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This relation to light and heat deserves particular notice ; but we must also remark, that, like quartz, it becomes phos phorescent by friction. The mean specific gravity of numerous specimens was 3931. The specific gravity of the purer coloured stones is the highest, 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-eye, 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 the author's own words.
To the above property must also be referred, that beautiful reflection of light, in the form of a star with six rays, which is frequently 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 proceeds, 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 already 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 en cabochen, 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 lamina 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 a much less perfect manner.
I have met with many fragments of sapphires, as well as of Oriental rubies, which naturally produced the effect here 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 lamina, and perpendicular to an axis passing through the two summits of the pyramid of the primitive
rhomboid; after which, the fragment had been a little rounded by friction.' P. 273.
The corundum, in its analysis, offers a large proportion of alumine, with a pretty steady 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 accompany the corundum, are next noticed. These, which occur in the corundum of different regions of the East, are particularly described, and furnish many very curious and 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 Corundum, and of some of the Substances which accompany it; with Observations on the Affinities which the Earths have been supposed to have for each other, in the humid Way. By Richard Chenevix, Esq. F. R. S. and MR.I.A.'
In this paper we find strong confirmation of the analysis of corundum, and a firm support of its small proportion of silex-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 experiments, that in the treatment of metallic substances, we should not use metallic crucibles; but, in the treatment of earthy bodies, they alone are to be depended upon. The easily oxidizable metals cannot be employed; 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 justify this preference, in all but a single instance. If a quantity of potash be kept for some time in fusion, in a platina crucible, 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 be saturated with muriatic acid, and evaporated, we shall find the well-known triple salt, formed by the combination of muriatic acid with potash and oxide of platina. This action of potash upon pla
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 salt 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 "Annales 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 easily 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 use, 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 also, that a little water increases the volatility of both potash and soda, as happens with boracic acid. This volatility of potash 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 that collection, which were opposed by M. Guyton. The latter was again opposed by a young chemist, whom we
have mentioned with the highest respect, M. Darracq; and our author now returns to the charge with additional experiments the result we shall transcribe.
From the experiments which I have related, it appears to be proved,
1st. That there exists an affinity between silica and alumina. 2dly. That there exists a very powerful affinity between alumina and magnesia.
3dly. That alumina shews an affinity for lime; but that the said affinity is not so strong as Mr. Guyton had supposed, nor, if pure reagents be used, is it to be perceived under the circumstances stated by him.
4thly. That Mr. Guyton was mistaken in every instance of affinity between the earths, excepting in the case of silica with alumina, which had been observed before his experiments; and that, in the other cases, he has attributed to a cause which does not exist, phenomena that must have resulted from the impurity of his reagents.
5thly. That neither the experiments of Mr. Guyton, nor the opinion maintained in the letter from Freyberg, are sufficient to diminish, in any degree, the value of the assistance mineralogy derives from chemical investigation.' P. 347.
XI. Description of the Anatomy of the Ornithorynchus Hystrix. By Everard Home, Esq. `F.R.S.'
This very singular animal again occurs in a new form, and its anatomy is peculiarly instructive and interesting. The ornithorynchus paradoxus, which seemingly formed a link between the birds and beasts-pretty certainly between the animals of the water and the earth, as the duck did between those of the water and the air-seems to have some congeners which merit particular attention. The O. hystrix belongs to the ant-eaters, and is noticed in the Zoology of Dr, Shaw (whose Lyncean eyes nothing escapes), under the name of myrmecophaga aculeata. This naturalist observes that it forms the link between the ant-eaters and the porcupines. It approaches also the manis; but this animal is found to be further removed from the ornithorynchus than the ant-eater: Even the myrmecophage are decidedly mammalia.
The peculiar characters of the ornithorynchus, as a genus, or more properly a tribe of animals, are,
The male having a spur on the two hind legs, close to the heel.
The beak being smooth, while the rest of the animal is covered with hair.
The tongue having horny processes, answering the purposes of teeth.
The penis of the male being appropriated to the passage of the semen; and its external orifice being subdivided into several openings,
so as to scatter the semen over an extent of surface, while the urine passes by a separate canal into the rectum.
The female having no common uterus; and the tubes which correspond to the horns of the uterus in other quadrupeds, receiving the semen immediately from the penis of the male.
These characters distinguish the ornithorynchus, in a very remarkable manner, from all other quadrupeds, giving this new tribe a resemblance in some respects to birds, in others to the amphibia; so that it may be considered as an intermediate link between the classes mammalia, aves, and amphibia; and, although the great difference that exists between it and the myrmecophaga, the nearest genus we are at present acquainted with, shows that the nicer gradations towards the more perfect quadrupeds are not at present known, the facts which have been stated may induce others to prosecute the inquiry, and render that part of the chain more complete.
Between it and the bird, no link of importance seems to be wanting.' P. 360.
Another species of ornithorynchus was shot at Van Diemen's Land it greatly resembled the O. bystrix.
XII. A Method of examining refractive and dispersive Powers, by prismatic Reflection. By William Hyde Wollaston, M. D. F. R. S.'
Our author's method may be styled, in a great measure, new it is at least new in its application. It depends on the principle of Newton's prismatic eye-glass; viz. the reflexion of light at the inner surface of a dense refractive medium.
Since the range of inclination within which total reflection takes place, depends not only on the density of the reflecting prism, but also on the rarity of the medium adjacent to it, the extent of that range varies with the difference of the densities of the two media. When, therefore, the refractive power of one medium is known, that of any rarer medium may be learned, by examining at what angle a ray of light will be reflected from it.
For instance, when any object is laid under a prism of flint-glass, with air alone interposed, the internal angle of incidence at which the visual ray begins to be totally reflected, and at which the object ceases to be seen by refraction, is about 39° 10′; but, when the object has been dipped in water, and brought into contact with the glass, it continues visible, by means of the higher refractive power of the water, as far as 57° of incidence. When any kind of oil, or any resinous cement, is interposed, this angle is still greater, according to the refractive power of the medium employed; and, by cements that refract more strongly than the glass, the object may be seen through the prism, at whatever angle of incidence it is viewed.
In examining the refractive powers of fluids, or of fusible substances, the requisite contact is easily obtained; but, with solids, which can in few instances be made to touch to any great extent, this cannot be effected without the interposition of some fluid, or cement,