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“Sxor. L] THE EARTH IN ITS PLANETARY RELATIONS. 9 
For many years the only evidence available as to the actual 
composition of other heavenly bodies than our own earth was 
furnished by the aerolites, meteorites, or falling stars, which from time 
to time have entered our atmosphere from. planetary space, and have 
descended upon the surface of the globe. Subjected to chemical 
analysis these foreign bodies show considerable diversities of com- 
position ; but in no case have they yet revealed the existence of 
any element not already recognised among terrestrial materials. 
Upwards of twenty of our elements have been detected in aerolites, 
sometimes in the free state, sometimes combined with each other. 
More than half of them are metals, including iron, nickel, 
manganese, calcium, sodium, and potassium. ‘T'here occur also 
carbon, silicon, phosphorus, sulphur, oxygen, nitrogen, and 
hydrogen. In some of their combinations these elements, as found 
in the meteoric stones, differ from their mode of occurrence in the 
accessible parts of the earth. Iron, for example, occurs as native 
metal, alloyed with a variable proportion (6 to 10 per cent.) of 
metallic nickel. But in other respects they closely resemble some 
of the familiar materials of the earth’s rocky crust. ‘hus we have 
such minerals as chromic iron, pyrite, apatite, olivine, augite, ensta- 
tite, hornblende, and labradorite. No more convincing proof could be 
desired that some at least of the other members of the solar system > 
are formed of the same materials as compose the earth.* 
But in recent years a far more precise and generally available 
method of research into the composition of the heavenly bodies has 
been found in the application of the spectroscope. By means of this 
instrument, the light emitted from self-luminous bodies can be 
analysed in such a way as to show what elements are present in their 
intensely hot luminous vapour. When the light of the incandescent 
vapour of a metal is allowed to pass through a properly-arranged 
prism, it is seen to give a spectrum consisting of transverse bright 
lines only. This is termed a radiation-spectrum. Hach element ap- 
pears to have its own characteristic arrangement of lines, which in 
general retain the same relative position, intensity, and colours. 
Moreover, gases and the vapours of solid bodies are found to intercept 
those rays of light which they themselves emit. The spectrum of 
sodium-vapour, for example, shows two bright orange lines. If 
therefore white light from some hotter light-source passes through 
the vapour of sodium, these two bright lines become dark lines, the 
light being exactly cut off which would have been given out by the 
sodium itself. This is called an absorption-spectrum. 
From this method of examination it has been inferred that many 
* Partsch, Die Meteoriten, Vienna, 1843; Rose, Abhand. kénigl. Akad. Berlin, 1863. 
Rammelsberg, Die Chemische Natur der Meteoriten, 1870. The student will find a 
valuable monograph on the structure and origin of meteorites in the second part of 
Daubrée’s tudes Synthétiques de Géologie Expérimentale, 1879. See also A Chapter on 
_ the History of Meteorites, by Dr. W. Flight, Geol. Mag. 1875, and a very interesting 
_ account of a recent meteoric shower, and of the microscopic constitution of the fragments 
__ by J. Galle and A. von Lasaulx in Monatsbericht kénigl. Akad. Berlin, July, 1879. 
