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thus to entomb the animal in such a condition as proves that at the utmost but a few days must have elapsed before it was so far encrusted as to completely preserve the form and position of the animal, not by a sudden immersion in supposititious silicious paste, impounding it instantly in its full vigour, but after by a slow and gradual decease; for this condition which I have described of semi-protrusion of the tentacula is that with which every one acquainted with recent zoophytes in a living condition is so familiar as an indication of slow and undisturbed death by exhaustion. In this condition of semi-protrusion of the tentacula I have seen the animals of Alcyonium digitatum, Alcyonidium parasiticum, Caryophyllacea Smithii, and numerous species of Sertularia and other zoophytes die, if allowed to do so without interference; but if touched or disturbed, the tentacula are slowly withdrawn, and never again extended. It does not appear to me to be necessary, for the production of a fossil, that the whole of the silex should have been deposited immediately. We may readily imagine, that after the rapid deposition of the first portion induced by the full exposure of the animal matter, and the consequently strong elective attraction exerted by the animal for the earthy particles, that the remainder of the deposit the filling up of the interstices of the network—would be more slowly and regularly completed in accordance with the laws of crystallisation, as we find that from the surface of this animal there are the same series of crops of radiating calcedonic crystals that characterise the structure of the great mass of the moss agates which I have described in my paper on those bodies, published in the "Annals and Magazine of Natural History," vol. x. page 9.

This prismatic semi-crystallisation, if we may reason from analogies afforded by the phenomena of crystallisation displayed by salts formed by acids with earthy or metallic bases, is a rapid, and perhaps irregular operation, compared with the slow formation of the regular and well-defined crystals of the respective substances under consideration, and which crystals are probably produced without the interference of any other agent than that which is necessary for their own construction. The specimens of fossil wood now shown prove that during the

process of silicification considerable ruptures of the vessels had taken place, and that the wood was in a recent state at the time of fossilisation. I may further state, that the late Robert Brown showed me a section of silicified wood which fully corroborated the reasoning of Dr Bowerbank, as it bore distinct evidence of having been alive, and struggling to live, during the process of silicification. That the solution of silica in the humid way was not so difficult as many geologists supposed, chemists' experiments all tend to prove that it is soluble in a large degree. I especially refer to the experiments of De la Rue, who succeeded in obtaining minute crystals of quartz by an aqueous method. De la Rue found that the gaseous body, fluoride of silicon (Si F), is decomposed by contact with water, one-third of the silicic acid being deposited in the form of jelly and silicated hydrofluoric acid (3HF + 2SiF) produced: thus, 3 (SiF, + 3 (HO) = Si0 ̧ (which deposits) + 3HF + 2SiF3 (which remains in solution).

The deposited silica is extremely soluble in water, and he observed that the silicated-hydrofluoric acid always retains a portion of uncombined silicic acid in solution, which deposits after the lapse of some months in minute crystals of artificial quartz. It thus appears that water is the solvent of the silica, which, when recently produced or separated from its combinations by the action of the atmosphere on the earthy silicates, is presented in the modification most favourable for solution. The conclusions to be drawn from these experiments simplify very much our ideas of the silicification of wood shells and other organic bodies. Geologists hitherto have sought the explanation of the phenomena of petrifaction in the action of intense heat aided by pressure. It is clear, however, from the facts adduced by Dr Bowerbank, that such agencies could not have obtained during the silicification of his Alcyonium; and further, it appears from the preservation of the most minute tissues in fossil woods, that pressure and extreme heat could not have been present.

With regard to the preservation of the minute tissues in various fossil woods, especially the pines and araucarians, as shown in their radial sections, I had always found a difficulty

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in accounting for their preservation on any hypothesis, until an accidental phenomenon enabled me satisfactorily to account for it.

While preparing chloride of gold, I had occasion to expose to the action of nitro-muriatic acid many small articles of jewellery which had been chased and variously-fashioned. After exposing them for many hours in the usual bath at the proper temperature, I was astonished to find that none of the articles had changed either in shape or size, although the solution showed the proper quantity of gold had been dissolved. After decanting the acid, and washing well the jewellery with distilled water, and again adding a fresh quantity of aqua regia, I found that no further portions of gold were dissolved, but that the articles still retained their form and size. Some of the rings, which were chased in high relief, exhibited, as when put into the flask, every mark of the workman's tool. The mystery was soon unriddled; the jewellery had all been alloyed largely with silver, every atom of which had been changed into a chloride, and as each atom of gold was dissolved, it was replaced by an atom of the worn silver, and so the form of each article was retained as when first put into the flask. On analysis, it was found that no particle of gold was left-all was converted into chloride of 'silver. This fortunate experiment at once enabled me to perceive, that when a ligneous atom was removed from a fossilising tree, an atom of silica was deposited, and hence the perfection with which the most minute structure was preserved.

This being the last meeting of the Session, the Society adjourned to the commencement of next Winter Session,

207

APPENDIX.

Description of the Bones found in a "Pict's House" in the Island of Harris. Being Notes to Communication read 22d February 1860. By JAMES M'BAIN, M.D., R.N.

The first fragment is a portion of the right side of the upper jaw of a dog of tolerably large size, with a part of the molar bone attached; the sectorial tooth and first tuberculated permanent molar remain in situ; and there are sockets for the second and third premolars in front, with a socket for the last true molar behind. The sectorial tooth in the upper jaw of the Canidæ is preceded by a deciduous tooth, and is therefore a premolar. The foramena present in this fragment are the inferior orbital foramen, the lachrymal canal, the sphenopalatine foramen for transmitting nerves and vessels to the nasal fossæ, and a slightly developed palato-maxillary canal. The next specimen also belongs to the dog, and is a part of the left lower jaw; it corresponds in size with the portion of the upper jaw just described, and probably belonged to the same individual-it contains the last, premolar, the sectorial tooth, the second true molar, and a socket for the third and last permanent molar. The sectorial tooth in the lower jaw is not preceded by a milk tooth as in the upper jaw, and is in fact the first true molar. The dental formula of the Canida 3-3 1-1 4-4 2-2 3-31-1pm

is i

m

4-43-3

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42. The depression formed

by the Masseter muscle in the lower jaw of the dog does not extend to the under edge of the bone as it does in the fox, and would enable the comparative anatomist to distinguish to which of the species this fragment of the lower jaw belonged. The third specimen is the left tympanic bulla of the common seal (Calocephalus vitulinus), broken off at the fissure that divides the bulla from the mastoid process. The mastoid process is very slightly developed in the section of the Phocida

to which the Calocephalus vitulinus belongs, and seems to form a part of the osseous bulla. The fourth fragment is the basal portion of a horn of the red deer (Cervus elephas), with a part of the brow antler cut and fashioned by a sharp instruThe fifth piece is a branchlet of the same species, cut and hacked apparently for the purpose of breaking it from the stem. The sixth fragment is a small bit of bone, evidently a portion of a lesser branchlet, which is also cut and polished. The seventh specimen is a portion of the left side of the lower jaw of the Bos longifrons, and contains the second and third deciduous molars and first permanent molar. The third de-, ciduous molar has three large vertical columns, each with an inner and outer acute summit, the inner summits rising higher than the outer. There are two accessory columns, one on each side of the outer surface of the large middle column, extending upwards to the base of its summit, with distinct concentric layers of crusta petrosa, enamel, and central dentine. This three-columned deciduous tooth is known to be succeeded, between the second and third year, by a permanent premolar with only two columns. The loose teeth also belong to Bos longifrons; the tooth marked No. 8 is the first true molar on the right side of the upper jaw; No. 9 is the last permanent molar on the left side; No. 10 is a third deciduous premolar on the right side; No. 11 is the second deciduous premolar, also on the right side, and all belonging to the upper jaw. The larger column in these deciduous premolars is situated posteriorly, and the smaller in front; which character serves to point out whether they belong to the right or left side of the jaw. The next two specimens likewise belong to Bos longifrons; No. 12 is a cancellous horn core; and No. 13 is the distal portion of the right humerus. The species to which the name of Bos longifrons has been applied by Professor Owen is considered to be the original stock from which the present domesticated breed of small Highland cattle are derived. The fourteenth specimen is a portion of the right lower jaw of a small sheep, containing the two last premolars and first true molar, with sockets for the first premolar and for the incisors. The external opening of the inferior maxillary canal is situated nearly midway between the anterior

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