ammonia, and consequently an ammoniacal mercurio-muriate. On the first supposition, charcoal must have been deposited; and on the second, carbonic acid must have been formed. But neither the one nor the other of these took place, though the liquid assumed a light-brown colour. "M. Gay-Lussac has not spoken of this phenomenon; probably because having employed less acid it did not take place in his experiments. However in another experiment, in which I employed only 30 grammes of muriatic acid, I obtained only about two grammes of hydrocyanic acid, possessed of all the properties described by Gay-Lussac. The residue of the operation contained likewise ammoniaco-mercurial muriate, though I had conducted the process with much caution. "It is singular that having some time afterwards repeated this process twice, I did not obtain the triple mercurial salt. I do not know to what I ought to ascribe this difference. It is possible that in the first processes, in which the apparatus was arranged the evening before, the cyadide of mercury having remained long in contact with the muriatic acid before being subjected to the action of heat, the hydrocyanic acid underwent a decomposition. "The observations which I have just stated respecting what passes sometimes between muriatic acid and cyadide of mercury, would be of little importance unless they were to lead to a better method of obtaining that acid. "Considering that mercury has a strong attraction for sulphur, and that cyanogen unites easily to hydrogen when presented in the proper state, I thought that sulphuretted hydrogen might be employed for decomposing dry cyadide of mercury. I operated in the following manner: I made a current of sulphuretted hydrogen gas disengaged slowly from a mixture of sulphuret of iron, and very dilute sulphuric acid pass slowly through a glass, tube slightly heated, filled with cyadide of mercury, and communicating with a receiver cooled by a mixture of salt and snow. As soon as the sulphuretted hydrogen came in contact with the mercurial salt, this last substance blackened, and this effect gradually extended to the furthest extremity of the apparatus. During this time no trace of sulphuretted hydrogen could be perceived at the mouth of a tube proceeding from the receiver. As soon as the odour of this gas began to be perceived, the process was stopped; and the tube was heated in order to drive over the acid which might still remain in it. The apparatus being unluted, I found in the receiver a colourless fluid, which possessed all the known properties of dry prussic acid. It amounted to nearly the fifth part of the cyadide of mercury employed. "This process is easier and furnishes more acid than the one by means of muriatic acid. I repeated it several times, and always successfully. It is merely necessary to take care to stop the process before the odour of the phuretted hydrogen begins to be perceived, otherwise the hydrocyanic acid will be mixed with it. However, we may avoid this inconvenience by placing a little carbonate of lead at the extremity of the tube. As dry hydrocyanic acid is only required for chemical researches, and as it cannot be employed in medicine, in which that acid in a dilute state begins to be used, I think it may be worth while to bring to the recollection of apothecaries a process of M. Proust, which, perhaps, has escaped their attention. It consists in passing a current of sulphuretted hydrogen gas through a cold saturated solution of prussiate of mercury in water,* till the liquid contains an excess of it; to put the mixture into a bottle in order to agitate it from time to time, and finally to filter it. "If the hydrocyanic acid, as almost always happens, contains traces of sulphuretted hydrogen, agitate it with a little carbonate of lead and filter it again. "By this process we may obtain hydrocyanic acid in a much greater degree of concentration than is necessary for medicine. It has the advantage over the dry acid of being capable of being preserved a long time, always taking care to keep it as much as possible from the contact of air and heat." Conclusions. From the important set of experiments of which we have just given an account, M. Vauquelin draws the following conclusions: "1. Cyanogen dissolved in water is converted into carbonic acid, hydrocyanic acid, ammonia, and a peculiar acid, which may be called cyanic acid, and into a charry matter. This happens in consequence of the decomposition of water. These new compounds arrange themselves in the following order: The ammonia saturates the acids, producing soluble ammonical salts; while the insoluble charry matter is deposited. "2. The alteration produced by the alkalies, strictly so called, in the constitution of cyanogen is exactly of the same nature as the preceding; that is to say, there are formed hydrocyanic acid, carbonic acid, probably cyanic acid, charry matter, and ammonia, which is disengaged in consequence of the presence of the other alkalies. This is the reason why the solu tion of cyanogen in an alkali gives at once (as Gay-Lussac has observed) prussian blue with the acid solutions of iron. "3. The common metallic oxides produce the same effects on cyanogen dissolved in water as the alkalies, but with different degrees of rapidity, according to the affinity which each of them has for the acids developed. But in this case triple salts are formed, as we have shown when treating of oxide of iron and oxide of copper; so that cyanogen, similar in this respect to * Experience has shown that a solution in the proportion of a gros (59.06 gr. troy) of cyadide of mercury to an ounce of water gives a hydrocyanic acid suffi ciently strong to be employed in medicine. This is the strength of the acid employed by MM. Hallé, Magendie, &c. chlorine, cannot combine directly with metallic oxides; and there are formed a hydrogenated acid and oxygenized acid, because cyanogen is a compound, while chlorine is simple. "4. Cyanogen is capable of dissolving iron without forming prussian blue. This is proved by the fine purple colour produced in the solution by the infusion of nutgalls. But as we find prussian blue in the portion of iron not dissolved, it is not quite certain that the iron is dissolved by the cyanogen: it is more probable that it is by the cyanic acid. On this supposition the water must have been decomposed: hydrocyanic acid must have been formed which would unite with the iron, and cyanic acid which likewise dissolving iron holds it in solution. Perhaps ammonia and carbonic acid are likewise formed. "5. Hydrocyanic acid forms prussian blue directly both with iron and its oxide without the presence either of acid or alkali; therefore, prussian blue is a hydrocyanate of iron. "6. Whenever cyadide of potash is in contact with water, ammonia is formed, which combines with carbonic acid formed at the same time. Hence it happens, that a great quantity of cyadide of potash gives only a small quantity of hydrocyanate; a great part of it being changed into ammonia and carbonic acid. "7. It appears to result from the preceding experiments, that the metals which, like iron, are capable of decomposing water at the ordinary temperature, form only hydrocyanates; while those incapable of decomposing that liquid form only cyadides. Among these last are silver and mercury; though mercury may possibly be an exception. "8. Finally, all my experiments confirm the beautiful results obtained by Gay-Lussac on the composition of cyanogen and hydrocyanic acid; extending the consequences of them." SIR, ARTICLE IV. On Parhelia, &c. By William Burney, LL.D. (To Dr. Thomson.) Gosport Observatory, Nov. 26, 1818. I AGAIN take the liberty of sending you some further remarks on Parhelia, to show that, with a vaporous atmosphere, they may be seen in the open day within a certain altitude, as well as early in the morning. I have been lately gratified with a sight of the Aurora Borealis, two Paraselenæ, and several meteors. Descriptions of these rare phenomena I herewith inclose for the Annals of Philosophy, should they be deemed deserving of a place; and am, Sir, your obedient servant, WILLIAM BURNEY. Parhelia and Paraselena, with Solar and Lunar Halos, and their Effects; the Aurora Borealis; and coloured Meteors; seen at Gosport. Oct. 17.-At half-past seven, a.m. a solar halo, 44° in diameter, appeared, and at its eastern edge there was a coloured Parhelion of the same altitude as the sun; a thin Cirrostratus was in the vicinity of, and a close corona round the sun at the time. The barometer, which had been rising, began to sink in two hours after the appearance of the Parhelion, till a shower of rain descended the next day, by the inosculation of Cirrostratus and Cumuli, thus indicating a change in the weight of the atmospheric column. 198 28. Parhelia. From a quarter till half-past eight, a. m. two Parhelia appeared, each being 22° 30′ distant from, and of the same altitude as, the sun: the first Parhelion in the S.S.E. S. was remarkably bright, with the usual prismatic colours (increasing and decreasing at intervals), and apparently as large as the moon in a horizontal view, and somewhat like her full illuminated disc when rising of a golden colour over a bank of haze near the horizon. The second in the E.S.E. E. progressively increased in size and colours till the first disappeared, but was not so large nor so bright, nor did it continue so long in sight: it was of the apparent size of the disc of the real sun when about 18° in altitude. The vivid red, yellow, pale blue, and silvery colours of the first, were no doubt increased from the sun being hidden, and from his direct rays being confined by a Cirrostratus, except at the very point of formation of the mocksun, which just cleared the edge of that cloud, in an apparently clear but vapourous space. No solar halo was perceptible at the time; but a circular, whitish light, or corona, about 34° in diameter, appeared round the sun, in consequence of the vapourous state of the lower atmosphere. Height of the barometer, 30.05 inches; of the thermometer, 53°; hygrometer of De Luc, 88°; and the wind at S.W. At nine o'clock an arched band of plumose Cirrus passed over to the eastward, followed immediately by an overcast sky, and some light rain fell in the afternoon. On the following day and night, the sky was completely shrouded with Cumulostratus. 31.-A stormy day, except two hours' sunshine in the afternoon. Aurora Borealis.-From 11 till midnight there was a fine display of the Aurora Borealis between the N.N.W. and N.E. points. Some of the beams were very brilliant, and of cylindrical and conical shapes; they ascended about 28° above the northern horizon, and varied in colour, according to the density of the medium through which they passed. The horizontal light was most extensive, tending to the magnetic east and west at 36 minutes past 11. During the appearance of these corusca tions, several small meteors fell almost parallel to the largest pillars of light-a circumstance much in favour of Mr. Dalton's theory of the phenomena. The air was serene at the time, and there were some dark longitudinal Cirrostrati interspersed in different parts of the sky. Nov. 3-At eight, a. m. a faint Parhelion appeared for a few minutes in the S.S.E. point on a Cirrus that was passing to a Cirrostratus cloud; it was 22° 30′ distant from, and of the same height as, the sun; and a small part of a halo passed through it perpendicularly. Some Cirrus clouds, just above the sun, were beautifully tinged with most of the prismatic colours at the time. Much Cirrocumulus passed over from the southward in the course of the day; and bright and dark hemispherical and pyramidal Cumuli appeared in different quarters. A copious dew at night, and a sinking barometer. 4, 5, and 6.-Rainy days and nights, with variable winds. 13. Parhelia.-At a quarter past nine, a. m. a Parhelion appeared in the S.E. by E. E. point, 22° 35' distant from the centre of the sun's disc, whose altitude was 12° 25′3′′. At 11 o'clock a Parhelion appeared in the S. by W. point, at the same distance as the first from the sun, which was at that time 20° 2′3′′ high. Part of a solar halo passed through the first; but no part of one could be traced at or near the second; the vésicular vapour upon which it was formed being scarcely perceptible. These Parhelia were of the same altitude as the real sun, and of the apparent size of his disc; but they enlarged as their colours (red, yellow, sea-green, and pale blue) approximated to perfection. At half-past three, p.m. another Parhelion appeared in the S.W. by W. point, 23° distant from, and perpendicular to, the sun, which was 8° above the horizon: this one was situated on the top of part of a solar halo upon an attenuated Cirrostratus cloud; but its colours were not so well defined as those which formed the Parhelia above-mentioned. There was a faint corona close round the sun during their appearance; and as a proof of the vapourous state of the atmosphere, the index of the hygrometer of De Luc kept within the range of from 80° to 88° all day. A sunny day, with plumose Cirri, Cirrocumuli, and Cirrostrati. Meteor.-At a quarter past seven in the evening, a low meteor moved slowly from the E. by N. to the N.E. by E. point, or through a space of about 220, in a direction parallel to the horizon: its densest part was like the bluish colour which surrounds the wick of a lighted candle, and it left some large electric sparks behind. Light rain and wind in the night. 14.-A stormy day; a strong gale, with heavy rain from the westward at night. 15. A continuation of the gale till noon, with sunshine, Cirrocumuli and Cirrostrati. Between six and seven, the moon rose under a semi-halo; and when she had ascended 23°, an entire coloured halo surrounded her, and continued perfect till after midnight, having th earance of a lofty, circular, darkhibiting at its extreme ish canopy, suspended in , and |