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nited. Mr. Pigott's comet, for instance, of the year 1781, seemed to have some kind of nucleus ; though its magnitude was so ill defined, that I probably over-sated it much, when, November 22, I guessed it might amount to 3 or 4" in diameter. But, even this, considering its nearness to the earth, proves it to have been very small.

That of the year 1783, also discovered by Mr. Pigott, I saw to more advantage, in the meridian, with a twenty-feet reflector. It had a small nucleus, which, November 29, was coarsely estimated to be of perhaps 3" diameter. In all my other pretty numerous observations of comets, it is expressly remarked, that they had none that could be seen. Besides, what I have called a nucleus, would still be far from what I now should have measured as a disk; to constitute which, a more determined outline is required.

1. In the second article, their motions differ much from that of comets; for, so far as we have at present an account of the orbits of these new stars, they move in ellipses which are not very eccentric.

• Nor are the situations of the planes of their orbits so much unlike those of the planets, that we should think it necessary to bring them under the third article of comets, which leaves them quite unlimited.

• In the fourth article, relating to the direction of their motion, these stars agree with planets, rather than with comets.

"The fifth article, which refers to the atmosphere of comets, seems to point out these stars as belonging to that class; it will, however, on a more particular examination, appear that the difference is far too considerable to allow us to call them comets.' P. 226.

In fact, the smallest coma of a comet exceeds that of Ceres or Pallas above a hundred times; and neither moves in orbs eren approaching the eccentricity of a parabola, or is distinguished by a tail. It is also highly probable that the neclei of comets are very small: they never disturb the planetary motions, though often disturbed by them.

Why then are not these bodies planets? We see no reason for any distinction : they revolve round the sun, and are not comets. We must discover another system, before we are allowed to change the appellation. Mr. Herschel would call them asteroids; but he labours for a distinction, which, in the end, will fail him.

I shall now give a definition of our new astronomical term, which ought to be considerably extensive, that it may not only take in the asteroid Ceres, as well as the asteroid Pallas, but that any other aste. roid which may hereafter be discovered, let its motion or situation be whatever it may, shall also be fully delincated by it. This will stand as follows.

• Asteroids are celestial bodies, which move in orbits either of little or of considerable excentricity round the sun, the plane of which may be inclined to the ecliptic in any angle whatsoever. Their motion may be direct, or retrograde; and they may or may not have considerable atmospheres, very small comas, disks, or nuclei.

.As I have given a definition which is sufficiently extensive to take in future discoveries, it may be proper to state the reasons we have for expecting that additional asteroids may probably be soon found out. From the appearance of Ceres and Pallas it is evident, that the discovery of asteroids requires a particular method of examining the heavens, which hitherto astronomers have not been in the habit of using. I have already made five reviews of the zodiac, without detecting any of these concealed objects. Had they been less resembling the small stars of the heavens, I must have discovered them. But the method which will now be put in practice, will completely obviate all difficulty arising from the asteroidical appearance of these objects; as their motion, and not their appearance, will in future be the mark to which the attention of observers will be directed.' P. 229.

We shall not extend our article by enlarging on our own original idea, that these bodies may have been comets constrained to revolve within less eccentric orbits; because, in reality, we know little of the muclei of comets, and have no criterion hy which we can neasure their density, nor indeed, very correctly, their diameters. The suspicion may remain on record, to be tried by future observations, with little solicitude, in the author, respecting its truth or fallacy.

IX. Description of the Corunduin Stone, and its Va. rieties, commonly known by the Names of Oriental Ruby, Sapphire, &c.; with Observations on some other mineral Substances. By the Count de Bournon, F.R.S.'

The count de Bournon has considerably enlarged our views in mineralogy, by tracing the adamantine spar in a great variety of precious stones. In fact, it is the basis of all that are stried Oriental--an appellation not perhaps exclusively confined to the country which offers them to our notice, but attributed to gems of a peculiar hardness: yet it will appear that these are generally of eastern origin. Our author describes the corundum somewhat too diffusely. The imperfect, which seems divided into laminæ, is of a greyish colour, and is found in the Carnatic. This is less hard than the more perfect coloured stones, which scarcely yield to the diamond in this respect. The red Oriental ruby is very hard, but somewhat exceeded by the sapphire.

• This substance emits pretty bright sparks, when struck with a piece of steel ; but they are by no means proportioned to its hardness. If a piece of flint be struck with the same force, the sparks it produces are more numerous, as well as more bright; and it is possible to obtain sparks from flint, by a very slight blow, such as would not be sufficient to prodace them from perfect corundum. It is also necessary, in order to obtain sparks from corundum, that the storie should have pretty sharp edges: if the part that is struck is obtuse, it is with some difficulty that any sparks can be obtained. The imperfect corundum, however, ha:, in this respect, some advantage over the perfect kind.' 8.249.

This relation to light and heat deserres particular notice ; but we must also remark, that, like quartz, it becomes phosphorescent by friction. The mean specific gravity of numel'ous specimens was 3931. The specific gravity of the purer coloured stones is the biglest, and may perhaps be reckoned at about 4000. The primitive crystal is a rhomboid : its angles 94-86. The various modifications of its crystals are very minutely described. The fracture is parallel to the faces, in the angles just mentioned. In the imperfect corundums, in particular circumstances, the terminal faces are chatoyant, sparkling like a cat's-ere, from the light seemingly pervading an external lamina, and reflected from an internal one. The following singular appearance we shall add, with the explanation, in tiie author's own words.

"To the above property must also be referred, that beautiful re. flection of light, in the form of a star with six rays, which is frequcally given, by cutting, to Oriental rubies, sapphires, &c. and which causes those stones to be then called by the name of star-stones. The manner of cutting which brings the perfect corundum into this state is, most commonly, on the part of the lapidary, rather the result of chance, than the consequence of any determined theory respecting the cause of the effect he means to produce. Accordingly, in the greater number of the stones which have this property, the point from whence the starry reflection procecds, instead of being in the middle of the stone, is observed to be situated in a part more or less near to its base; a circumstance which considerably diminishes the beauty of the star-stone. The reflection which produces this effect, arises from the same cause as that of which we have alrcady spoken, and proceeds from the same part of the stone; consequently, when an Oriental ruby, or a sapphire, which has the qualities necessary for the purpose, is intended by the lapidary to be formed into a star-stone, he ought to make his section pass below that part of the stone which he has found to correspond with the summit of the primitive rhomboid. As the kind of cutting most proper to produce this effect in the stone, is that rounded form which is called on cibochon, with as high an ellipsis as is possible, the lapidary ought, at the same time, to take great care that the summit of this ellipsis be situated exactly under the point which corresponds with the summit of the rhomboid ; in that case, the light reflected in the interval of the lamine upon the three edges of the primitive rhomboid, and upon the middle of its three faces, will trace upon the stone, a star, the six rays of which will include the circumference of the rounded part, or ellipsis. The same effect may also be made to take place on one of the solid angles of the base, but in 2 much less perfect manner.

"I have met with many fragments of sapphires, as will as of Ori. ental rubies, which naturally produced the effect he're spoken of, in consequence of their having been broken, by chance, in a manner proper to occasion it; that is, they were broken, accidentally, in a direction contrary to that of the laminæ, and perpendicular to an axis passing through the tio summits of the pyramid of the primitive rhomboid; after which, the fragment had been a little rounded by friction.' P. 273.

Thc corundum, in its analysis, offers a large proportion of alumine, with a pretty stedy proportion of iron, both in the ruby and sapphire, but with a more variable one in the imperfect kinds. The iron, in the Chinese corundum, amounts to more than 0.06, while in that of the Carnatic, it exceeds only in an equal quantity 0.01.

The compact corundum, which has no traces of crystallisation; its matrix, which is a sand-stone approaching a coarse marble; and the substances which occasionally ac. company the corundum, are next noticed. There', which OCCII! in the cornndum of different regions of the East, are particularly described, and furaish many very curious anil important mineralogical remarks. We regret that it is impossible to pursue them within any reasonable limits. This singular stone seems peculiar to India, though there are reasons to think that it occurs in America and some of the mountains of France.

X. Analysis of Corundun, and of some of the Substances which accompany it; with Observations on the Affivities which the Earths have been supposed to have for each other, in the humid Way. By Richard Chenevix, Esq. F.R.S. and M R.I.A.'

In this paper we find strong confirmation of the analysis of corundum, and a firm support of its small proportion of siler--a circumstance which powerfully supports the idea, long since suggested in this journal, that hardness does not depend on ingredients, but on the rapidity and close union of the crystallisation. The following chemical remarks, though not peculiarly or immediately connected with the subject, merit particular attention,

As the greater part of the above substances were fusible without difficulty in potash, I preferred using a silver crucible to any other. It may be laid down as a general rule, with respect to delicate experie ments, that in the treatment of metallic substances, we should not use metallic crucibles ; but, in the treatment of eartby bodies, they alons are to be depended upon. The easily oxidizable metals cannot be emplo; ed ; but silver and platina present advantages which no other metals seem to possess. Theory would certainly give a general preference to platina, from its resistance both to heat and to acids; and practice will justifv this preference, in all but a single instance. If a quantity of potash be kept for some time in fusion, in a platina crucia ble, it will be found that the crucible has lost several grains of its weight. The platina so dissolved may be looked for in the potash; and, if this he saturated with muriatic acid, and evaporated, we shall find the well-known triple salt, formed by the combination of muriaric acid with potash and oxide of platina. This action of potash upon plaa

tina, does not depend upon any mechanical cause, such as friction, the force that determines it being purely chemical. If a salt formed by potash, or a sait formed by ammonia, be mixed with a salt of platina, a precipitate ensues, which is a triple salt; and it is by this method, that the Spanish government detects the platina, in the ingots of gold sent from their American possessions. It is therefore evident, that an affinity does exist between potash and platina, in a certain state ; and I imagine it to be this affinity, which causes the oxidizement of the platina, when potash is kept in fusion upon that metal. I must however observe, that my crucible was prepared by Janetty, in Paris, according to a method he has published in the “ linnales de Chimie ;” and that he always employs arsenic, a little of which certainly remains united to the platina. What influence arsenic may have, remains to be determined. Soda does not form a triple salt with the oxide of platina ; for I have frequently kept this alkali in fusion, in a platina crucible, for a long time; yet very little action was produced upon the metal. This fact seems to corroborate my assertion, that the affinity of potash for oxide of platina, determines the oxidizement of the metal.

" Whenever I suspected that platina had been dissolved, I could ea. sily detect the smallest portion of it. A solution of platina, so dilute as to be nearly colourless, manifests, in a very short time, the colour of a much more concentrate solution, and becomes reddish, by the addition of a solution of tin in muriatic acid. This I have found to be, by many degrees, the most sensible test for platina; and it would answer the purposes of the Spanish government, much better than that they usually employ.

* The alkalis have no immediate action upon silver: but I have observed, that crucibles of this metal, after they have been a long time in nse, become somewhat more brittle than they were before.

• Potash and soda have long been termed fixed alkalis; and it is certain that, if we compare them with ammonia, they are so. But fixed is an sbsolute term, and cannot admit of degrees. If potash, such as we obtain from Mr. Berthollet's method of preparing it, be kept in fusion at a very strong heat, it may be totally volatilized. The vapour of the alkali may be perceived in the room; and vegetable colours will undergo the change which is usually produced by alkalis. Indeed, in preparing Mr. Berthollet's potash, the vapour of the alkali may be easily perceived. Soda is not quite so volatile; though far from being fixed. It appears aiso, that a little water increases the volatility of both potash and soda, as happens with boracic acid. This volatility of poiash has been advantageously applied of late to the art of bleaching.' P. 336. . The article concludes with some remarks on the affinities the earths have been supposed to possess for each other in the humid way. This section relates to a slight controversy, carried on in the Annales de Chymie, which we have already noticed. M. Chenevix communicated some experiments to tlat collection, which were opposed by M. Guyton. The Jatier was again opposed by a young chemist, whom ve

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