18 Firtp CotumBiAN MusrumM—GEo .ocy, VOL. 1. 
prevail upon the earth, in the absence of air or free oxygen and of 
water. The lack of the first is indicated by the phosphide of iron, 
schreibersite, which would, in the presence of oxygen, have been 
changed to a phosphate; also by the fact that the iron and nickel are 
in the elementary condition and not oxidized, as they are upon the 
earth’s surface. The absence of water is proved bythe fact that no 
hydrous minerals are present in meteorites. 
It is known that either the atmosphere in which the meteorites 
were formed or one through which they at some time passed, con- 
tained a large amount of hydrogen, from the fact that it can be ex- 
tracted in large quantities from some of the metallic meteorites. It 
was, too, under a much higher pressure than is that of the earth’s 
atmosphere, since Graham obtained from the Lenarto meteorite 2.85 
times its volume of mixed gases, of which hydrogen formed 85%. 
Under the pressure of the earth’s atmosphere it is difficult to make iron 
absorb more than its own volume of this gas. The reducing action 
of this hydrogen-laden atmosphere must be very great and in gen- 
eral it may be said that meteorites differ from analogous terrestrial 
rocks in containing in a reduced state substances which occur as 
oxides upon the earth. 
Considered as mineral aggregates, meteorites may be conveniently 
divided into three classes according as they are made up chiefly of 
iron, partly of iron and partly of stone or chiefly of stone. 
The meteorites of the first class have been called by Maskelyne 
aerosiderites (from ayp, air, and ofdypes, iron) or by Daubrée holosider- 
ztes, (odog, Whole, ofdypos, iron). The term is frequently shortened to 
stderite but the abbreviation is objectionable on account of the liabili- 
ty of its confusion with the mineral of the same name. The meteorites 
of the second class are called by the same authors aerosiderolites (ayp, 
air, otdnpos, iron, and A¢os, stone) or sysszderites (ob», with, ofdypos, iron). 
Those of the third classare known as aerolites (ajp, air and Aros, stone, ) 
or by Daubrée are divided into the two groups of sporado-siderites 
(ozopds, scattered, ofdnpos, iron,) and aszderites (4 without, ofdjos, iron.) 
The aerosiderites, as is indicated by their name, are made up 
chiefly of iron. ThiS is, however, always alloyed with nickel. The 
percentage of iron in the mass varies between 87% and 97% and that 
of nickel from a fraction of one per cent to 15%. 
Two exceptions to this are known among irons supposed to be 
meteoric. One is that of Octibbeha Co., Miss., which contains 38% 
of iron to 60% of nickel and the other that of Santa Catarina, Brazil 
(97-108) which bears 64% of iron to 34%of nickel. It is possible, how- 
ever, that the latter is of terrestrial origin. 
