Philosophical Transactions. 383 
a peculiar substance which, upon the authority of Vauquelin, 
exists in, and confers its peculiarities upon, tobacco. When. 
this pitch-stone is distilled per se, a small quantity of a pecu- 
liar bituminous matter which it contains, and which the author 
proposes to call Newrine, is separated, probably in an altered 
state, appearing as an oily fluid, smelling exactly like a * to- 
bacco-pipe long in use.”” The residue in the retort was a pale 
ash-gray, porous, semi-vitrified substance, exactly resembling 
pumice. The artificial formation of a substance, having, as our 
author proves, all the properties, even the magnetic property of 
voleanic pumice, is an interesting circumstance, and may throw 
some light upon the natural formation of that body. Mr. Knox 
adds the following interesting observations : 
It appears to be a condition, in converting a stone into pumice, that it 
should contain a volatile substance, which can only be removed by the 
same degree of heat which is at the same time necessary for producing that 
sort of semi-vitrification in the mass which renders it coherent, hard, and 
porous. Ifa stone contains only water, pumice is not formed, because 
that is driven off by a red heat, which does not act upon the earths of 
which the stone is composed. Drive off the water ata red heat, and you 
have a harder but incoherent mass ; increase the heat, and you have, as in 
the case of alumine,a more compact and denser substance, or else, when 
the ingredients of the stone favour vitrification, a glass. 
There are many valuable remarks connected with chemical 
analysis scattered through this communication, but Mr. K. ap- 
pears to have fallen into some errors in his determination of the 
quantity of soda contained in pitchstone; we, at least, cannot 
follow him in his conclusions. The Newry pitchstone is cha- 
racterized by its large proportion of bitumen, a peculiar oily 
smell, a tendency to divide into thin lamin, and disintegrate 
spontaneously into rhomboidal fragments. Mr. Knox gives the 
following as its components : 
Silica . a f shih 22000 
Alumine 3 : - 11.500 
Lime . } R . 1.120 
Protoxide of iron . POG OBYE 
Soda . p : ee 2S iam 
Water and bitumen . 8.500 
99.813 
7. Observations on the Changes the Egg undergoes, during 
Incubation in the common Fowl, illustrated by Microscopical 
Drawings. By Sir E. Home, Bt., V.P.R.S. 
Among the numerous obligations under which science lies to 
the author of this paper, there is none for which we feel more 
grateful than having enlisted Mr. Bauer into the service of 
animal physiology. We by no means intend to underrate the 
importance of that gentleman’s contributions to botanical 
science, but we sincerely believe that, without Sir Everard 
