Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/236

Rh 222 Percentage. Chloride of sodium (eonnnon salt) ............. .. 75786 Chloride of magnesimn ......................... .. 9‘159 Chloride of iotassium ............................. .. 3'65? Sulphate of ime (g_vpsum) ....................... .. 4'61? Sulphate of magnesia (Epsom salts) ........... .. 5‘597 Bromide of sodium ................................ .. 1 '184 100 ‘O00 Total percentage of salts in sea-water.. 3'52? Besides these chief ingredients, sea-water has yielded minute traces of iodine, ﬂuorine, silica, phosphoric acid, carbonate of lime and magnesia, silver, lead, copper, arsenic. Doubtless more perfect analysis will greatly increase this list. In addition to its salts sea-vater always contains dissolved atmospheric gases. From the researches conducted during the voyage of the “Bonité” in the Atlantic and Indian Oceans it was estimated that the gases in 100 volumes of sea-water ranged from 1'85 to 3'04, or from two to three per cent. From observations made during the “Porcu- pine” cruise of 1868 it was inferred that the proportion of oxygen was greatest (251 per cent.) in the surface water, and least (19'5) in the bottom water, while that of carbonic acid was least at the top (207) and greatest (279) at the bottom, and that the action of the waves was partially to eliminate the latter gas and to increase the amount of oxygen. More recently, however, during the voyage of the “Challenger,” Mr J. Y. Buchanan ascertained that the proportion of carbonic acid was always nearly the same for similar temperatures, the amount in the Atlantic surface water, between 20° and 25° C., being 00466 gramme per litre, and in the surface Paciﬁc Water O'0'3G8. He points out the curious fact that, according to his analyses, sea-water contains sometimes at least thirty times as much carbonic acid as an equal bulk of fresh water would do, and he traces the greater power of absorption to the presence of the sulphates. II. THE SOLID GLOBE. 1. General Considerations.—Vithin the atmospheric and oceanic envelopes lies the inner solid globe. Reference has already been made to the comparative density of the planet among the other members of the solar system. In all speculation about the history of the earth, the density of the whole mass of the planet as compared with water- the standard to which the speciﬁc gravities of terrestrial bodies are referrerl—is a question of prime importance. Various methods have been employed for determining the earth’s density. The deﬂexion of the plumb-line on either side of a mountain of known structure and density, the time of oscillation of the pendulum at great heights, at the sea-level, and in deep mines, the comparative force of gravitation as measured by the torsion balance—each of these processes has been tried with the following various results :— Plumb-line experiments on Schichallicn (Maskelyne and Playfair) gave as the mean density of the earth..... 4'713 Do. on Arthurs Seat, Edinburgh, (James) ........... 5'316 Pendulum experiments on Mont Cenis (Carlini and Giulio).. 4'95O Do. in Harton coal-pit, N eweastle (Airy) ................. .. 6'565 Torsion balance experiments (Cavendish) ....................... .. 5'480 Do. do. (Baily) ............................. .. 5'66O Though these observations are somewhat discrepant, ve may feel satisﬁed that the globe has a mean density neither much more 11or much less than 5'5 ; that is to say, it is ﬁve and a half times heavier than one of the same dimen- sions formed of pure water. N ow the average density of the materials which compose the accessible portions of the’ earth is between 2'5 and 3; so that the mean density of the whole globe is about twice as much as that of its outer part. We might therefore infer that the inside consists of GEOLOGY [IL G1-10G.'OSY. much heavier materials than the outside, and consequently that the mass of the planet must contain at least two dis- similar portions—an exterior lighter crust or rind, and an interior heavier nucleus. But the effect of pressure 1nust necessarily increase the speciﬁc gravity of the interior as will be alluded to further on. 2. The C2-ast.—It was formerly a prevalent belief that th_e exterior and interior of the globe difl'e1'ed from each other to such an extent that, while the outer parts were cool and solid, the vastly more enormous inner part being intensely hot was more or less completely fluid. Hence the term “ crust” was applied to the external rind in the usual sense of that word. This crust was variously computed to be 10, 15, 20 or more miles in thickness. For 1‘I;asn11s 'llll‘ll will be afterwards given, the idea of internal liquidity has been opposed by eminent physicists and is now abandoned by most geologists. The term “ crust, ” however, Colltllllles to be used as a convenient word to denote the cool, upper, or outer layer of the earth’s mass, accessible to human observation. It is in the structure and history of this crust that the main subjects of geological investigation are co11tained. It will therefore be fully treated of in the following parts of this article. There are, however, some ge11eral views as to its composi- tion and the arrangement of its materials, which may appropriately ﬁnd a place in this preliminary section. Evidently our direct acquaintance with the chemical con- stitution of the globe must be limited to that of the crust, though by inference we may eventually reach highly pro- bable conclusions regarding the constitution of the interior. Chemical research has discovered that sixty-four simple or as yet indecomposable bodies, called elements, in various proportions and compounds, constitute the accessible part of the crust. O.f these, however, the great majority are comparatively of rare occurrence. The crust, so far as we can examine it, is mainly built up of about sixteen elements, which may be arranged in the two following groups, the most abundant bodies being placed first in each list :— Mctalloicls. Jldals. Atomic .tomie Weight. 'eI:.'|If. Oxygen ....................... .. ]5'96 Aluminium ................. .. ‘_’7‘3H Silicon ....................... .. 28‘00 Calcium ....................... . 3!|'$'0 Carbon ....................... .. 11'97 Magnesium. . . . . . . . . . . . . . . . .. ‘_’J3't'»t Sulphur ...................... .. 31 '98 Potassium .................... . 391'!-1 Hydrogen (really a metal) 1 '00 Sodium ...................... .. ‘3‘_"99 Chlorine ..................... .. 3537 Iron ........................... . :'»5'£I0 I’hosphorus ................. .. 30'96 Manganese .................. .. 54'-‘s‘H Fluorine ..................... .. 19'10 ll-ariuni ...................... ..13t3'tH By far the most abundant and important of these elements is oxygen. It forms about 23 per cent. by weight of air, 8888 per cent. of water, and about a half of all the rocks which compose the visible portion or “crust” of the globe. Another metalloid, silicon, comes next in abundance. It is always united with oxygen, forming the mineral silica which, either alone or in combination with various metallic bases as silicates, constitutes a half of all the known mass of the globe. Of the remaining metalloids carbon and sulphur sometimes occur in the free state, but usually in combination with oxygen or some base or metal. Chlorine and ﬂuorine are found associated with metallic bases. Hydrogen is properly a metal, and occurs ehiell y in combination with oxygen as the oxide, water. Phosphorus occurs with oxygen principally in phosphate of lime. Of the metals by far the most important in the architec- ture of the exterior of the earth is aluminium. In con- junction with oxygen and silicon it forms the basis of most crystalline rocks. Calcium, magnesium, potassium, and sodium, combined with oxygen, enter largely into the com- position of rocks. Iron is the great colouring material in nature, most of the yellow, brown, red, and green hues of