Transactions of the Geological Society, 1st series, vol. 4/Analysis of a bituminous Limestone
member of the royal academy of sciences at berlin
and honorary member of the geological society
[Read 21st June, 1816.]
The superiority which has been observed in the architecture of the ancient Greeks and Romans, may in some measure be ascribed to the materials used in the construction of their edifices. This remark is especially applicable to the works of the Romans; because a very principal part of the materials of their architecture consisted of substances that were in their nature artificial. Their aqueducts, walls, and foundations, often consisted of bricks and mortar; and in the making of mortar, by the judicious use of the pulvis Puteolanus, a cement was prepared which had the property of becoming indurated under water, in such a remarkable manner, that, in many instances, it acquired a greater degree of hardness than the substances themselves exhibit, which this cement was intended to hold together. To this property are owing the specimens of polished mortar, which exist in the cabinets of antiquaries, derived from ruins upon the coast of Baia, of Putéoli, and of Naples, and wherever else the pulvis Puteolanus was used in the fabrication of mortar, which has subsequently been exposed to the action of water. “ Puteolanus pulvis,” says Seneca,  “ si aquam attigit, Saxum est.”—It was a property so well known to the ancients, that the ashes of Putéoli were exported to very distant parts of the Roman Empire, to be used in the preparation of mortar for all public works, such as moles, bridges, and ramparts, situate in rivers, lakes, or in bays, and upon the borders of the sea. The excavations carried on in search of it, caused the spacious caverns and extensive subterraneous galleries, afterwards used as catacombs, in the neighbourhood of Naples and of Rome;  and the same arenaceous substance has sometimes been brought even into Great Britain, to be used in the fabrication of mortar, both in ancient and in modern times. It may therefore be considered as a discovery of some importance, that we possess, in this country, a species of limestone which, when used for purposes of extracting lime, and in the preparation of mortar, is capable of communicating to the cement all the properties of the pulvis Puteolanus.
This species of limestone is found in North Wales, in the parish of Whitford, in Flintshire. Some specimens of it were sent to me by my friend David Pennant, Esq. son of the celebrated naturalist of the same name. Its specific gravity, estimated in pump water, at a temperature of 50° of Farenheit, equals 2.670. It is of a dark brown colour, and, when breathed upon, it exhales an earthy odour, denoting the presence of iron oxide, in combination with, alumine; but its colour is owing to bitumen, rather than to iron, as will appear by the following analysis, undertaken at the request of Mr. Pennant, for ascertaining the chemical constituents of this limestone.
1. One hundred grains being placed upon red hot iron for the expulsion of the water of absorption, were thereby diminished of a grain.
2. The remainder being reduced to powder in a porcelain mortar and exposed to diluted muriatic acid until all effervescence ceased, there remained an insoluble residue of the original dark colour of the limestone, which when carefully washed and dried, weighed 10 grains; allowing therefore for the weight of the carbonic acid and lime, after the expulsion of the water of absorption, 89 grains.
3. The supernatant acid used in this experiment being decanted, and neutralized by the addition of an alkali, yielded no precipitate of iron to the tincture of galls; but the prussiate of potass threw down a blue precipitate upon which however no reliance can be placed; as it is well known that the prussiate of potass is not a satisfactory test of the presence of iron when this metal exists in an inconsiderable portion.
4. The ten grains of dark brown powder mentioned in No. 2, being collected, washed and dried, were exposed to the heat of a flame of a candle urged by the common blow-pipe, when combustion instantly ensued, accompanied by a lambent flame, which continued during some seconds, the powder thereby losing its colour and becoming white; attended also by a loss of weight, amounting to of a grain. Hence it is manifest that the colour is owing to bitumen.
5. To ascertain the proportion of alumine (which from its chemical combination with silex remained insoluble in the muriatic acid) a plan recommended by Mr. Holme was adopted. One hundred other grains of the same limestone were calcined in a platinum crucible, and the loss of weight owing to the expulsion of the carbonic acid was found to equal 40 grains.
6. The calcined residue being placed in muriatic acid, a solution now took place both of the lime and of the alumine, and there remained at the bottom of the vessel only an insoluble portion of pure silex, in the form of a white powder, which when carefully washed and dried weighed of a grain. Deducting therefore this weight of the silex, from the weight of the silex and alumine, which remained in No. 4, after the combustion of the bitumen, the weight of the alumina is ascertained; which of course equals 8, grains.
From all the preceding observations, it is therefore evident that the constituents of this limestone are as follow:
And the valuable property of the mortar prepared from this limestone, is owing to the presence and proportion of alumine; and to its property of rapidly absorbing water.
- Natur. Quest. Lib. 3. Cap. 20. See also Pliny. Hrst. Nat. Lib. 35. Cap. 13.
- The pulvis Puteolanus was also used by the ancients in constructing the streets of Rome, and in all the great roads of the empire. Sec Winkelmann, Hist. de l'Art, tom. 2, p. 553.
- Vinkelmann, Observ. sur L'Archict. des Ancient, &c. ubi supra.