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pletely carbonic acid from sulphurated hydrogen. It is particularly in those cases in which we wish to use hot solutions, that the elastic-gum-bottle will be found a valuable addition to the eudiomettical apparatus.

On the Quantity of Carbon in carbonic Acid, and on the Nature cf the Diamond, 'by W. Allen, Esq. F.L.S. and W. H. Pepys, Esq.—One of the most important of the labours of the celebrated Lavoisier was the decomposition of carbonic acid. The process which he employed was apparently conducted with much accuracy, and the result which he obtained from it was for some, time universally received. It appeared to be confirmed by the rc searches of Mr. Tennant on the nature of the Diamond: but, in consequence of the experiments afterward performed by M. Guy ton on the combuftion of this body, it was imagined that Livoisier had been mistaken in his idea of the nature of carbon; and a different estimate of the composition of carbonic acid, deduced from Guyto;i's experiments, was generally adopted. As the determination of this question is not only in itself of considerable consequence, but also leads to many" important conclusions on other parts of chemical science, the authors of this memoir proceedtd to its investigation, after having taken every possible precaution to ensure the accuracy of the results.

The apparatus .which they employed consisted of two gazo* meters, connected together by a tube of platina, in which the carbonaceous matter might be subjected to combustion. Into one of the gazometers, a quantity of oxygrnous gas was introduced; and while the carbon was heated, the gas was brought into contact with it, by being several times in succession passed from one gazometer to the other. Charcoal, diamond, stonecoal,' and plumbago, Were each subjected to this kind of process. In the actual execution of the experiments, the greatest nicety appears to have been observed; and after an attentive perusal of the paper, we do not perceive a single circumstance' which can warrant the suspicion of inaccuracy. We shall not attempt to enter on a detail of the individual experiments; deeming it sufficient to quote the results of the whole, as stated by the authors, and only remarking that the conclusions appear to be fairly deducible from the facts:

'ist. That the estimate given by Lavoisier, of a8 parts of caibon in every 100 parts of carbonic acid, is very nearly correct; the mean of our experiments makes it 28,60.

* 2dly. That the diamond is pure carbon ; for had it contained any notable proportion of hydrogene, it must have been discovered, either by detonating with the oxygene, as in the case of animal charcoal, or by diminishing the quantity of oxygene gas.

• 3dly. That

• jdly. That well burnt charcoal contains no sensible quantity of hydrogene; but if exposed to the air for a few hours it absorb* moisture, which renders the results uncertain.

'4tlily. That charcoal can no longer be considered as an oxide of carbone. because, when properly prepared, it requires quite as much oxygene for its combustion as the diamond. This is also the case with stone coal and plumbago.

* r,thly. It appears that diamond and all carbonaceous substances (as far as our present methods of analysis are capable of demonstrating their nature) differ principally from each other in the state of aggregation of their particles Bkkihollet has well remarked, that in proportion as this is stronger, decomposition is more difficult; and hence the variety of temperatures requiied for the combustion of

different inflammable substances.' x

An Account of the Relistian Tin-Mine, by Mr. Jos. Carne, of Penzance. The circumstance which renders this tin-mine an object of curiosity is the occurrence of a body of chlorite pebbles, imbedded partly in chlorite schist, and partly in the chrystallized oxyd of tin.

An Analysis of the Waters of the Dead Sea, and the River Jordan, by Alex. Marcet, M.D., and one of the physicians of Guy's Hospital. — Although the nature of the water of the Dead Sea is in itself an object of mere curiosity, yet the account of its analysis which is given in this paper is so well drawn up, and the process seems to have been conducted with so much accuracy, that we have perused it with considerable interest. The most remarkable property of the water of the Dead Sea is its great specific gravity, which Dr. Marcet found to be 1.2115 'a degree of density,' he observes, • scarcely to be met with in any other natural water.' The following propositions contain an account of its other peculiarities:

* 2. The water of the Dead Sea is perfectly transparent, and does not deposite any crystals on standing in close vessels.

'3. Its taste is peculiarly bitter, saline and pungent.

• 4 Solutions of silver produce from it a very copious precipitate, showing the prrsence of marine acid.

'5. Oxalic acid instantly discovers lime in the water.

* 6. The lime being separated, both caustic and carbonated alkalies readily throw down a magnesian precipitate.

4 7. Solutions of barytes produce a cloud, showing the existence of sulphuric acid.

• 8. No alumine can be discovered in the water by the delicate test of buccinic acid combined with ammonia.

'9. A small quantity of pulverised sea salt being added to a few drops of the water, cold and undiluted, the salt was readily dissolved with the assistance of gentle trituration, showing that the Pcad Sea is not saturated with common salt,

'10. None

'10. None of the coloured infusions commonly used to ascertain the prevalence of an acid or an alkali, such as litmus, violet, and turmeric, were in the least altered by the water.*

In order to discover with precision the quantity of the saline ingredients, the author made some experiments for the purpose of ascertaining the Composition of the different salts which it was supposed to contain, particularly the magnesian salts. Proceeding on these data, he found that 100 grs. of the water contain muriate of lime 3.920 grains, muriate of magnesia 10.146 grs., muriate of soda 10.360, and sulphate of lime 0.054 grs., amounting in the whole to very nearly one quarter of the weight of the water itself.—The water of the Jordan seems to contain the same kind of ingredients, but in a very diluted state.

Philosophy and Astronomy-.

Experiments for investigating the Cause of the cilottref con'centric Rings discovered by Sir Isaac Newton, between tw* Object Glasses laid upon each other, by William Herschell, LL-D. F.R.S.— Probably all of our philosophical readers are acquainted with the curious experiments made by Newton on the rings of colours which are produced by pressing together a convex and a concave lens nearly of the same curvature. Round the point of contact, alternate coloured and dark rings appear: the former produced by reflected light, the latter indicative of light transmitted. That illustrious author, who never stopped at the mere statement of a fact, but speculated concerning its cause, attributed these phenomena to certain affections or constitutions of rays of light, which he called fits of easy refection and transmission; and the fitsof the two sorts would take place at certain thicknesses of the air through which the rays had to pass: thus, at the thicknesses o, 2, 4, 6, f>, &c. the rays would be transmitted, and at the thicknesses I, 3, 5, 7, 9, they would be reflected. As the invention of a name is not the invention of a cause, this hypothesis and explanation leave us where they found us, at the fact; and certainly, a very curious fact it would be, if, at the respective thicknesses of air above specified, light should be alternately transmitted and reflected.

The learned and indefatigable author of the present memoir Is not satisfied with Newton's explanation; and he records a number of experiments instituted and varied with considerable skill, which are intended to make manifest some modifications of li^ln hitherto unobserved, to overthrow Newton's hypotheses, and to afford the ground and basis of a more just system.

In the beginning of his experiments, Dr. Herschell followed Newton's plan and direction, and he observed all the phenomena of concentric rings such as Sir Isaac had described: but, since Newton's experiments appeared to Dr. H. to be too much confined, he extended them; and of his own methods he gives an instance, and then what he calls a generalization. This pare will be understood by an extract:

First Method. On a table placed before a window I laid down a slip of glass the sides of which were perfectly plain, parallel, aud highly polished. Upon this I laid a double convex lens of »6 inches focal length, and found that this arrangement gave me a set of beautiful concentric rings.

4 I viewed them with a double convex eye lens of 2$ inches focus mounted upon an adjustable stand, by which simple apparatus I could examine them with great ease; and as it was not material to my present purpose by what»obliquity of incidence of light I saw the rings, I received the rays from the window most conveniently when they fell upon the lens in an angle of about 30 degrees from the perpendicular, the eye being placed on the opposite side at au equal angle of elevation to receive the reflected rays.

'Centralization. Instead of a plain slip of glass, the plain side of a plano-concave, or plano-convex lens of any focal length whatsoever may be used : and when the convex side of any lens is laid upon it, whatever may be the figure of the other surface, whether plain, concave, or convex, and whatever may be its focal length, a set of concentric rings will always be obtained. I have seen rings with lenses of all varieties of focus, from 170 feet down to one quarter of' an inch. Even a common watch glass laid upon the same plain surface will give them.'

In his second method, instead of the plane glass or the plane side of a plano-convex or a plano-concave glass, Dr. H. puts a convex reflecting mirror, and on that mirror he placed the double convex lens: then, again, instead of the plane mirror he substitutes a convex reflecting mirror; next a concave mirror, &c; and in all these cases, the concentric rings are seen. After a very particular description of the experiments, and oi every circumstance relating to them, the author in the latter division of his memoir approaches to the most interesting part of his disquisition, and states «considerations that relate to the cause of the formation of concentric rings ;' which cause, he intimates, is to be sought in the action of the surfaces, the Newtonian hypothesis of alternate fits being rejected as a merely plausible supposition. To shew that this hypothesis ought to be rejected, it is proved in the 30th article that concentric rings cannot be formed by an alternate reflection and transmission of the rays of light: in the 31st, that alternate fits of easy reflection and easy transmission, if they do

exist, exist, do not exert themselves according to various thicknesses" of thin plates of air; and in the 32d that these fits if" they exist, do not exert themselves according to various thicknesses of thin plates of ghss.—The 33d article relates an*experiment which seems destined, according to the intimation of the author, to conduct us to the true cause of the phenomena of coloured rings: 1

* The experiment I am now to relate was at first intended to be reserved for the second part of this paper, because it properly belongs to the subject of the flection of the rays of light, which is not at present under consideration; but as it p.irticulatly opposes

, the admission of alternate fits of easy reflection and easy transmission of these rays in their passage through plates of air or glass, by proving that their assistance in the formation of rings is not required, and also throws light upon a subject that has at different times been considered by some of our most acute experimentalists, I have used it at present, though only in one of the various arrangements, in which I shall have occasion to recur to it hereafter.

'Sir I. Newton placed a concave glass mirror at double its focal length from a chart, and observed that the reflection of a beam of light admitted into a dark room, when thrown upon this mirror, gave " four or five concentric irises or rings of colours like •* rainbows."* He accounts for them by alternate fits of easy reflection and easy transmission exerud m*their passage through the glass-plate of the concave mirrorf.

* The Duke De Chaulnes concluded from his own experiments of the same phenomena, "that these coloured rings depended upon the first surface of the mirror, and that the second surface, or that which reflects them after they had passed the first, only served to collect them and throw them upon the pasteboard, in a quantity sufficient to make them visible \."

* Mr. Brougham, after having considered what the two authors I have mentioned had done, says, "that upon the whole there appears every reason to believe that the rings are formed by the first surface out of the light which, after reflection from the second surface, is scattered, and passes on to the chart. |j"

* My own experiment is as follows. I placed a highly polished 7 feet mirror, but of metal instead of glass, that I might not have two surfaces, at the distance of 14 feet from a white screen, and through a hole in the middle of it one-tenth of an inch in diameter I admitted a beam of the sun into my dark room, directed so as to fall perpendicularly on the mirror. In this arrangement the whole screen remained perfectly free from light, because the focus of all the rays which came to the mirror was by reflection thrown back into the hole through which they entered. When all was duly

« * Newton's Optics, p. 265. f Ibid, p. 277.'

* X Priestley's History, &c. on the Colours of thin Plates, p. 515.' « || Phil. Trans, for 179(3, p. 21S.'

prepared,

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