Highgate Rezin, or Retinasphaltum
- Bitumen resiniferum. B. M. 186.
- A resinous substance. &c.
- Thompson’s Annals, Vol. 2. p. 9.
- Gillman in Monthly Magazine, &c. 31. p. 535.
- A Member of the Lambeth Chemical Society in Do. Vol. 32. p. 108.
- Fossil Copal.
- Aikins Manual, p. 64.
- Mawes Catalogue.
The Retinasphaltum figured in tab. 186, leads us to admire the progress of Nature in the various chemical changes in the system. The discovery of an analogous substance in the Clay of Highgate, by B. G. Snow, Esq. Surgeon, of that place; so extraordinary in the Annals of Mineralogy, must raise our admiration, and the more so as it is considerably different from that of Bovey.
At first sight it seems to resemble Resin, and it acquires a low degree of resinous electricity by friction, so characteristic of that division. Its specific gravity when pure is 1.0375; it is in flatted pebble shaped lumps, and seems to have subsided both regularly and irregularly, as if it had been partly in a melted state; part, especially the outside, is of a soft and somewhat corky texture, and whitish; other parts are of a compact resinous texture, from a light yellowish to a dark brown colour, with the glassy fracture, &c. of resin. The light, part when in flamed, gives out much smoke of a pleasant odour, melts, froths, and when blown out, subsides rather solidly and like the brown part, which generally has the pleasantest and most permanent odour, melting and burning quietly. Rubbing or crumbling it between the fingers or scraping with a knife, expresses an odour like that of lemon thyme; a very few specimens are rather fœtid. It is softer than rosin in the darker parts, losing its angles and polish by handling; and it differs but little in the other parts which are rather crumbly when not broken or perfect. The lumps are sometimes found independant, at others mixed with pyrites, with various appearances,* sometimes in polished globules, filling a few of the vesicles, of which some pieces have many. A few specimens have been found attached to the remains of stumps of trees perforated by Teredo antenautæ, and to Septaria, but in general they are separate among the Clay from 10 to 90 feet deep. Some globular pieces have been found as large as ones fist, but they are more commonly about the size of broad beans: when boiled in water it communicates a very slight taste to it and swells much, becoming porous, but no part seems to be dissolved. Cold Alcohol dissolves a part of it slowly, and boiling Alcohol digested upon its powder extracts about 36 per cent of an orange brown odoriferous resin from it, without materially altering its appearance,† except that it is rendered more arid to the touch, for the unaltered powder adheres to the fingers almost as much as that of rosin. Turpentine dissolves it completely, forming a lighter coloured solution, than a similar one made from the Retinasphaltum of Bovey. It is easily distilled; when a small portion of gas, an orange brown oil, and a very small cinder are the whole products, but no acid, (analogous to the Succinic) or alkali is obtained. It is soluble in Sulphuric and Nitric acids, and convertible by them into Tannin like other resinous substances. The part insoluble in Alcohol seems to be intermediate between elastic Bitumen and Asphaltum; when melted it is of a dark bottle green colour, semi-transparent and Tree from scent. When inflamed it smells like Bitumen mixed with some vegetable matter.
Mess. Trimmers have occasionally found the same substance in the Brick Clay (blue Clay) at Brentford, and I have had a Specimen from Hyde-park, with the Nautili found there, in 1812. Dr. Woodward seems to have described it (Vol. 1. p. 168. No. 45) as “two samples of Amber, brown and foul, found at least 30 feet deep in the pit where they dig clay to make tiles, at Richmond, Surrey,” and says, “the workmen call it Rosin, there is in some pieces of it a salt that I take to be Vitriol, which starting and shooting makes the mass very apt to dissolve and tali to pieces, of which I have seen several instances, one of these samples being very little broken, and is covered With an exterior crust, alter the manner of all true nodules. Exposed to fire this sort burns and emits an oil and a smell, exactly like that of Amber, but exerts no electric attractive power when rubbed or heated.” Again Bk. 2. p. 18. b. 36. “A resinous matter found lying between the bark and the wood of some of the trees dug up in Wilmeslow mosses; they call it frankincense.” We are not sure what these are, the former seems most likely to be our substance. I find, by Writing to my friend, the Woodwardian professor, that neither of these substances remain in the collection at Trinity College, Cambridge.
The upper figure, or No. 1, is a large piece of the best sort, or such as is not liable to come to nieces or decompose, being free from pyrites; it is a dark rick brown within, and outwardly only coloured by the clay in which it was found: it is about half of a lump which weighed nearly five ounces, and was broken ere it fell into the hands of my kind friend Mr. Snow: it breaks nearly like rezin. No. 2. is a curious variety arrested apparently while dropping like melting rosin; of a duller colour, and has pyrites or sulphuret of iron about it. No. 3. has hollows like bubbles, that seem to have been filled with brilliant pyrites that has passed out, one piece in situ excepted; in this piece the lighter and darker varieties are partly mixed. No. 4. is in some parts quite white and crumbly, mingled with a Lightish amber coloured variety, its analogy to the light coloured amber, tab. 273. is curious. No, 5. shows a lump of the earthy appearance or variety. No. 6. part of a mass found within a ball of pyrites, surrounded by shells, and which has the earthy texture, having apparently undergone a kind of decomposition or disintegration, probably owing to the pyrites outside it.‡ No. 7. is a yellow brittle inflammable substance found in the clay at Crockerton, which much resembles the yellow variety from Highgate; the quantity hitherto found has been very inconsiderable, but I have ascertained that it contains an acid, so that it is between the retinasphalt and amber: should the acid prove to be the succinic, (which I rather doubt,) it can only be a variety of amber.
An hasty analysis of the Highgate resin, by digesting 100 parts in alcohol, afforded the following results, viz.
|Bitumen, yielding 4 parts of cinder||65.|
|Resin, not quite free from alcohol||37.5|
|Oxyde of iron and earthy matter||2|
The alcohol adheres strongly to the resin, which is easily decomposed: to this circumstance may be attributed the increase of weight.
I formerly observed in a note that the Highgate resin was sometimes found in contact with the septaria: this was thought a mistake, and was contradicted. I still have a specimen, shewing it with the remains of wood and Teredo antenautæ, Min, Con. tab. 102; thus proving that it accompanies both animal and vegetable remains.
- * Although a great quantity of resin has been found at Highgate, perhaps a peek or two, little now remains, because almost all has decomposed. Brongniart thinks some has been found lately on the Continent, and Sir J. .Banks and Sir Charles Blagden have procured him specimens from Highgate for comparison, to which I have had the pleasure of adding a specimen from Crockerton, that comparison and analysis might be made to a greater perfection.
- † Powdered copal boiled in Alcohol unites into a stiff elastic mass somewhat resembling white caoutchouc, a character that at once distinguishes it from the Highgate Retinasphaltum.
- ‡ See a figure of this whole specimen m Mineral Conrhology, tab. 27.