count of its dearness, the latter on account of its injurious effect on the workpeople; and yet no substitute for them was discovered. M. Gaultier de Claubry has found out a very simple mode of rendering these dyes soluble in water. It may be remarked that their solution is attended with peculiar difficulties, on account of their being, in many instances, composed of constituents of very different solubility. Thus the aniline violet contains red elements which are soluble in various fluids, and blue, which are with difficulty soluble at all. During his researches, M. Claubry ascertained that there are many substances which impart to water the power of dissolving them. Thus gums, mucilages, soaps, glucose, dextrine, the jellies of different feculæ, lichens, and fuci. When, for example, Egyptian soap-wort is used, and it answers extremely well, it may be triturated with the colouring matter. Boiling water is then to be added in successive doses-each being removed before the next is added-as long as anything remains undissolved; the action of the different portions of fluid being aided by shaking, and the clear solutions obtained being removed by decantation. All the solutions must, in the end, be mixed together, as the earlier ones will take up chiefly the more soluble constituents: and thus a perfectly pure tint, quite unchanged by the process, will be obtained. The mode of proceeding may be modified in various ways. Thus, if it is considered desirable to use some alcohol, the soap-wort may be employed first, and the alcohol towards the end of the solution, or

vice versa.

PORTABLE SUBMARINE LIGHT.-The electric light has very often been used for producing illumination under water; the charcoal points being inclosed in a water-tight receiver, and the battery being placed in a boat, etc. But this arrangement is objectionable; it is very costly, the apparatus is extremely liable to disarrangement, and is difficult to be manipulated; moreover, the light, for many purposes is far too intense. M. Paul Gervais has devised a method of applying an apparatus, very similar to that proposed for use in mines, to submarine illumination. It consists of a Geisler tube, of an appropriate form, filled with carbonic acid, and inclosed in a stout water-tight glass cylinder, and capable of being put in connection, at pleasure, with a battery and coil, which are within a water-tight case that is easily portable. In constructing this apparatus, M. Gervais obtained the assistance of Ruhmkorff, who is also at present carrying out some improvements, that are expected to render it still more practically useful. The light emitted by it is said very closely to resemble that of phosphorescent animals, except that it is more intense; even when several yards under water it can be perceived at a considerable distance. The instrument used has been kept nine hours at a time under water, and six of these in action; indeed, there does not seem any limit to the time during which it may be in operation.

* INTELLECTUAL OBSERVER, No. xxxviii. p. 146.

IS LIGHT IMPONDERABLE?

WHAT do we mean when we speak of any object around us, and call it a ponderable substance, or one that has weight? Simply that the body in question attracts, and is attracted by the mass of our earth. No doubt it also attracts, and is attracted by, the moon, the sun, and other celestial bodies; but from their great distance their attraction is so much diminished, that we take no account of it in relation to masses of moderate dimensions. The tides result, as we all know, from the attractive force exerted upon the waters of our globe by the sun and moon; and if we could put the Pacific or the Atlantic ocean into a scale, the weight of those fluid masses, or in other words the amount of force with which they pressed towards the earth's centre, would noticeably vary, as it was more or less counteracted by opposing forces exerted by the sun and moon. The weight of bodies on the globe, or on any other planet whose form differs from a true sphere, varies according to its position. Our globe is a little flattened at the poles, and thus any body taken from the equator to either pole approaches a little nearer to the earth's centre, and is more powerfully attracted. The projecting equator, being larger in circumference than a circle near the poles, has to move faster than the latter as the earth rotates, or it would be left behind, which we know is not the case. Now the centrifugal force arising from rotation tends to throw any body off, while the gravitation attraction of the earth tends to pull it towards its centre. Hence the weight of a body, measured by the force with which it tends to the earth's centre, is diminished at any point at which the centrifugal force is increased. Thus two circumstances cause a difference in the weight of bodies moving from the equator to the poles. "Owing to the elliptical form of the earth, alone, and independent of the centrifugal force, its attraction ought to increase the weight of a body in going from the equator to the pole by almost exactly 9th part, which together with th due to the centrifugal force, makes up the whole quantity,th, observed."*

289

Weight, then, is simply a result of attraction; and if we desire first to weigh a body at the equator and then at the poles, in order to ascertain the difference, we must obtain an invariable standard of comparison. Counterpoising it with a weight or lump of metal clearly would not do. For, suppose we had one pound of iron as the thing to be weighed, and another pound of iron as the thing to weigh it by, it is obvious

*Sir J. Herschel, Outlines of Astronomy, p. 150, seventh edition.

that as they counterbalanced each other at the equator they would do so at the poles, for whatever alteration in weight resulted from their change of place would affect both alike. The tension exerted by a spiral spring affords, as Sir J. Herschel points out, an illustration of a kind of force not affected by change of position; and such an instrument might enable us to compare the attraction of the earth upon a given mass in the two situations described.

These preliminary remarks may help those who have not reflected on the subject to consider weight simply as the result of attraction; and, as our instruments are all comparatively clumsy, it is not right for us to assume that a particular body possesses no weight at all because we have not succeeded in weighing it. If light be a mode of motion of some fluid, that fluid may not, as is commonly supposed, be absolutely imponderable, that is, not at all affected by the earth's attraction. We have, indeed, no right to assume that gravitation is an essential property of all matter under all conditions. Astronomers trace what we ordinarily call the universality of this force; but that it is really universal—that is to say, that it exists wherever matter exists-is more than we know. It is common to speak of light, heat, electricity, etc., as "imponderable agents," but we apprehend no thinking man is satisfied with such a phrase. If they are all modes of motion of some kind of matter, either that matter must be ponderable, or gravity only a property manifested under certain conditions.

Mr. Balfour Stewart and Professor Tait consider, in the technical language employed by the former, "that to this time it has been assumed, without proof, that the change in the co-efficient of terrestrial gravity does not in itself alter any other co-efficient of a body; and if a reason be asked none can be given, since gravity is a force of the nature of which men of science are confessedly ignorant."

Now, if gravitation acts upon light so as to have any share in determining the position of any rays emerging from a prism, and forming a spectrum, a considerable change in the position of such a prism and of such light rays, involving a change in the force of gravitation, might cause a dark line in the spectrum to take a new position, more or less differing from that which it first assumed.

When this notion was propounded to Mr. Gassiot, he acted with his accustomed liberality in the cause of science, and requested Mr. Browning to construct a "rigid spectroscope." That is, such a spectroscope that the position of any given line could be exactly measured with minute accuracy, and with the certainty that its position would not be changed by the action of heat, or by the jolting inevitable in transport

ing the instrument from place to place. The difficulties of making such a spectroscope were very great; but they have been surmounted with marvellous dexterity and skill. A full description of this instrument will be found in the Proceedings of the Royal Society, No. 76.

It consists of two prisms of dense flint glass, made by Chance, and having a specific gravity of 3.9. These prisms are mounted on a bed of slate. Light reaches them through a slit, and straight through a portion of a right-angled prism, where the refraction is neutralized by a small prism cemented on to it. After passing through the two large prisms, and once suffering refraction and dispersion, it is sent back through them a second time by a third prism, having one side silvered, and thus it is again refracted and dispersed, one set of prisms being compelled to do double work. On its return journey, the light enters the prism over the slit and is reflected to a telescope at right angles to it, and carrying a micrometer. A series of trials in Mr. Browning's workshop, at Kew, and in the apartments of the Royal Society, have shown that the variation of the D line is quite infinitesimal, in spite of great changes of temperature and motion from place to place.

It was at first intended to send a "rigid spectroscope " up in a balloon; but the weight of the present form, and the uncertainty in balloon experiments of the prisms acquiring a uniform temperature, caused this idea to be abandoned, and Mr. Gassiot invites the Royal Society to request Her Majesty's Government to send the instrument on board some ship sailing between points on the earth's surface differing considerably in latitude, and, consequently, in the force of terrestrial gravitation. Should such experiments succeed, light must no longer be considered as the motion of an imponderable substance. Should they fail, it may still be held that success would be possible, if stations much nearer to, and much remoter from, the earth's centre, could be reached.

We must not be understood as stating that the contemplated experiments are attempts to weigh light; but if it can be proved that any change in the direction and force of gravitation affects the position of any line in the rigid spectroscope, the ponderability of that fluid of which light is a mode of motion might be probably assumed. Is gravitation merely a mode of force, correlative with other forces known and unknown?

ARCHEOLOGIA.

MR. J. T. BLIGHT, a zealous and careful antiquary, and very clever artist and engraver, of Penzance, has contributed to the July number of the Gentleman's Magazine an excellent though short paper on CORNISH BARROWS. The barrows to which Mr. Blight directs attention in this essay are of a peculiar class, large tumuli, containing at least one rather extensive chamber, and sometimes several smaller ones, formed of massive stones, and they are found chiefly in the West of England, in the Scilly Islands, in the Channel Islands, and in Brittany. Our own impression is that these tumuli are not of an extreme antiquity-they do not belong to a very barbarous population, but to one which has made advance in social progress, and has attained to about the social condition of the Anglo-Saxons, a little before they entered Britain, or of the Icelanders in the time of Burnt Njal. They belong probably to the Britons of the South-West, from no very remote period before the Roman period to some time after the establishment of the Roman power, and were the burial-places of the patriarchal heads of families of importance, where, probably, generation after generation was interred. Such barrows are not found in the central and then more barbarous parts of the island, because they were held by the older populations, while these districts had received an immigration of more civilized peoples. In Scilly they are called giants' graves a common popular name for anything ancient. In its simplest form, the receptacle of the dead in these tumuli is a mere square chamber, an expansion of the idea of the ruder cromlech, though more elaborately built; in its more enlarged character it took the form of a great subterranean gallery, as at Bolleit and Pendeen, in Cornwall. The chamber of the barrow at Pennance, in the parish of Zennor, described by Mr. Blight, is nine feet six inches in length, by four feet in width, and four feet four inches high. It is constructed in that bold sort of Cyclopean masonry which has prevailed in Cornwall down to the present day, the more massive stones forming the basement of the walls. That of the end of the chamber is formed almost entirely of one single slab. The roof consists of large slabs of granite thrown horizontally across from wall to wall. This chamber lay in a direction from north-west to south-east, the entrance being from the northeastern end, where there was no wall, and access was easily obtained by clearing away a part of the side of the mound. This mound is twenty-three feet in diameter, and eight feet in height. It is bounded by a circle of retaining stones, some of them of large dimensions, and is filled up with stones chiefly, mixed with some earth, piled around and over the cell. No remains were found in the chamber, or cell, of this barrow, which has been the case with others of the same class, probably in consequence of the depredations of people who, at some perhaps remote period, opened them in search of treasure. Mr. Blight describes the opening, at which he assisted, of two other Cornish barrows, on Conquer