Sorby on the Physical History of Meteorites. 447 
V. On tHE PuysicAL History or METEORITES. 
By H. C. Sorsy, F.R.S. 
HOUGH Iam most willing to admit that much remains to be 
learned before we can look upon the following theory as any- 
thing more than provisional, yet at all events it serves to unite a 
great number of facts, and is not opposed to any with which I am 
now acquainted. I shall describe the facts and discuss the objections 
to this and other theories in a communication to the Royal Society. 
As shown in my paper in the ‘ Proceedings of the Royal Society,’ 
(xiii. 833), there is good proof of the material of meteorites having 
been to some extent fused, and in the state of minute detached par- 
ticles. I had also met with facts which seemed to show that some 
portions had condensed from a state of vapour; and I expected that 
it would be requisite to adopt a modified nebular hypothesis, but 
hesitated until I had obtained more satisfactory evidence. The 
character of the constituent particles of meteorites and their general 
microscopical structure differ so much from what is seen in terres- 
trial volcanic rocks, that it appears to me extremely improbable that 
they were ever portions of the moon, or of a planet, which differed 
from a large meteorite in having been the seat of a more or less 
modified volcanic action. A most careful study of their microsco- 
pical structure leads me to conclude that their constituents were 
originally at such a high temperature that they were in a state of 
vapour, like thatin which many now occur in the atmosphere of the 
sun, as proved by the black lines in the solar spectrum. On cool- 
ing, this vapour condensed into a sort of cometary cloud, formed of 
small crystals and minute drops of melted stony matter, which 
afterwards became more or less devitrified and crystalline. This 
cloud was in a state of great commotion, and the particles moving 
with great velocity were often broken by collision. After col- 
lecting together to form larger masses, heat, generated by mutual. 
impact, or that existing in other parts of space through which they 
moved, gave rise to a variable amount of metamorphism. In some 
few cases, when the whole mass was fused, all evidence of a previous 
history has been obliterated; and on solidification a structure has 
been produced quite similar to that of terrestrial volcanic rocks. 
Such metamorphosed or fused masses were sometimes more or less 
completely broken up by violent collision, and the fragments again 
collected together and solidified. Whilst these changes were taking 
place, various metallic compounds of iron were so introduced as to 
indicate that they still existed in free space in the shape of vapour, 
and condensed amongst the previously formed particles of the meteo- 
rites. At all events, the relative amount of the metallic consti- 
tuents appears to have increased with the lapse of time, and they 
often crystallized under conditions differing entirely from those 
which occurred when mixed metallic and stony materials were 
metamorphosed, or solidified from a state of igneous fusion in such 
small masses that the force of gravitation was too weak to separate 
the constituents, although they differ so much in specific gravity. 
