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greatest height attained by a terrestrial aurora and by a solar red flame, in order to be able to assign the limit, not only of our own atmosphere, but also of that of our luminary.

One other point remains to be noticed in connection with magnetic disturbances, and this is, that there appear to be two separate disturbing forces, nearly opposite in character, both connected with the sun, which act simultaneously upon the magnet; the position which the latter assumes being due to the combined effect of both. This has been shown to be true by General Sabine, who has observed that the curve which exhibits the daily range of the east component of the disturbing force, is in many places very different in character from that which exhibits the same for the west component. And this difference between the two curves is of one kind at one station, and of another kind at another station. This duality of the disturbing forces may also be observed directly in the Kew disturbance-curves. Here it was shown, by means of models, kindly constructed by Mr. Beckley, and also by reference to the parallelopiped of forces, that whenever the corresponding peaks and hollows for the different components continue to bear a definite proportion to one another, these then denote the action of a disturbing force, varying in intensity, but always preserving the same type.

A set of curves were exhibited in which this proportion held, and in which the disturbing force, whose variations were denoted by the peaks and hollows, was one which affected the north and south component twice as much as the other two. It was then shown by reference to the normal line, or line of no disturbance, that there was also in action at that time another disturbing force, which was not however of the same variable character as that which caused the peaks and hollows.

The attention of foreign men of science has been much directed to the problem of terrestrial magnetism, and five sets of magnetographs, similar to those in operation at the Kew Observatory, have been already procured by foreign governments. These, however, will be placed in the northern hemisphere, and it is to be desired that some of our colonies in the southern hemisphere may come forward in order that by the next epoch of maximum disturbance (1869), there may be such a network of magnetic observatories as may enable us to obtain the solution of this interesting and important problem.

[B. S.]

WEEKLY EVENING MEETING,

Friday, March 27, 1863.

Sır HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. Vice-President,

in the Chair.

WILLIAM CROOKES, Esq. F.C.S.

On the Discovery of the metal Thallium.

THE speaker commenced by remarking that the discovery of a new metal was no novelty in this century. Since its commencement, our knowledge of the material world had been increased by the discovery of no less than thirty-two of these elements. Glancing rapidly at the already known metals, the speaker said that seven of these were known to the ancients, and we have no knowledge of their first discovery. We know, however, that many are found native, and the others are separated from their ores either by heat alone, or by heat and the simplest chemical agencies. Most of those of more recent discovery have been obtained by exclusively chemical means, the exceptions being those, the separation of which, by means of voltaic electricity, by Sir Humphry Davy, makes the commencement of this century a marked epoch in the history of chemistry, and also of the Royal Institution. From the time of Sir Humphry Davy no new method for the recognition of elements had been discovered until the researches of Bunsen and Kirchhoff gave to the scientific world a definite knowledge of analysis by means of the spectrum. By this means Bunsen discovered the two new alkali metals cæsium and rubidium, and by the same means the speaker was led to the discovery of Thallium.

The special history of the discovery of the last-named metal offered a curious parallelism to the discovery of selenium by Berzelius. The great Swedish chemist was engaged on the examination of a residue from a sulphuric acid manufactory, in which he was induced to suspect the presence of tellurium. A complete examination proved to him that tellurium was absent; but in the course of his experiments he succeeded in separating a new element, belonging to the sulphur group, to which he gave the name of Selenium. The speaker, three years ago, was occupied in the analysis of a similar residue from some sulphuric acid works at Tilkerode, also with a view to the separation of tellurium. The sulphuric acid residue had originally been placed at his disposal by Professor Hofmann, in the year 1850, for the purpose of extracting selenium from it. In the distillation of the crude

selenium, there was left behind in the retorts a residuum which a few chemical tests led the speaker to conclude contained tellurium. This was placed aside for further examination, and remained unnoticed until, in the beginning of the year 1861, it was re-examined with the object of preparing tellurium from it. Not succeeding in finding evidences of the presence of this metal by chemical means, the speaker had recourse to the recently discovered means of spectrum analysis ; but instead of noticing the alternate bands of light and shade characteristic of tellurium, he was surprised to observe a single green line of remarkable brilliancy and intensity, which had hitherto been unnoticed, and was communicated by no known element which could have been present in the residuum. (The spectrum of thallium was here projected on the screen, a 40-cell Grove's battery being used to produce the electric arc. Its green band appeared perfectly sharp and brilliant upon a dark background. Further researches soon proved to the speaker that he was, in fact, dealing with an entirely new element; but the quantity contained in the material under analysis was so minute that the complete isolation was a matter of great difficulty. In September, 1861,* however, and by means of the galvanic battery, he succeeded in precipitating the metal in a pure form ; to which, in consequence of the green band it communicates to the spectrum and to flame, he had previously given the name of Thallium, from Barros, a bud, the colour of early vegetation most nearly resembling the shade of green it gives. The metal was shown to friends soon after isolation, and its nature freely communicated; but no formal publication of this fact was made until the opening of the International Exhibition, on May 1, 1862, where, in a case deposited some days before, were displayed several grains of the new body, labelled and described as a heavy metal. . By the kindness of numerous friends, among whom may be mentioned Dr. Thornthwaite, Professor Chandelon, and Mr. Peter Spence, and by the munificence of the Royal Society, he has since found more prolific sources of the metal, and been able to work on a large scale.

While speaking of the metallic nature of Thallium, the speaker noticed the fact of the observation of the green line and separation of the metal, on the 16th of May last, independently of himself, by a skilful Belgian chemist, M. Lamy, who deposited a specimen in the International Exhibition in June, 1862.

Thallium, the speaker said, belongs to the class of heavy metals. Its specific gravity is 11:9, very nearly that of lead. In colour it most resembles cadmium. It possesses considerable lustre, but quickly tarnishes in the atmosphere. It is very soft, being easily scratched by lead. It is very malleable, and the speaker showed that it could easily be forced into wire, several lumps welding together in the cold into one solid rod. It marks paper like plumbago, but the mark quickly disappears by the formation of a light-coloured oxide, being reproduced however by the action of a soluble sulphide. Thallium, the speaker

• At the Lecture, the year 1862 was given inadvertently for 1861.-[W. C.]

showed, was strongly diamagnetic, being only inferior to bismuth in this respect; and Dr. Matthiessen had determined that its power of conducting electricity was very near that of lead. It is precipitated from its solution by means of the battery in beautiful crystals, the form of which has not yet been determined ; and when the experiment is carefully conducted, these spread over the dish in branches like a delicate seaweed, or with a stronger solution like the club-moss. The atomic weight of thallium is about 203. Its chemical properties are at first sight rather anomalous, and seem to justify the remark of M. Dumas, that it is the ornithorhynchus of metals. That eminent French chemist ranks it with the alkaline metals, with which indeed it has some common properties : but the speaker is inclined to class it with the heavy metals, lead and silver, to which it seems more nearly allied both by its chemical and physical properties : several of the chemical reactions of thallium were shown in corroboration of this view. Like the metals potassium and sodium, it forms a soluble oxide which is endowed with strongly alkaline properties; like them too it is not precipitated by sulphuretted hydrogen, nor in its lower state of oxidation by an alkali ; it also forms an insoluble platino-chloride, but here the resemblance ceases. All the other reactions are similar to those of the heavy metals. The soluble oxide is in reality also allied to the oxides of silver and lead (which are both soluble in water and alkaline), inasmuch as it has no affinity for water, being rendered anhydrous even at the common temperature in a vacuum. It is readily precipitated in the metallic form from its saline solutions by zinc. It forms an insoluble peroxide, sulphide, iodide, bromide, chromate and sulphocyanide, and a slightly soluble protochloride, sesquichloride, ferrocyanide and chlorate.

The alloys of Thallium have not yet been much studied. Th euses of the metal the speaker said he was hardly yet in a position to dwell upon. At present it would appear only to confirm in the most striking manner the value of spectrum analysis as a guide in chemical research. As an agent for the production of colour in pyrotechny, it was only equalled in brilliancy by the two other elements which give a monochromatic light, lithium and sodium. The speaker here illustrated the monochromatic nature of the thallium light, by throwing the magnified image of its electric arc on a bouquet of variously coloured flowers ; and concluded by thanking the audience for the indulgence they had extended to his first public discourse.

[W.C.]

Royal Institution of Great Britain.

WEEKLY EVENING MEETING,

Friday, February 6, 1863.

COLONEL PHILIP JAMES YORKE, F.R.S. in the Chair.

JAMES GLAISHER, Esq. F.R.S.
On Scientific Experiments in Balloons.

In the introductory part of the discourse, Mr. Glaisher gave a brief history of aërostation, from the discovery of the fire-balloon in the year 1782, by the two brothers Montgolfier, noticing some of the principal ascents made with fire-balloons. He then spoke of the discovery of Cavendish in 1776, viz. that hydrogen gas was fully ten times lighter than common air; when it immediately occurred to Dr. Black, of Edinburgh, that if so, a thin bladder filled with this gas ought to rise of itself: yet a period of several years elapsed before this obvious property was applied to the inflation of balloons ; not in fact till the success of the fire-balloon was established. He then spoke of the more remarkable ascents and experiments which were made with air-balloons within a few years of its discovery, and remarked, that the result of the several experiments having proved that the balloon would raise great weights in the air, and remain for a long time thus suspended, caused a general desire to navigate the lofty regions of the atmosphere, and to pursue meteorological and other investigations in those regions, and the invention of the balloon was looked upon as likely to be followed by great consequences. He then spoke of the different ascents which have been made in the interests of science : the first of which was that of Mr. Boulton, well known as the partner of the famous Watt, who constructed a balloon to which a match and serpent were attached, that the gas might explode in the air. The object was to determine whether the reverberating sound of thunder was caused by echo or by successive explosions ; the point remained unsettled owing to the shouting of the people; but it was thought that the sound did resemble thunder.

This experiment was made on December 26, 1784. No further experiments were made, so far as he knew, till the beginning of the present century, when in the years 1803 and 1804, Mr. Robertson Vol. IV. (No. 38.)

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