OX METEORIC STOXES. 77 



In physical as well as in chemical characters aerolites resemble at 

 the first aspect some terrestrial volcanic rocks. 



The minerals of which they are composed are nearly entirely 

 crystalline, as is evinced by the colors in polarized light of such as are 

 transparent. These minerals are usually aggregated with slight cohe- 

 sion, and they present in by far the greatest number of cases a peculiar 

 spherular or " chondritic " structure. 



In these the spherules are composed of similar minerals to those 

 which enclose them, and even contain metallic iron sometimes in 

 microscopically fine grains disseminated through them. 



A section of an aerolite was exhibited by the microscope in which 

 some of the spherules had been broken before being cemented by the 

 surrounding mass, and in another fissures were seen which had been 

 filled with a fused material after one side of the fissure had slidden 

 along the other ; facts pointing to events in the history of the meteorite 

 subsequent to its first formation. 



The chemical composition and the mineral constitution of aerolites 

 were illustrated by tables showing the elements met with in these 

 bodies, and the minerals in which they were distributed. The former 

 comprised about one-third of the known elements ; among them 

 magnesium, iron, silicon, oxygen, and sulphur, were conspicuous ; cal- 

 cium, aluminium, nickel, carbon, and phosphorus, coming next in im- 

 portance, the basic elements of most importance by their amount being 

 the same as those which are found by spectroscopic analysis to be 

 present in the sun and in those stars which have been the best ex- 

 amined. 



The minerals most frequent in aerolites besides nickeliferous iron 

 or troilite (iron monosulphide) and graphite, are bronzite (a ferriferous 

 enstatite) and olivine, both of the latter being essentially magnesium 

 silicates. Augite and anorthite also occur (more particularly in the 

 eukritic aerolites of Rose) and some minerals unknown in terrestrial 

 mineralogy have also been met with ; such are the different varieties 

 of Schreibersite (phosphides of iron and nickel) ; calcium sulphide, 

 asmanite (a form of silica crystallizing in the orthorhombic system and 

 having the specific gravity of fused quartz), and a cubic mineral with 

 the composition of labradorite. The crystalline form of bronzite was 

 first determined from the crystals in a meteorite, and was found to 

 confirm the conclusion Descloizeaux had arrived at as regards its 

 system from observations on the distribution of the optic axes in the 

 terrestrial bronzite and enstatite. 



The question as to whence the meteorites come is one that we are 

 not yet in a position to answer with certainty. The various hypotheses 

 which suppose for them an origin in lunar volcanoes, or in our atmos- 

 phere, or again in a destroyed telluric satellite, or that would treat 

 them as fragments of an original planet of which the asteroids are 

 parts, or as masses ejected from the sun; all these hypotheses seem to 



