Account of a coloured Test, much used in the Arts, named Tournesol by the French, and Litmus generally by the English; with some Remarks. By Mr. T. HENRY.

Communicated in a Letter to the Editors.

GENTLEMEN,

IN consequence of the remarks in your valuable work *,

by Mr. Smithson, I send this account, taken from the English translation of Thenard's very useful Treatise on Chemical Analysis, just published in 8vo.

"Litmus has a violet-blue colour. It exists in commerce in two different states, namely, in cakes and in rags. The latter is prepared in the South of France, at the village of Grand Gallangues, near Montpellier, by impregnating rags with the juice of the croton tinctorium, an annual and herbaceous plant, and exposing them to the vapour of putrid urine. It is exported to Holland, where it is said to be employed for colouring the outside of cheese.

"The cake litmus is made in Holland. For a long time it was believed that the Dutch made use of the rag litmus for that purpose; but it is known at present that they prepare it with the lichen rocella of the Canaries, or Cape Verd, or with Swedish moss. They mix the lichen, reduced to powder, with half its weight of crude tartar, well pounded, and moisten the mixture with human urine, to excite fermentation. When the mass has acquired a red tint, they sprinkle it again with urine, and stir it. By these means it is rendered of a violet-blue colour in a few days: they then divide the paste to moderate the heat, mix it with a third part of good potash, and a new quantity of urine, which deepens the colour * See vol. XXXIII. page 334.

considerably; and add afterward chalk, to diminish the price, mould and dry it. Chaptal has obtained an analogous colour, but less beautiful, from the lichen parellus. (See Bancroft on Permanent Colours).

"Litmus is very alterable by even the weakest acids; and on this account is employed in solution, to discover their presence. Its primitive colour is red; and if it exhibits a blue tint, that is owing to the alkalies with which it is always combined. When reddened by acids, its blue colour is restored by most of the oxyds. Litmus appears to be only a combination of a red colour, with an alkali or a metallic oxyd: consequently, it must be conceived, that by pouring an acid into the solution of litmus, the acid combines with the alkali, and liberates the red colour; and that, by adding afterwards an oxyd to the li quid, that oxyd combines with the acid or the red colour, and renders it again blue. These effects then depend on a real affinity of the colouring matter for the oxyd, and of the acid for the oxyd. In general, the latter is stronger than the former; and this is the reason why one of the characters of acids is to redden litmus."

"Bergman directs the solution to be prepared by enclosing lakmus' (lac-moss?) in clean linen cloth, and steeping it in distilled water; and litmus paper, by tinging unsized white paper in a saturated solution, with a little starch boiled in it. Nicholson (Chem. Dict.) states erroneously, that the solution may be made with spirit instead of water. TRANSLATOR."

I find the solution to be a far more sensible test than litmus-paper; but the latter is frequently the more convenient. See the Edinb. Encyc. art. dyeing and lichen. It is to be lamented, that such fugacious colours should be ever employed as dye stuffs.

Yours, &c.

T. HENRY.

An

An Account of Experiments for determining the Length of the Pendulum vibrating Seconds in the Latitude of London. By Capt. HENRY KATER, F. R. S.

With a Plate.

From the PHILOSOPHICAL TRANSACTIONS of the
ROYAL SOCIETY of LONDON.

To determine the distance between the point of sus pension and centre of oscillation of a pendulum vibrating seconds in a given latitude, has long been a desideratum in science. Many experiments have been made for this purpose, but the attention of all who have hitherto engaged in the enquiry (excepting Whitehurst) appears to have been directed to the discovery of the centre of oscillation. The solution of this problem depending, however, on the uniform density and known figure of the body employed, (requisites difficult if not impossible to be ensured in practice,) it is not surprising that the experiments made by different persons should have been productive of various results.

When I had the honour of being appointed one of the committee of the Royal Society for the investigation of this interesting subject, I imagined that the least objectionable mode of proceeding would be to employ a rod drawn as a wire, in which, supposing it to be of equal density and diameter throughout, the centre of oscillation, as it is well known, would be very nearly at the distance of two-thirds of the length of the rod from the point of suspension; and I purposed by inverting the rod, and taking a mean of the results in each position, to obviate any error which might arise from a want of uniformity in density or figure. After numerous trials, however, and as frequent disappointments, I was at length

convinced

convinced of the impracticability of obtaining a rod sufficiently uniform, and I was besides aware, that under certain circumstances errors might arise from this cause which it would be impossible by any method to detect.

Not feeling at all satisfied with the prospect which the use of a rod presented, I endeavoured to discover some property of the pendulum of which I might avail myself with greater probability of success; and I was so fortunate as to perceive one, which promised an unexceptionable result. It is known that the centres of suspension and oscillation are reciprocal; or in other words, that if a body be suspended by its centre of oscillation, its former point of suspension becomes the centre of oscillation, and the vibrations in both positions will be performed in equal times. Now the distance of the centre of oscillation from the point of suspension, depending on the figure of the body employed, if the arrangement of its particles be changed, the place of the centre of oscillation will also suffer a change. Suppose then a body to be furnished with a point of suspension and another point, on which it may vibrate, to be fixed as nearly as can be estimated in the centre of oscillation, and in a line with the point of suspension and centre of gravity. If the vibrations in each position should not be equal in equal times, they may readily be made so, by shifting a moveable weight, with which the body is to be furnished, in a line between the centres of suspension and oscillation; when the distance between the two points about which the vibrations were performed being measured, the length of a simple pendulum, and the time of its vibration, will at once be known, uninfluenced by any irregularity of density or figure*.

* In the Connoissance des Temps for 1820, is an article by M. de Prony on a new method of regulating clocks. At the conclusion of

An unexceptionable principle being thus adopted for the construction of the pendulum, it became of considerable importance to select a mode of suspension equally free from objection. Diamond points, spheres, and the knife edge, were each considered; but as it was found. difficult to procure diamond points sufficiently well executed, the knife edge was preferred, after many experiments had been made with spheres, the result of which it may not be useless for a moment to dwell upon.

of this article is a short note, in which the author adds, "J'ai proposé en 1790 à l'Academie des Sciences un moyen de déterminer la longuer du pendule en faisant osciller un pendule composé sur deux ou trois axes attachés à ce corps. (Voyex mes Leçons de Méchanique, art. 1107 et suivans). Il paroit qu'on a fait ou qu'on va faire usage de ce moyen en Angleterre." On referring to the Leçons de Mécanique, as directed, I can perceive no hint whatever of the possibility of determining the length of the seconds pendulum by means of a compound pendulum vibrating on two axes, but it appears that the method of M. de Prony consists in employing a compound pendulum having three fixed axes of suspension, the distances between which, and the time of vibration upon each, being known, the Length of three simple equivalent pendulums may thence be calculated by means of formulæ given for that purpose. M. de Prony indeed proposes employing the theorem of Huygens, of which I have availed myself, of the reciprocity of the axis of suspension and that of oscillation, as one amongst other means of simplifying his formulæ, and says, “ J'ai indiqué les moyens de concilier avec la condition à laquelle se rapportent ces formules, celle de rendre l'axe moyen le reciproque de l'un des axes extrêmes; J'emploie pour les ajustemens qu'exigent ces diverses conditions un poids curseur dont j'ai exposé les propriétés dans un mémoire publié avec la Connoissance des Temps de 187." Now it appears evident from this passage, that M. de Prony viewed the theorem of Huygens solely with reference to the simplification of his formulæ; for had he perceived that he might thence have obtained at once the length of the pendulum without further calculation, the inevitable conclusion must instantly have followed that his third axis and his formulæ were wholly unnecessary.