reflector. No dark division can as yet be seen on the ring of Saturn; but it is hardly open enough to expect it to be visible. Aug. 5. 1791; 20-feet reflector. The black list, on this side of the ring of Saturn, is exactly in the same relative place where it was seen on the northern plane. - Sept. 25. 1791; 20-feet reflector. The black division goes all around the ring, as far as he can trace it, exactly in the same place where he used to see it on the north side. - Oct. 13. 1791; 10-feet reflector. The black division on the southern plane of Saturn's ring is in the same place, of the same breadth, and at the same distance from the outer edge, that he had always seen it on the northern plane. With a power of 400, he saw it very distinctly: it is of the same kind of colour as the space between the ring and the body, but not so dark. · Oct. 24. 1791; seven-feet reflector. With a new machine-polished, most excellent speculum, he saw that the division on the ring of Saturn, and the open spaces between the ring and the body, are equally dark, and of the same colour with the heavens about the planet. 20-feet reflector. The black division on the ring was as dark as the heavens. It was equally broad on both sides of the ring. With a 40-feet reflector, he saw the division on the ring of Saturn of the same colour as the surrounding heavens. It was of equal breadth on both sides, and he could trace it a great way towards the body of Saturn. With a 20-feet reflector, and power of 600, he could trace the division very nearly as far as the place, where a perpendicular to the direction of the ring would divide the open space between the planet and the ring into two equal parts.

From these observations, added to what has been given in some former papers, Dr. H. thinks himself authorised now to say, that the planet Saturn has two concentric rings, of unequal dimensions and breadth, situated in one plane, which is probably not much inclined to the equator of the planet. These rings are at a considerable distance from each other, the smallest being much less in diameter at the outside than the largest is at the inside. The dimension of the two rings and the intermediate space are nearly in the annexed proportion to each other.

Inside diameter of the smaller ring
Outside diameter

Inside diameter of the larger ring
Outside diameter

Parts. - 5900

7510 - 7740

- 8300

805

280

115

Admitting, with M. de la Lande, that the breadth of the whole ring, as formerly supposed to consist of one entire mass, is near one third of the diameter of Saturn, it follows that the vacant space between the two rings, according to the above statement, amounts to near 2513 miles.

By way of forming more easily a comparative idea of the stupendous size of this ring of Saturn, Dr. H. calculated the proportion it bears to the earth, and found that its diameter is to that of the latter as 25.8914 to 1. From the above proportions we also compute that this ring must be upwards of 204,883 miles in diameter.

Experiments and Observations on the Production of Light from different Bodies, by Heat and by Attrition. By Mr. THOS. WEDGWOOD.-[1792.]

He found that the phosphorism of almost all bodies might be made apparent either by heat or by attrition; he therefore divides the subject of this paper into two parts. i. On the light produced by heat.-ii. On the light produced by at

trition.

i. The best general method of producing the light by heat is, to reduce the body to a moderately fine powder, and to sprinkle it, by small portions at a time, on a thick plate of iron, or mass of burnt luting made of sand and clay, heated just below visible redness, and removed into a perfectly dark place. The following is a list of such bodies as he found to be luminous by this treatment, arranged according to the apparent intensity of their light.

1. Blue fluor, from Derbyshire, giving out a fetid smell on attrition.

2. Black and grey marbles, and fetid white marbles, from Derbyshire. Common blue fluor, from Derbyshire. Red feldspat, from Saxony.

3. Diamond. Oriental ruby. Aerated barytes, from Chorley, in Lancashire. Common whiting. Iceland spar. Sea shells. Moorstone, from Cornwall. White fluor, from Derbyshire.

4. Pure calcareous earth, precipitated from an acid solution. Pure argillaceous earth (of alum). Pure siliceous earth. Pure new earth, from Sydney Cove. Common magnesia. Vitriolated barytes, from Scotland. Steatites, from Cornwall. Alabaster. Porcelain clay of Cornwall. Mother of pearl. Black flint. Hard white marble. Rock crystal,

from the East Indies. White quartz. Porcelain. Common earthern ware. Whinstone. Emery. Coal ashes. Sea sand.

5. Gold, platina, silver, copper, iron, lead, tin, bismuth, cobalt, zinc. Precipitates by an alkali from acid solutions of gold, silver, copper, iron, zinc, bismuth, tin, lead, cobalt, mercury, antimony manganese. Vitriolated tartar, crystals of tartar, borax, alum, previously exsiccated. Sea-coal. White paper, white linen, white woollen, in small pieces, white hair powder. Deal saw-dust. Rotten wood (not otherwise lu minous). White asbestos. Red irony mica. Deep red porcelain.

6. Antimony, nickel. Oils, lamp, linseed, and olive, white wax, spermaceti, butter, luminous at and below boiling.

The duration of the light thus produced from different bodies is very unequal; in some the light is almost momentary, in others it lasts for some minutes, and may be prolonged by stirring the powder on the heater. It soon attains its greatest brightness, and dies away gradually from that point, never appearing in a sudden flash, like the light of quartz-pebbles rubbed together. If blown on, it is suddenly extinguished, but immediately re-appears on discontinuing the blast.

The light of bodies is, in general, uncoloured; there, are however, some exceptions. Blue fluor, of that kind which gives out a fetid smell when rubbed, first emits a bright green light resembling that of the glow-worm so exactly, that when placed by the insect just as it has attained its greatest brightness, there is no sensible difference in the two lights, either of colour or intensity. This bright green quickly changes into a beautiful lilac, which gradually fades away. Fetid marbles, and some kinds of chalk, give a bright reddish or orange light; pure calcareous earth, a bluish white light; Cornish moorstone emits a fine blue light; powder of ruby gives a beautiful red light, of short continuance.

For the most part, the softest bodies require the least heat to become luminous; marble, chalk, fluor, &c. give a faint light when sprinkled on melted tin just becoming solid. As the temperature of the heater is raised, they continue to give out more and more light. Vitriols of iron, copper, and zinc, previously exsiccated, when thrown on earthern ware or metal made nearly red-hot, give minute flashes of light of momentary duration, such as appear from some of the metallic precipitates, particularly zinc, on a similar treatment; with this difference, however, that the light of most of the precipitates is of a reddish hue. The light of the metals is white, and exactly similar to that of some earths.

ii. The experiments on the light produced by different bodies by attrition were chiefly made by rubbing in the dark two pieces of the same kind against each other: all that were tried, with a very few exceptions, were luminous by this treatment. The following is a list of them, arranged in the order of the apparent intensity of their light, and as the lights are either white, or some shade of red, figures are affixed to denote these differences; (0) denoting a pure white light; (1), the faintest tinge of red, or flame colour; (2), a deeper shade of red; (3) and (4), still deeper shades.

Rock-crystal, quartz, flint-glass, and many other hard bodies during attrition, emit now and then reddish sparks of a vivid light, which retain their brightness in a passage of one, two, and even three inches, through the air.

Bodies give out their light the instant they are rubbed on each other, and cease to be luminous when the attrition is discontinued. Colourless, transparent, and semi-transparent bodies emit a flashing light, their whole masses being, for a

moment, illuminated; opaque bodies give little more than a defined speck of red light, and are not luminous below the part struck. The greatest apparent quantity of light is produced by hard uncoloured, transparent, and semi-transparent bodies, whose surfaces soon acquire an asperity by rubbing together, as quartz, agate, &c.

From an examination of the table, it appears that white lights are emitted from colourless transparent bodies; faint red, or flame-coloured, from white semi-transparent bodies; deeper red from more opaque and coloured bodies, and the deepest red from opaque and from deep-coloured bodies. Extremely faint lights, such as those given by fluor, marble, &c. are of a bluish white; quartz, very lightly rubbed, gives a very faint light of a bluish hue; when rubbed a little harder, it emits a flame-coloured light; when rubbed with violence, its light approaches to whiteness. Opaque red feldspat gives a deep red light by attrition; exposed to a strong heat in the furnace, it becomes white, and somewhat transparent, and when cool, gives out, on attrition, as white a light as quartz; clear, blackish flint, made opaque by heat, gives a redder light than before; deep-coloured glass gives out a red defined light without any flash, while clear uncoloured glasses emit a white flashing light of some brightness.

Bodies are not luminous by simple pressure; but when they are at all broken by the pressure, the fragments rubbing on each other produce some light. Mr. Boyle, indeed, found a particular diamond to emit light when pressed by a steel bodkin; but the diamond is phosphorescent in so many ways, and is so curious and singular a body, both in properties and constitution, that it can scarcely be expected to exhibit the same appearances as the common class of earthy bodies.

All hard earthy bodies emit a peculiar smell on attrition.. The most remarkable for this property are chert, quartz, feldspat, biscuit earthen-ware, and rock-crystal: this smell does not differ much in kind, though it does considerably in intensity. Many of the softer bodies yield the same smell, but in a less degree, and probably none are entirely without it. It appears to be strongest where the friction is greatest: it has no dependence on the light produced by attrition, as it is often very strong when no light is emitted. Rock-crystal, quartz, feldspat, white biscuit earthen-ware, and probably all such hard bodies, produce this smell under water. Quartz stones, violently rubbed on each other for a few minutes in a cup of water, communicate this smell, and a peculiar taste, to tlre