PRELIMINARY OUTLINE. 5 



consist almost wholly of metal, chiefly iron alloyed with a small per- 

 cent, of nickel (holosiclerites) ; some consist of metal and rock inti- 

 mately mixed (syssiderites and sporadosiderites) ; and some consist 

 wholly of rock (asiderites) . The rock is usually composed of the heavier 

 basic minerals; +hough some meteorites consist largely of carbonaceous 

 material. B^^' "^. meteorites, there is little doubt that wandering 

 gaseous partic. strike the earth, but this is beyond the reach of present 

 demonstration. The amount of substance added to the earth by these 

 meteorites and gases in recent times is relatively slight compared with 

 the whole body of the earth. What contribution may have come to 

 the earth in earher times from such sources is a matter of hypothesis 

 which will be discussed later. 



Geognosy. 



The constitution of the earth. — Turning from its external relations 

 to the earth itself, a natural threefold division is presented: (1) the 

 atmosphere, (2) the hydrosphere, and (3) the lithosphere. 



/. The Atmosphere. 



The atmosphere is an intimate mixture of (1) all those substances 

 that cannot take a liquid or solid state at the temperatures and pres- 

 sures which prevail at the earth's surface, together with (2) such tran- 

 sient vapors as the various substances of the earth throw off. The 

 first class form the permanent gases of the atmosphere, and consist 

 of nitrogen about 79 parts, oxygen about 21 parts, carbon dioxide 

 about .03 part, together with small quantities of argon, neon, xenon, 

 krypton, hehum, and other rare constituents. The second class are 

 the transient and fluctuating constituents of the atmosphere, chief 

 among which is aqueous vapor, which varies greatly in amount accord- 

 ing to temperature, pressure, and other conditions. To this are to be 

 added volcanic emanations and a great variety of volatile organic 

 substances. Theoretically, every substance, howcA^er solid, discharges 

 particles which may transiently become constituents of the atmosphere. 

 Practically, only a few of these exist in such quantity as to be ap- 

 preciable. Dust and other suspended matter are usually regarded as 

 impurities rather than constituents of the atmosphere, but they play 

 a not unimportant part by affecting its temperature and luminosity, 

 and by facilitating the condensation of moisture. 



