ARTICLE III.

Description of a Resinous Substance lately dug out of the Earth at Highgate. By Thomas Thomson, M.D. F.R.S.

DURING the late attempt to make a tunnel through Highgate-hill, in the neighbourhood of London, a very considerable number of curious fossils were discovered. The beds dug through consisted partly of gravel and partly of clay. The number of shells thrown out, and the round masses of limestone, could not escape the most careless observer; but one of the most remarkable substances detected was a resinous body, in shapeless masses of various sizes. Mr. Sowerby had the goodness to send me some specimens of this curious substance; and as I am not aware that any account of it has hitherto been published, I conceive that the following observations which I made upon the specimen that I received will prove acceptable to my chemical and mineralogical readers.

I. The colour of Highgate resin is of a dirty yellowish light brown. It is semitransparent. Its lustre is resinous, and its surface smooth; though not perfectly so; but having the appearance of having been rubbed, as would have happened had it been mixed with gravel upon the margin of the sea-shore, or a lake.-Brittle; not so easily broken as common resin ; but much more so than copal: softer than copal; has a resinous and aromatic smell, especially when heated; this smell is peculiar, though it has some faint resemblance to the smell of camphor.

II. Its specific gravity at the temperature of 60° is 1046. This agrees almost exactly with the specific gravity of copal as determined by Brisson; but on trying the specific gravity of copal, I found it 1.069. Hence either copal differs considerably in its specific gravity, or the resin called copal by Brisson was not the same to which we give that name in Britain.

III. When heated it melts, and may be rendered as liquid as water without alteration in its colour. It catches fire at the flame of a candle, and burns with a clear yellow flame, and emitting abundance of smoke, as is the case with other resins. At the same time it emits a strong aromatic odour.

IV. When in lumps it is insoluble in all the reagents I tried, namely, water, alcohol, potash ley, acetic acid; except ether, nitric acid, and sulphuric acid, which act upon it more or less.

Ether renders it opake, and white, and quite tender; so that it has lost its cohesion, and crumbles into powder upon the least pressure between the fingers. The ether at the same time dissolves a portion of it which it deposites, and becomes milky when agitated with water.

Nitric acid acts upon it slowly when assisted by heat, and partly dissolves it, and partly converts it into a red coloured substance. The acid itself becomes red, and when diluted with water lets the resin again fall in white flocks. These flocks when dry are in the state of a light yellow coloured powder, having a bitter taste. I could not dissolve it in water; but it dissolved in alcohol, at least as easily as the unaltered resin.

Sulphuric acid readily chars this resinous body when assisted by heat.

V. When reduced to the state of a fine powder alcohol readily dissolves a small portion of it, and lets it fall again when mixed with water; but alcohol is a bad solvent of this resinous body. The same observations apply to ether.

VI. I cannot find that either potash or subcarbonate of potash dissolve this resin, though boiled with it for some time in the state of powder. This is the property which distinguishes Highgate resin from every other with which I am acquainted. Even amber is partially acted upon by alkaline leys, and tinges them yellow very speedily.

VII. Nor do I find that acetic acid dissolves any perceptible portion of this resin after a week's digestion in it, when in the state of a fine powder. I even triturated them together for a considerable time in a mortar, and then boiled them in a glass tube, but no solution was effected. Here, again, another character which Mr. Hatchett has assigned to the resinous bodies fails when applied to the Highgate resin.

VIII. I have not tried the action of oils upon Highgate resin; but from the properties above described I conceive there is reason to presume that, like copal, it will not dissolve in any of them.

IX. It burns all away before the blow-pipe upon a piece of metal without leaving any perceptible ash behind it, when we make choice of pieces quite free from any earthy matter attached

to them.

Such are the properties of this substance, as far as I have examined them. They are sufficient, I think, to distinguish it from all the vegetable substances hitherto observed. It approaches nearest to copal and amber; but is distinguished from the first by its solution in alcohol, and its non-solution in potash ley; from the second, by its readily melting when heated, and by its melting without any perceptible change of its properties. Thus the chemical properties of this singular substance throw no light upon the source from which it was derived; and cannot, therefore, facilitate our inquiries into the revolutions which the southern part of this kingdom has undergone, and the various animal and vegetable remains so thickly scattered in its bowels.

ARTICLE IV.

On a new Variety of Ulmin. By Thomas Thomson, M.D. F.R.S.

THE experiments detailed in the first number of these Annals upon the ulmin from the elm show us that it is a peculiar vegetable principle, distinguished by the following properties :1. Of a dark brown colour, and little taste.

2. Soluble in water, but insoluble in alcohol and ether. 3. Nót precipitated by solution of gelatine.

4. Precipitated brown by iron, tin, mercury, and lead, when these metals are in the state of saline solutions.

5. Precipitated by acids.

6. Swells greatly when heated, as is the case with gum, but does not melt.

These characters do not apply to any other vegetable principle at present known. Hence it is obvious that we must constitute this substance a new vegetable principle. Some chemists are inclined to refer ulmin to extractive; but it is high time to render that very indefinite class of vegetable substances somewhat more precise. Nobody has ever examined extractive in a state of purity. Hence its properties are unknown, and it has been customary to refer to that class all substances which could not be referred to any other. It constitutes a kind of sink, or common sewer, in vegetable chemistry; but such an indefinite mode of proceeding is highly injurious to the progress of this branch of the science. If we wish to make ourselves accurately acquainted with the vegetable kingdom, we must distinguish every substance which possesses peculiar properties by a peculiar name. No risk of error results from multiplying the number of vegetable principles; the error into which we are most likely to run is classing the most dissimilar substances under the same name; thus enabling us to satisfy ourselves and others with giving a substance a name without being in the least aware of its distinguishing characters. What, for example, could be more preposterous than to give the same name to vegetable substances, some of which are soluble, and some insoluble, in alcohol? Yet this has been proposed by some of the most eminent chemists, both in this country and on the continent.

Now that the vegetable principle which constitutes the subject of this article has been distinguished by a name, and that the attention of men of science and observation has been turned to it, we may expect to find different species of it exuding from different species of trees besides the elm. The variety which I am going to describe is an example of this. I got it from Mr. Sowerby, who informed me that he collected it from the oak,

It possesses the following properties, which distinguish it from the ulmin of the elm :

1. Its colour is a very dark brown, almost black, and it leaves a chocolate coloured stain on paper. It readily crumbles to powder between the fingers. Lustre, resinous. Taste, more astringent than the ulmin from the elm, and inclining to bitter. 2. Dissolves readily in water. The colour of the solution is dark brown, and so intense as to be opake. When left to spontaneous evaporation upon a watch-glass the ulmin remains, divided into a great number of minute portions by sections, which issue in rays from the centre, and the ulmin adheres but weakly to the glass.

3. Insoluble in alcohol and ether.

4. When the aqueous solution is dropped into alcohol of the specific gravity 0-809, brown flocks precipitate, but the alcohol retains a brown colour, and of course a portion of the ulmin remains in solution.

5. The aqueous solution is not precipitated by the solution of gelatine in water.

6. Some of the precipitates which the aqueous solution of this variety of ulmin forms with the metalline salts differ in colour from the precipitates formed by the ulmin from the elm. The following were the metalline salts tried :

(1.) Sulphate of iron is thrown down dark green, and the colour is permanent, though the liquid be left 24 hours in an open glass.

(2.) Sulphate of copper is precipitated also green.

(3.) Sulphate of zinc is precipitated brown, and the colour of the precipitate speedily deepens, and becomes at last a dirty black.

(4.) Nitrate of silver is precipitated in brown flocks.

(5.) Acetate of lead is precipitated in brown flocks.

7. The aqueous solution is precipitated in brown flocks by a few drops of nitric acid. This solution being evaporated to dryness in a watch-glass, a yellow tasteless powder remained, insoluble in alcohol, but soluble in water. This powder was charred at a very moderate heat; owing probably to the action of the nitric acid which it still retained.

Acetic acid does not precipitate this variety of ulmin from

water.

8. Neither potash, carbonaté of potash, nor ammonia, precipitate it from water.

9. When exposed to heat it swells up like gum, and readily burns away before the blow-pipe, leaving behind it a minute portion of white matter, which did not melt by the continuation of the heat. This matter dissolved with effervescence in nitric acid, except some hardly perceptible flocks, which had the

aspect of silica, but were too minute to be subjected to a chemical examination. The addition of ammonia to the nitric acid solution occasions the separation of a few flocks, which redissolve by agitation. Carbonate of potash occasions a more copious precipitate. Hence it would appear that the ash consists of carbonate of lime, with some traces of magnesia and silica.

This variety of ulmin resembles the bitter principle from coffee described by Chenevix, and the tannin of Kino, in striking a green colour with sulphate of iron. Its effect upon sulphate of zinc I consider as its most remarkable property. Zinc is usually precipitated of a white colour from its solutions; but the ulmin of the oak throws it down almost black.

The appearance of the ulmin of the oak, its taste, and the tree from which it was obtained, led me to expect that it would contain tannin; but if not forming a precipitate with gelatine be characteristic of the absence of that principle, as we consider it at present, we must conclude that the ulmin of the oak contains no tannin whatever.

Mr. Sowerby likewise collected ulmin from the hornbeam; but as he unfortunately mixed it with ulmin from the elm, it was not possible to determine its peculiar characters. I mention the circumstance to induce such of my readers as are interested in the progress of vegetable chemistry, and have an opportunity of examining the trunks of trees, to look for exudations from them, that we may have information as speedily as possible of the various trees that yield this hitherto neglected vegetable principle.

ARTICLE V.

On Sir H. Davy's Theory of Chlorine, and its Compounds.* By Mr. William Henderson, Member of the Royal Medical Society of Edinburgh.

THE reasons which may be alleged in proof of the simple nature of oxymuriatic gas seem easily reducible to four heads, viz. :

I. It is converted into muriatic acid by union with hydrogen; and this change is unaccompanied by the evolution of any

aqueous vapour.

II. The products of its action on combustibles, and on metals, differ essentially from those which arise from the action of oxygen on the same bodies.

*This essay was honoured with the prize medal of the Medical Society of Edinburgh for 1812,