methyl-morphium, &c. Indeed it may be stated generally that, as far as observation goes, compounds of trebly related nitrogen exert an action totally different in kind from similar compounds of fivefold-related nitrogen, that a similar difference exists between the triatomic and pentatomic compounds of other members of the nitrogen family, and that this principle appears to be of still wider, and probably general application. The speaker, in conclusion, drew attention to the peculiar interest attaching to those regions of science which lie on the frontiers between two distinct departments, as on their successful exploration would depend the ultimate fusion of all physical sciences into one, the science of dynamics, the science which treats of matter and energy and their relations to one another. Such a fusion is probably very remote; but we now see in the border-land between chemistry and physics that slow process of absorption going on which has already converted the once independent sciences of sound, light, heat, electricity, and magnetism into more or less completely subjugated provinces of the great empire of applied mathematics. If we believe in the unity of the plan of creation, we must believe that this process will advance and ultimately triumph. LVII. Intelligence and Miscellaneous Articles. HAVING ON A MIRAGE IN THE ENGLISH CHANNEL. BY JOHN PARNELL, M.A., F.R.A.S.* AVING had the good fortune to witness an extraordinary mirage at Folkestone on the 13th of April in the present year, I venture to think that the following description of the phenomenon, so far as it passed under my observation, may not be uninteresting. During the morning, and up to 2 o'clock P.M., a dense fog had hung over the sea; but apparently it was not very deep, as the sun's rays penetrated it pretty freely. At the hour above mentioned the fog opened towards the S.E., disclosing the cliffs on the French coast; and in the course of a few minutes the fog had disappeared, leaving the atmosphere in a state of unusual transparency. The French cliffs were apparently so lofty and with every indentation so clearly visible, that one might easily have imagined that they were but ten miles distant. On examining the objects in view through a small telescope with a 25-power, it was at once apparent that this arose from something more than common looming. The French coast could be seen from near Calais towards the E. to far away and many miles beyond Boulogne towards the S.W., the land in the latter direction being ordinarily invisible, as it is situated below the horizon. Immediately under the erect image of the coast was an inverted one, of about double the height of the former. The lighthouse at Cape Gris Nez gave five images in a vertical line :-the lowest erect but somewhat magnified; above that and separated from it a pair of images of the centre and highest portion of the building only, one erect and the other inverted; and over these another pair, the inverted image being like the former one, but the * Communicated by the Author. erect image showing the whole building. Over Boulogne, in the air, were two images of the double funnels and the mast of a tugboat, the lower image being erect and the upper inverted, the two lines of smoke bending, the one upwards and the other downwards, and both towards the W., till they joined together. The only tugboat near Boulogne at the time, so far as I could learn, was in the harbour. The cathedral was plainly visible, but only gave a single image. Towards the S. W., and beyond the French coast, some fishing-luggers were observed hull down, so that the position of the horizon could be ascertained; up to 3 o'clock they presented no unusual appearance; but over these were pairs of images of vessels which ordinarily would have been invisible. In some instances three and even four pairs could be observed placed in a vertical line, the lower image in each pair being inverted. With the exception of the uppermost pair, the images seemed to represent the maintopgallant sail only, and that considerably elongated; but the highest erect image showed the mizen- and the fore masts and the jib, but in no instance could the hulls be seen. In all cases the inverted images were of about twice the height of the erect. Soon after 3 o'clock vessels between the observer and the horizon began to be affected. The Varne light-ship, which is about 8 miles from the English coast, had her mast-flagstaff and stanchions elongated to some three times their proper length: this effect lasted for about ten minutes, when they shrank to less than half their usual size, and the hull began to rise till it was nearly as high as it was long, and formed a most conspicuous object even to the naked eye. I then looked towards Dover: the pier seemed completely disorganized; it appeared to be divided in half longitudinally, with the sea in the midst, and the stone coping moved as if huge waves were agitating it. A steam-boat entering Dover harbour was shrunk to less than half her proper vertical dimensions, but elongated horizontally. Captain Paull, of the S.E.R. steam-boat Napoleon III.,' crossed the Channel between the hours of 2 and 4; and he told me that he saw Beachy Head during the passage, a circumstance which had never previously occurred during the many years that he has been on the Folkestone and Boulogne route. At 4 o'clock the atmosphere had returned to its normal condition. The place of observation was about 30 feet above high-water mark. The barometer on the day in question stood at 30.33 in., thermometer 58° F.; wind S.W., very light at 2 o'clock and dropping to a calm; the electricity of the air unusually high, 185 by a Thomson's portable electrometer (with which five cells of Grove give a potential of 3); and the ozone was 7 of Negretti's scale. Hadham House, Upper Clapton. April 20, 1869. ON THE PRODUCTION OF A BEAUTIFUL PATINA ON BRONZES IN LARGE TOWNS. In almost all large towns, especially in those in which coal is used as a combustible, it has been found that bronzes exposed in public places, instead of becoming coated with a patina (vert antique), have Phil. Mag. S. 4. Vol. 37. No. 250. May 1869. 2 D a dark and dirty appearance like that of cast iron. The desire to remedy this led the Berlin Verein zur Beförderung des Gewerbfleisses to cause comparative experiments to be made with a view to find, if possible, a remedy. First of all the question had to be solved, whether the production of a beautiful patina is dependent upon the composition of the bronze. For this purpose ten samples of bronzes, remarkable for their beautiful patina and taken from different places, were analyzed. Each of the samples was divided into two, and intrusted to two different chemists. The results were given in the Proceedings of the Verein for the year 1864. They showed that the bronzes investigated were of very varying composition. The proportion of copper varies between 94 and 77 per cent. The quantity of tin amounts in one to 9 per cent., in others to only four; while others, again, only contain 0.8 per cent., but up to as much as 19 per cent. of zinc. The other accidental admixtures, such as lead, iron, nickel, vary in like manner. With the most different composition these bronzes have all a very beautiful green patina. The composition may possibly have some influence on the time requisite for the production of the patina; but the experiments leave no doubt that it ensues even with the most different composition. To ascertain the effect of other influences, a number of bronze busts were placed in a part of the town in which particularly unfavourable exhalations take place, and in which various bronze statues in the neighbourhood are destitute of any trace of patina, but have the above-mentioned unpleasant black exterior. It was observed that, on several monuments in places accessible to the public, the parts liable to be handled had, if not a green, an otherwise beautiful patina, while all the other parts were black and unsightly. This led the Commission to the idea that grease probably influenced the production of the patina. Hence one of the busts was rinsed every day, with the exception of rainy days, and was, moreover, once a month painted over with bone-oil, which was immediately rubbed off with woollen cloths. Another bust was washed daily with water, but was not oiled. A third, which was also cleaned daily with water, was only oiled twice a year. The fourth was left unpurified, and, indeed, was not at all touched. The first- and the last-named bust have been set up since 1864, and treated in the manner described; the second and third since 1866. The supposition as to the action of the fat has been most completely confirmed. The one which has been oiled once a month possesses a dark green patina, which is considered to be very beautiful by all connoisseurs. The one which is only rubbed twice a year does not look so well; and that only cleansed with water has none of the beautiful appearance which bronzes obtain by the deposit of patina. The one which was not at all cleansed is quite dull and black. Hence we may expect that if a bronze exposed in the open air, after being purified, be rubbed down with oil, it will assume a beautiful patina. How far this rubbing, which in many cases will be difficult to perform, may be restricted is to be decided by continued experiments which have already been commenced. The Verein has also exposed two bronzes which have been artificially patinated by chemical means, in order to ascertain how they are influenced by similar treatment. How the oil works in the formation of the patina, cannot with certainty be stated. Experiments have shown that any excess of oil is to be avoided, and that which has been painted on must be rubbed off as much as possible. If excess of oil remains, the dust deposits on it, and the bronze acquires an unsightly appearance. It cannot be assumed that the residual small quantity of oil enters into a chemical combination with the layer of oxide, especially as bone-oil acts just as well as olive-oil in the experiments. Probably the film of oil only acts in preventing the adherence of moisture, by which dust adheres, gases and vapours are absorbed, and in which vegetation forms. Whatever be the mode of action, the experiments leave no doubt that the fat is essentially connected with the formation of the patina. It is probable that in other respects it will be advantageous. It has been observed that bronzes covered with a beautiful patina, in those parts where water trickles down assume a white, opaque, chalky surface, which in course of time is more and more washed away by the water. Proper treatment with oil will doubtless prevent the formation of these chalky places; yet only long-continued experiments can settle this point. In any case this use of oil justifies the hope that for the future we may retain beautifully patinated monuments even in large towns. Where coal is the only combustible, they will not be bright, but dark green, and perhaps even black; but they will have the other beautiful property of the patina, the peculiar transparent condition of the surface.-Poggendorff's Annalen, April 1869. TYNDALL'S COMETARY THEORY. To the Editors of the Philosophical Magazine and Journal. GENTLEMEN, Streatham Hill, April 8, 1869. Professor Tyndall's interesting cometary theory, which appeared in your last Number, turns on an assumption which is hardly compatible with the laws of motion. His assumption is that the tail of a comet is "matter precipitated on the solar beams traversing the cometary atmosphere." If so, the linear dimensions of a comet must exceed the length of its tail-that is, in some cases 60 millions of miles or more. Now it seems utterly incredible that such a bulk of vapour can whirl round the sun at the perihelion passage unbroken. A very slight modification of Tyndall's theory avoids this difficulty, and equally well explains all the observed appearances. If there be an extremely attenuated solar atmosphere extending considerably further than the earth's orbit, and if, when a comet approaches the sun and is therefore exposed to intense heat, its volume becomes comparable with the volume of the sun, the heat of the sun will be shut off from that portion of the solar atmosphere in the shade of the comet (which though transparent to light is opaque to heat), and actinic clouds will be formed in the solar atmosphere, thus giving the appearance of a tail to the comet. If the shape of the comet be irregular there may be more than one tail. The comet after its perihelion passage recedes from the sun and at the same time gradually cools and contracts; the tail therefore ought to be greatest and brightest at or shortly after the perihelion, and then slowly diminish and fade away. This is in accordance with observation. Your obedient Servant, ERNEST CARPMAEL, Scholar of St. John's College. GENTLEMEN, In the last Number of the Philosophical Magazine Professor Tyndall has propounded the theory that the visible head and tail of a comet is an actinic cloud, resulting from the decomposition of vapour by the solar light. This theory accounts for the extreme tenuity of comets, their polarization, and the motion and development of their tails. Now if all our knowledge of comets were derivable from observations with the unassisted eye, this theory would account for the observed phenomena; and, as it is, it is valuable by increasing our knowledge of matter which, if not cometary matter, at all events has striking analogies with it; but I do not think it affords any explanation of cometary phenomena as observed by the telescope. In the first place, immediately behind the nucleus, where, according to this theory, we should expect a very luminous region, we commonly have a dark space. Also the matter which forms the tail usually streams out from the head towards the sun; this was especially noticed in Halley's comet in 1836. Bond also, speaking of Donati's comet, says, "the material, after being thrown off from the nucleus, instead of at once being driven into the tail, formed a dense cloud of nebulosity into which the luminous matter continued for some time to stream. This cloud extended itself on the sunward side, remaining in its vicinity for several days. When it had acquired a certain stage, the discharge took place mainly from the corners or cusps on either side in two streams, which, coalescing with those issuing from other envelopes, formed the two branches of the tail." Then, too, we have several series of envelopes which rise up towards the sun. In Donati's comet seven were detected; in the great comet of 1861 no less than eleven were noticed: the force which causes the ascent of these is intermittent and finally dies away. Mr. Webb, in 1861, noticed the descent of the envelopes on the nucleus; also Herschel and Schroeter noticed the same in the comet of 1811. From these facts I am disposed to think that although Professor Tyndall's hypothesis accounts for some of the phenomena, yet the true theory is still to be discovered. I am, Gentlemen, Talfourd Road, Peckham, Your obedient Servant, W. B. GIBBS, F.R.A.S. |