of air, and of the different sub- stances which the air n1ay contain. Being caught by the min, and held in solution or suspension, these substances em be best examined by analysing rain-water. I11 this way ammonia, nitric, sulphurous, and sulphuric acids, chlorides, various salts, solid carbon, inorganic dust, and organic matter have been detector]. M. J. J. Pierre found as the result of his analysis that in the neighbourhood of Caen, in France, a hectare of land receives annually from the atmosphere, by means of rain—— Chloride of sodium ................. .. 3'/"'5 kilogrammes. ,, potassium .............. .. 8'2 ,, , , niagnesinm . . . . . . . . . . . .. 2 '5 , , , , calcium ................. .. 1 ‘S , , Sul )ll:ll:‘. of soda ..................... .. 8'4 9) , , potash .................. . . S ‘O , , ,, l1me...._ ................ .. 6‘2 ,, , , magnesia .............. .. 5 '9 , , To these ingredients nmst be added traces of ammonia, various salts, and organic substances, besides others still undetermined."-’ The powerful oxidizing agent ozone is present in variable but always minute quantities in the air. The comparatively small but by no means unimportant proportions of these various components of_ the atmosphere are much more liable than the 1nore essential gases to great variations. Chloride of sodium, for instance, is, as might be expected, p Irticularly abundant in the air border- ing the sea. Nitric acid, ammonia, and sulphuric acid appear in the air of towns most conspicuously. The organic subst-1nces present in the air are sometimes living germs, such as probably often lead to the propagation of disease, and sometimes more fine particles of dust derived from the bodies of living or dead organisms.3 2. The 0 -mus.—About three-fourths of the surface of the globe (or about 144,712,000 square miles) is covered by the irregular sheet of water known as the sea. Within the last ten years much new light has been thrown upon the depths, temperatures, and biological conditions of the ocean-basins, more particularly by the “Lightning,” “Porcupine,” and “ Challenger” expeditions fitted out by the British Govern- ment. It has been ascertained that few parts of the Atlan- tic Ocean exceed 3000 fathoms, the deepest sounding obtained there being one taken about 100 miles north from the island of St Thomas, which gave 3875 fathoms, or rather less than miles. The Atlantic appears to have an average depth in its more open parts of from 2000 to 3000 fathoms or from about 2 to 3; miles. In the Pacific Ocean the “ Challenger” got soundings of 3950 and 4475 1 The quantity of aqueous vapour depends upon the temperature, wa1'm air being able to retain more than cold air. Air at a tempera- ture of 10° C. is saturated when it contains 9362 grammes of vapour in a cubic metre of air. 2 Cliiwnie Agrioole, quoted by Dr Angus Sinitl1,.-fir and Ila)’-:1, p. 232. 3_ The air of towns is peculiarly rich in impurities, especially in manufacturing districts, where much coal is used. These impurities, however, though of serious consequence to the towns in a sanitary point of view, do not sensibly affect the general atmosphere, seeing that they are probably in great measure taken out of the air by rain, even in the districts which produce. them. They possess, however, a special geological significance, and in this respect, too, have important economic bearings. See on this whole subject Dr Angus Smith’s work already cited. GEOLOGY 221 fathoms, or about and rather more than 5 miles. But these appear to mark exceptionally abyssal depressions, the average depth being, as in the Atlantic, between 2000 and 3000 fathoms. We may therefore assume, as probably not far from the truth, that the average depth of the ocean is about 2500 fathoms, or nearly 3 miles. The water of the oceans is distinguished from the ordinary terrestrial waters by a higher specific gravity, and the presence of so large a proportion of saline ingredients as to impart a strongly salt taste. The average density of sea- water is about l'026, but it varies slightly in difi'erent parts even of the same ocean. According to the recent observa- tions of .Ir J. Y. Buchanan during the “Challenger ” expedi- tion, some of the heaviest sea-water occurs in the pathway of the trade-winds of the North Atlantic, where evaporation must be comparatively rapid, a density of l'0278l being registered. Where, however, large rivers enter the sea, or where there is n1ucl1 melting ice. the density diminishes; Mr Buchanan found among the broken ice of the Antarctic Ocean that it had sunk to l'024l8.4 The greater density of sea-water depends of course upon the salts which it contains in solution. There seems no reason to doubt that these salts are, in the main, parts of the original constitution of the sea, and thus that the sea has always been salt. It is also probable that, as in the case of the atmosphere, the composition of the ocean water has in former geological periods been very different from what it is now, and that it has acquired its present character only after many ages of slow change, and the abstraction of much mineral matter originally contained in it. There is evidence indeed among the geological formations that large quantities of lime, silica, chlorides, and sulphates have in the course of time been removed from the sea.5 But it is evident also that, whatever may have been the original composition of the oceans, they l1ave for a vast section of geological time been constantly receiving mineral matter in solution from‘ the land. Every spring, brook, and river removes various salts from the rocks over which it moves, and these substances, thus dissolved, eventually find their way into the sea. Consequently sea—water ought to contain more or less traceable proportions of every substance which the terrestrial waters can remove from the land, in short, of probably every element present in the outer shell of the globe, for there seems to be no constituent of this earth which may not, under certain circumstances, be held in solution in water. Moreover, unless there be some counteracting process to remove these mineral ingredients, the ocean water ought to be growing, insensibly perhaps, but still assuredly, salter, for the supply of saline matter from the land is incessant. It has been ascertained indeed, with some approach to certainty, that the salinity of the Baltic and Mediterranean is gradually increasing!‘ The average proportion of saline constituents in the water of the great oceans far from land is about three and a half parts in every hundred of water. But in enclosed seas, receiving much fresh water, it is greatly reduced, while in those where evaporation predominates it is correspondingly augmented. Thus the Baltic water contains from one- seventh to nearly a half of the ordinary proportion in ocean water, while the Mediterranean contains sometimes one- sixth more than that proportion. The mineral constituents include the following average ratios of salts7:— '* Buchanan, Proc. Roy. Soc. (1876), vol. xxiv. 5 Dr Sterry Hunt even supposes that the saline waters of Canada and the northern States derive their mineral ingredients from the salts still retained among the sediments and precipitates of the ancient sea in which the earlier Palzcozoic rocks were deposited.——O'eolo_r]ical and ('l1mn1'crzl ]s'ssa_:/s, p. 104. " Paul, in Vatts’s l)z'rtz'onar3/ Qf ('lzemz'str_I/, v. 1020.
7 Bischof, Chemical Geology, i. 379.