But it is not only with respect to the parallax that the Dublin and Dorpat observations agree. Dr. Brinkley, (Phil. Trans. 1821,) by a mean from several stars, finds the constant of aberration 20′′37. He finds (in his paper on solar nutation) for a Cygni the constant =20"33, and for a Lyræ 20/35. This extraordinary coincidence in results, obtained by methods so entirely different as the Dublin and Dorpat observations, appears well worthy the attention of astronomers, and places in a strong light the perfection of modern instruments, and the accuracy of modern observations.

M. Struve proposes to extend his observations for parallax to several other stars, of which he has given a list. Several of these are of the fourth magnitude; but he hopes for success in making the observations, from the favourable time of day of culmination, when the parallax in right ascension is a maximum. a Lyræ is also among those stars; but there appears to be no circumpolar star nearly opposite in right as cension, greater than between the fourth and fifth magnitudes. M. Struve's future observations will greatly interest astronomers. A vast field is still open to him; and one almost feels sorry that any other pursuit should divert him from the transit instrument, with which he has made such accurate and important observations. We must, however, expect that the circle of Reichenbach, which he had just received when he published his third volume, will share a good deal of his attention.

ART. IV.—An Account of the Janji, or Valisneria Alternifolia of Dr. Roxburgh, the plant used in India in refining sugar, by FRANCIS HAMILTON, M. D. F. R. S. and F. A. S. Lond. and Edinb. &c. Author.

Communicated by the

Ir is well known that one of the most common processes for refining sugar is by filtering water slowly through small quantities of it, contained in pots, with an aperture in the bottom. The water carries along with it the extractive matter, and the minute saccharine particles united with it, which constitute what is called treacle; and leaves behind the more pure crystalline sugar. In our West Indian islands the

water is supplied by a cake of moist clay placed on the surface of the sugar; but in India it is supplied by covering the upper surface with a layer of moist aquatic plants.

The plant selected is commonly that, which in the Hindos tan language is called Janji, and which Dr. Roxburgh, in his manuscript, called Valisneria alternifolia, although I doubt much the propriety of placing it in that genus. A similar plant, indeed, by the younger Linnæus was called Serpicula verticillata, and was described by Dr. Roxburgh under this name; although Willdenow had called it Hottonia serrata. The latter botanist, however, having found out that his Hottonia serrata was the Serpicula verticillata, adopted this name. Before his death, Dr. Roxburgh became sensible that the plant was neither a Serpicula nor a Hottonia, and that it belonged to the same genus with the Janji, which he called a Valisneria. In the catalogue of the plants in the Botanical Garden at Calcutta, the Serpicula verticillata was therefore called Valisneria verticillata; but, if I understand M. Poiret aright, it has been proposed by M. Richard to form for it a new genus called Hydrilla. (Enc. Meth. Sup. v. 136.) ·

Like the Valisneria, the Janji no doubt is a diœcious plant, and in order to impregnate the female stigma, which projects above water, from the summits of the branches, the male flowers separate from the plant before the antheræ burst, and, floating on the surface, are entangled among the female branches, before the pollen explodes. In other respects, in even the manner by which the female flowers are elevated above the surface of the water, and in a habit resembling the caulinia of Willdenow, or the Tonina of Jussieu, the Janji totally differs from the Valisneria, which is a plant with leaves like grass, and without any stem; whereas the Janji has a very branchy long stem, covered to the very extremities with leaves, much like several species of Potomogeton, some which are indeed applied to the purpose of refining s! and in this country might supply the place of the Janji. preference indeed given to the latter seems to be owing abundance; as great quantities of it grow in almost pond in India that is not kept clear of weeds. Exc the few circumstances above mentioned, in which the resembles the Valisneria spiralis, it has a much greater

ity to the Commelina dubia of Jacquin, called Schollera graminifolia by Willdenow, Heteranthera graminea by Vahl, and Leptanthus gramineus by Michaux and Hooker. The dif ferences above mentioned, however, seem to me sufficient to separate the Valisneria alternifolia and V. verticillata from the genus Valisneria; and, notwithstanding the utmost affinity to the Schollera, to establish the genus Hydrilla proposed by Richard..

Dr. Hooker (Exotic Flora, No. 94.) places his Leptanthus or Schollera in the natural order of JUNCEE; but I doubt the propriety of separating it from the HYDROCHARIDES, in which M. Poiret places the Hydrilla, and to which Mr. R. Brown (Prod. Nov. Hol. i. 345.) approximates the Serpicula verticillata, which he admits to be a distinct genus, although he does not give it a name.

So far as I know, no description of the Janji has yet been published, and I shall therefore give one.

Radix fibrosa.

Caules submersi, filiformes, glabri, ramosi.

Folia alterna, approximata, amplexicaulia, linearia, acuta, integerrima, enervia, venis longitudinalibus confertis striata. Flores axillares, sessiles, plerumque gemini, dioici.

Masc. Spatha capsuliformis, compressa, acutangula, diaphana, ore bifariam dehiscens. Spadix in spathæ fundo brevissimus, ramosissimus, flosculis semina mentientibus tectus. Flosculi plures, pedicillati, spatha hiante ante antherarum maturitatem disruptis pedicellis enatantes, minuti, albidi, quadrivalves. Antheræ in flore natante dehiscentes, tres, sessiles, obovatæ.

Fam. Spatha longitudine germinis vaginans, ore obliquo dehiscens. Calyx minutus triphyllus, superus. Germen anceps, mucrone calycem gerente longissimo (digitali) subulato terminatum. Stigmata sex, per paria basi unita, sessilia, oblonga.

Fructus membranaceus, vix dehiscens, sublanceolatus, anceps, acutangúlus, acuminatissimus, unilocularis, filamentis umbilicalibus repletus. Semina plura, oblonga, inter filamenta nidulantia. Seminum structura in planta recente non conspicua, germinantia non observavi.

ART. V.-Contributions to Popular Science.

THE common objects of scientific inquiry are generally far withdrawn from popular apprehension. Even the systematic details of our elementary works require a degree of preliminary knowledge, and of patient attention, which are not with in the reach of ordinary readers; while the researches of those who are permitted to penetrate into the mysteries of nature, and to develope her laws, are accessible only to minds of similar capacities and attainments.

In those branches of science which are susceptible of popu lar illustration, and in those applications of it which have been so happily made to the wants and purposes of life, the public have long been accustomed to take a deep interest. The mind enjoys a peculiar gratification, not only when it obtains from a striking experiment both the proof and the explanation of a scientific principle, but even when it succeeds in referring the most ordinary facts to some general law, the truth of which it has been accustomed to recognise. From these causes the topics of popular science have always commanded an unusual degree of attention; and while the general reader is amused and instructed by their details, the philosopher often reads them with an interest of scarcely less intensity,

In the following series of papers which we propose to publish in successive numbers, either the facts, or the reasonings, or both, will, we trust, be found to be new; and, with the assistance of some of our able correspondents, we are not without the hope that they will often contain facts and views of some importance to science.

No. I-On the Revival of the Inscriptions on Coins and Medals by Unequal Oxidation.

It has been long known, though we have not been able to ascertain to whom we owe the discovery, that a coin, from which the inscription and the figures have been entirely ef faced, so as not to present the slightest trace of an impres sion, may have the inscription and figure partly or wholly restored, by placing it upon a hot iron. In order to perform this experiment with the fullest effect, the coin em-ployed should be one equally worn down, and in which very

little of the metal has been worn off the hollow parts by which the letters are surrounded.

When a coin of this kind, or what is still better, a coin on which an illegible trace of the letter still remains, is placed upon a heated iron, it will be seen that an oxidation takes place over its whole surface, the film of oxide changing its tint with the intensity or continuance of the heat. The parts, however, where the letters of the inscription had existed, oxidate at a different rate from the surrounding parts, so that these letters exhibit their shape, and become legible in consequence of the film of oxide which covers them having a different thickness, and therefore reflecting a different tint from that of the parts adjacent. The tints thus developed sometimes pass through many orders of brilliant colours, particularly pink and green, and settle in a bronze, and sometimes a black tint, resting upon the inscription alone. In some cases the tint left on the trace of the letters is so very faint that it can just be seen, and may be entirely removed by a slight friction of the finger.

When the experiment is often repeated with the same coin, and the oxidation successively removed after each experiment, the film of oxide continues to diminish, and at last ceases to make its appearance. It recovers the property, however, in the course of time. When the coin is first placed upon the heated iron, and consequently, when the oxidation is the greatest, a considerable smoke rises from the coin, and diminishes like the film of oxide by frequent repetition. A coin which had ceased to give out this smoke, smoked slightly after twelve hours exposure to the air, having been removed from the hot iron at the beginning of that interval, and replaced upon it at the end of it by a pair of pincers.

From a great number of experiments I have found that it is always the raised parts of the coin, and in modern coins the elevated ledge round the inscription that oxidate first. This ledge, in an English shilling of 1816, began by exhibiting a brilliant yellow tint before it appeared on any other part of the coin.

In examining a number of old coins, a brilliant red globule, accompanied with a smell of sulphur, appeared on one or two points of the coin; and sometimes small globules, like those of quicksilver, exuded from the surface. Other coins exhal