It must not be forgotten that the liquid state of these aqueous combinations is often an accident of temperature; alum and the rhombic phosphate of soda are liquids at 212° F., and bi-hydrated sulphuric acid is a crystalline solid below 46° F. The ease with which many of these compounds are destroyed by evaporation, and even by changes of temperature, is not to be urged as an objection to the chemical nature of the union. We need only compare the corresponding silver salts with the chlorid and iodid of gold, or the hydrochlorates of morphia and ammonia with those of caffeine and piperine, which lose their acid by a gentle heat, to learn how variable is the stability of admitted chemical compounds. Chemical affinity may be very feeble in degree.

According to Gay-Lussac one part of oil of vitriol will absorb from air saturated with moisture, fifteen parts of water, or more than eighty equivalents; terchlorid of arsenic requires eighteen equivalents of water to dissolve it, and the saturated solution unites with as much more water, evolving heat and forming a stable solution.* According to the experiments of Mr. Griffin in the paper cited above, the condensation which takes place in the solution of the acid is still perceptible with 6000 equivalents of water to one of SO3. There appears however to be with many bodies a limit beyond which the affinity for water is satisfied, and the liquids being then mechanically mixed, gradually separate by reason of their difference in density, as is observed in dilute alcohol, and probably in some saline solutions† and metallic alloys.

Solution is a result of that tendency in nature which constantly leads to unity, condensation, identification. I have elsewhere with Kant defined chemical union to be interpenetration, but the conception is mechanical, and therefore fails to give an adequate idea. The definition of Hegel, that the chemical process is an identification of the different, and a differentiation of the identical, is however completely adequate. Chemical union involves an identification not only of the volumes, (interpenetration mechanically considered,) but of the specific characters of the combining bodies, which are lost in those of the new species. Such is equally the case in aqueous solution, and we may say that all chemical union is nothing else than solution; the uniting species are as it were, dissolved in each other, for solution is mutual.

* Penny and Wallace, L., E. & D. Phil. Mag., Nov., 1852, p. 363.

+ See Gmelin's Handbook, Eng. ed., vol. i, p. 111. Gmelin throws a doubt upon these experiments; but the satisfactory results obtained on a large scale, in applying this principle to the rectification of spirit of wine by a recently patented process, were communicated to the American Association for the Advancement of Science, at Washington in May, 1854, by Dr. L. D. Gale.

Stallo's Philosophy of Nature, p. 453. See also p. 67, where Stallo insists upon the same view, To Hegel belongs the merit of having first among modern philosophers obtained a just conception of the nature of the chemical process, although in its application he was misled by the received terminology of the science.

Solution being then identification, the discussion as to whether metallic chlorids are changed into hydrochlorates when dissolved in water, is meaningless. Such a solution is a unity, in which we can no more assert the existence of the chlorid or of water, than of chlorine, hydrochloric acid, or a metallic oxyd, although these and many others are conceivable results of its differentiation. If the solution be one of chlorid of potassium, evaporation resolves it into water and the chlorid, but if chlorid of aluminum, it is decomposed by boiling into water, hydrochloric acid, and alumina, or in the case of the magnesian salt, into hydrochloric acid and an oxychlorid.

The precipitation of the sulphates of cerium, lanthanum and lime from their solutions by heat, and of most other salts by cold, is chemical decomposition or differentiation. Dilution may also effect decomposition in solutions; we have already said that the combination of terchlorid of arsenic AsCl3, with 36HO is stable at ordinary temperatures, but a further addition of water causes the solution to divide into aqueous hydrochloric acid, and crystalline oxyd of arsenic. The precipitation of chlorid of antimony, and many salts of bismuth and mercury by water, is an analogous process. This decomposition of the solution of chlorid of arsenic is an example of what is called double elective affinity, (attractio electiva duplex,) and is generally explained by saying that the attraction of arsenic for oxygen, and that of chlorine for hydrogen, enable the chlorid and water to decompose each other. But these elemental species do not exist in the solution, although they are possible results of its decomposition, and to explain the process in this manner is to ascribe it to the affinities of yet unformed speries.

I have elsewhere asserted that double decomposition always involves union followed by division,* although we cannot in every case arrest the process at the first stage. Under some changed conditions of temperature and pressure, the decomposition may be the counterpart of the previous union, and thus reproduce the original species, as in the case of mercuric oxyd, which is decomposed into mercury and oxygen at a temperature a little above that at which it was formed. When the division takes place in a sense different from the union, giving rise to new species, we have double decomposition. In the case of chlorid of arsenic, the aqueous solution exhibits the first stage of the process. similar condition of unstable union is observed in many other instances; thus binoxyd of manganese gives with cold hydrochloric acid, a brown solution, but the combination is by a gentle heat resolved into chlorine gas, and a rose-red solution of protochlorid of manganese. So a mixture of equivalent parts of

* Consderations on the Theory of Chemical Changes, etc., cited above.

A

chlorid of benzoyl and benzoate of soda combines at a temperature of 130° C., to form a limpid solution, and it is only on raising the temperature that the precipitation of sea-salt indicates the commencement of that decomposition which yields at the same time anhydrous benzoic acid.* It is only when looked upon as a momentary combination followed by a decomposition, that the theory of double decomposition becomes intelligible, and in accordance with known facts.

From the narrow limits of temperature which often include the two processes, and from the ease with which light, warmth, friction and pressure excite the decomposition of such bodies as the chlorid of nitrogen, the nitrite of ammonia, the oxyds of chlorine, and the metallic fulminates, we may conceive that within still narrower limits, and under conditions as yet undefined, many bodies may exhibit affinities for each other, which are reversed by a very slight change of condition. In this way we may explain many of those obscure phenomena hitherto ascribed to action by presence or catalysis.

Montreal, Nov. 10, 1854.

ART. XIV.-Correspondence of M. Jerome Nicklès, dated Paris, Nov. 3, 1854.

Obituary. The patriarch of French Botanists, M. Brisseau de Mirbel, has just died at an advanced age. For many years he had been dead to science as well as to his family and friends. He came out, like many others illustrious in science, during the French Revolution, and was active in promoting the progress of the Science of Botany at the commencement of this century. He first introduced into France the study of the microscopic anatomy of plants. The microscope which more. than a century before had furnished important results to Grew and Malpighi, had long been left, in France especially, among physical apparatus, and was hardly applied to the Natural Sciences. M. de Mirbel, engaged in this fertile line of research, with very imperfect instruments, and from the commencement of his investigations in 1801, aimed to found the department of the comparative anatomy of plants, by study. ing for this object a number of families of acotyledonous and monocotyledonous plants.

In early youth he devoted himself with success to painting, and was intimately acquainted with the celebrated artist Girard. His knowl edge of painting was afterwards of great use to him, enabling him to sketch well what he observed, as may be seen especially in his researches on the structure of the seed and embryo of different plants of the family of Labiatæ, etc.

M. Mirbel was a member of the Academy of Sciences from the year 1808. His work entitled "Eléments de Botanique" in 1815, led to his

* Gerhardt, Ann. de Ch. et de Phys. 3me Serie, tom. xxxvii, p. 299.

appointment as Professor in the Faculty of Sciences at Paris, succeeding Desfontaines. This was the first period of his scientific life. His intimate friend Duke Decaze having been named Minister of the Interior (in 1816), he accepted the position of general Secretary, which he held till 1824. If he did not publish works during this time, he performed an important service to science by using his influence in bringing back from exile men of science who had become victims of political vicissitudes at the Restoration; and through him also funds were given to the Museum of Natural History, to render the institution useful to travelling naturalists.

Returning to private life, he took up again his researches in physiol. ogy. His new labors possessed a novelty, an exactness, and perfection, which was hardly expected of a savant, who had been so long a stranger to the progress of the science. What was especially surprising, was the profound difference between his new views and those of his youth, and also his noble frankness in acknowledging any inexactness or too positive assertions in his former works. His memoirs on the development of the ovule, on the structure of the Marchantia, on the formation of the embryo, on the arrangement and mode of formation of the tissues in the stems and roots of monocotyledonous plants, on the cambium, were elaborated in this new period; and they were the occasion of spirited discussions with M. Gaudichaud, then young, whom science has lost during the present year. M. Mirbel did not long continue in this new career. He fell into imbecility, and continued in this state until his death.

Astronomical Refraction.-A memoir by M. Faye, in which he endeavors to show a defect in the existing theory of astronomical refraction and proposes a formula for correcting it, has led to an interesting discussion which has already continued two months. All the astronomers and the principal physicists have taken part. M. Biot does not adopt the innovation, and his third memoir has just been read, opposing the view that it is necessary to add to the theory the coefficient of terrestrial refraction. M. Faye has nevertheless many partisans, and the issue of the discussion does not appear doubtful.

Constitution of the Sun; Solar Magnetism.—Mr. Thomson, one of the physicists, who with Carnot, Clapeyron, Joule, Meyer and others, have most largely contributed towards establishing the relations between heat and mechanical force, has extended his researches to the heat emitted by the sun; and he observes that this heat corresponds to a development of mechanical force, which, in the space of about 100 years is equivalent to the whole active force required to produce the movement of all the planets. The author examines successively the different sources of heat, and ends by concluding that the solar heat can have no other than a meteoric origin, and that it results from the motion of meteors which fall into the sun-an idea first put forth by M. Waterston at the meeting of the British Association at Hull. Whatever may be the value of this hypothesis, we may ask whether it would not be more simple to admit that the solar heat proceeds simply from the rotatory movement of the sun; Mr. Thomson admits himself that the rotation is necessary to the production of the heat. It is known that the sun moves on its axis, and what use is this intervention of ineteorites, which nothing justifies ?

This idea of deriving the heat from motion, which was rejected more than thirty years ago, suggests the hypothesis which assigns an analogous origin to terrestrial and hence to planetary magnetism, an hypothesis of which we have spoken on several occasions in this Journal. But, at that time, the question of solar magnetism was still under discussion, which, researches undertaken by M. Secchi, director of the Observatory at Rome, have now established on evidence. The sun, which is a source of heat, and a source of light, is then a source also of magnetism; heat, light, electricity and magnetism, have then a common origin-matter in motion.

Optics-Manufacture of Glass for Objectives.-In the manufacture of glass for the lenses of telescopes, the vitreous mass whep brought to a liquid state in a crucible, is stirred in order to render it homogene. ous, and to expel the air it may enclose. But this result is never fully attained; there are always numerous streaks in the mass, which cause the loss of a large part of the material, and hence the difficulty of obtaining lenses of large dimensions.

M. Peyronny, captain in the corps of Engineers at Cherbourg, proposes to avoid these difficulties, by giving the crucible a rapid rotatory movement around a vertical axis; the centrifugal force tends to bring all the bubbles of air about the centre of the melted mass, whilst the streaks caused by the stirring mostly disappear, and those remaining are circular and feeble, and also little objectionable if the axis of the mass be made the axis of the lens.

Polarization of the Atmosphere, &c.-There are several kinds of apparatus for exhibiting the phenomena of polarization. Besides the po lariscopes of Biot, Arago, Savart, Guérard, Delezenne, Soleil, there are the polariscope of Babinet and the chromatic clock of Wheatstone: but we have not, properly speaking, an apparatus for measuring easily and rigorously the quantity of polarized light contained in a ray or in a given luminous field. M. Bernard, Professor of Physics at Bordeaux, has resolved this problem in a manner satisfactory to the most critical physicists of the Academy of Sciences of Paris, and those at the recent session of the British Association, where his apparatus was exhibited. Through the assistance of the theories of Fresnel and Arago, and also profiting by the discoveries of Babinet and Beer of Bonn, M. Bernard has constructed an instrument of extreme delicacy, which is managed with great ease, and requires but two minutes for an observation. The same physicist has constructed a Refractometer, for determining, to the 4th decimal, the index of refraction of solid bodies, and for liquids arranged as a medium with sensibly parallel surfaces. He has also contrived a universal photometer, of which we shall give details in another communication.

Microscopes for Micrographic demonstrations, by Nachet.-Those who use the microscope in instruction, know the difficulty of adjusting in the field of vision, the objects to which they would call attention, and are aware how convenient it would be could they exhibit to several students at once, the part of an object to which attention may be directed with a needle. The microscopes of Nachet realize this object, and have been employed by Prof. Milne Edwards for a year in his lec* January 1854, p. 116, and November, 1854, p. 386. SECOND SERIES, Vol. XIX, No. 55-Jan., 1855.

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