of oblique solids, by introducing the idea of polarity in the axes of crystallization, Mr. Dana has successfully applied this molecular theory to crystallography, yet he goes no farther; and the most important and difficult steps in this branch of physical science still remain to be made, and many phenomena in crystallization, with the cause of which we are at present wholly unacquainted, still require to be explained by the theory. The author particularly refers to the important facts discovered by MM. Leblanc and Beudant, of the influence that solutions or mediums in which bodies crystallize have on the secondary forms which these bodies take; and states, that the present views of crystallography afford not even a glimpse of the least relation between such forms and the properties of the mediums. Why, he asks, does pure water appear, in general, to tend to simplify the forms, precisely as do certain mixtures, those of chlorite in axinite, quartz, felspar, &c., and why, on the contrary, do other mediums, acid or earthy, complicate them? Impressed with the importance which must attach to the solution of such questions, M. Necker offers some ideas which long meditation on this important subject has suggested to him. Adopting the ellipsoid as the form of the molecule, he remarks, that the more complicated the form of the crystal, the more the number of its faces increases, and the more, at the same time, does it approach to the ellipsoidal form of the molecule; and, on the contrary, the simpler the form becomes, the more does it recede from that with a curved surface. All crystalline forms may be considered as making a part of one or more series, which, in each system of crystallization, have for extreme terms, on the one side, the most simple solid of the system, or that which has the least number possible of faces, and on the other, the solid having the greatest number, namely a sphere or an ellipsoid. Although it is more convenient in the calculation of forms to start from the most simple polyhedral forms in order to arrive at the more complex, nothing proves that such has been the route which nature has followed. As long as we considered the integral molecules as polyhedral, it appeared natural to view them as grouping in polyhedrons; but when once we cease to admit polyhedral molecules, it then becomes most natural to suppose, that ellipsoidal molecules should have a tendency, more or less decided, to group in solids of the same form as themselves, when no extraneous circumstances interpose an obstacle to this tendency. In order to give an idea of the kind of effect which would be produced on the form of the solid by these obstacles, such as the nature of the medium in which crystallization takes place, a hurried or tumultuous crystallization, &c., the author conceives that each molecule, as well as each solid formed by their union, has different axes of attraction, endued with different degrees of energy, and symmetrically disposed in groups, the weaker and the most numerous round the stronger, which are, at the same time, the smallest in number; all, in short, symmetrically arranged around the principal axes of crystallization, which are the most energetic of all. Thus we shall conceive that sort of polarity by which crystallization is distin guished from molecular attraction. The effect of obstacles, such as the attraction exerted by mediums, by interposed bodies, by the molecular attraction of the molecules themselves, when they arrive both in too great numbers and too rapidly towards the same point, will be the annihilation of the weaker axes; whence will follow the formation of a tangent plane to the spherical or elliptical surface. If the action of the obstacle goes on increasing, axes of attraction, which, by their intensity, had resisted the first obstacles, are destroyed by the new ones; and new tangential planes are produced, in which those that had been first formed finish by being confounded: thus it will happen that, by the increase of obstacles, the surface of the solid from being curved has become polyhedral, and finishes by presenting only an assemblage of a small number of plane faces, separated by edges, and placed tangentially at the extremity of the axes whose forces have longest resisted the action of the obstacles. But since the most energetic axes are necessarily the least numerous, the greater the energy they possess, the number of faces which bound the solid will continually decrease according as the obstacles increase; until, at length, the solid, reduced to its most simple form, no longer presents any but that constituted by the principal axes of crystallization, terminating at the summits of the solid angles of the simple polyhedron, which axes alone have been capable of withstanding the action of all the obstacles opposed to the tendency of the molecules to unite in the form of an ellipsoid. On this hypothesis, the author explains how common salt, alum, sulphate of iron, &c., crystallize in pure water in the most simple forms, the reciprocal attraction of their molecules being controlled and diminished by the affinity exerted on them by the molecules of the water; whilst if some of these molecules of water are neutralized by mixture with another soluble principle, they cease to act as an obstacle to the crystallization of the body, which then takes forms more complicated and approaching nearer to that of the normal solid with a curved surface. M. Necker considers that the new views he has sketched require, for their complete developement, many ulterior details, as well as many new experiments and new facts; but that the tendency which the crystals of all systems present, to progress towards the curved surface form appropriate to each system, by the complication of their forces, is a fundamental fact of the first importance; and that an advance has been made by showing the bearing of the important experiments of MM. Leblanc and Beudant, and by having brought the theory of crystallography nearer to those views which the progress of chemistry and of physics have led us to adopt, relative to the form of the elementary molecules of bodies. January 24, 1839. FRANCIS BAILY, Esq., V.P., in the Chair. Charles Darwin, Esq., was elected a Fellow of the Society. A paper was read, entitled, "Experiments made on a piece of Peña silver, saved from the Lady Charlotte, wrecked on the coast of Ireland in December 1838, as to its capability of holding water." By W. D. Haggard, Esq. Communicated by Sir Henry Ellis, K.H., F.R.S. Plata Peña, so called, is silver collected by quicksilver after the ore is pounded; it is then placed in a mould, and by great force the quicksilver is squeezed out, when it forms a mass, resembling dry mortar, of great porosity. 5 1 17 into it Total weight of water contained in the piece A paper was also read, entitled, "On the Application of the Conversion of Chlorates and Nitrates into Chlorides, and of Chlorides into Nitrates, to the determination of several equivalent numbers." By Frederick Penny, Esq. Communicated by H. Hennell, Esq. F.R.S. The researches which form the subject of this paper were suggested by an inquiry into the most effectual method of ascertaining the quantity of nitrate of potassa existing in crude saltpetre. The author found that by the action of hydrochloric acid the nitrate of potassa was converted into the chloride of potassium; and conversely, that the chloride of potassium might, by the proper regulation of the temperature, be reconverted into the nitrate of potassa by the action of nitric acid. These mutual conversions afforded excellent means of determining, with great exactness, the relative equivalent numbers, in the theory of definite proportions, belonging to these salts, and to their respective constituent elements. The author, accordingly, pursued the investigation of these numbers by several successive steps, of which the details occupy the greater part of the present paper. He first determines the equivalent of chloride of potassium by decomposing chlorate of potassa into oxygen and chlo For ride of potassium; the proportion between which gives the ratio which the respective equivalent numbers of each bear to one another, and also to that of chlorate of potassa. The equivalent of nitrate of potassa is next obtained by converting the chlorate and the chloride of potassium into that salt; and from these data the equivalents of chlorine and of nitrogen are deduced. A similar train of inquiry is next instituted with the corresponding salts having sodium for their base: chlorate of soda being decomposed into the chloride, and into the nitrate; nitrate of soda into chloride; and chloride of sodium into nitrate of soda. The results of these different series of experiments coincide so closely with one another as mutually to confirm their general accuracy in the most satisfactory manner. the purpose of determining the equivalent numbers of the elementary bodies themselves, (namely, chlorine, nitrogen, potassium, and sodium,) the author employed the intermedium of silver, the several saline combinations of which with chlorine and with nitric acid were found to afford peculiar advantages for the accurate determination of the relative weights of the constituents of these salts, when subjected to various combinations and decompositions. The conclusions to which the author arrives with regard to the equivalent numbers for the six elementary bodies in question, tend to corroborate the views of the late Dr. Turner, and to overturn the favourite hypothesis that all equivalent numbers are simple multiples of that for hydrogen. He finds these numbers to be as follow: The author intends to pursue these inquiries, by applying similar methods to the investigation of other classes of salts. January 31, 1839. JOHN W. LUBBOCK, Esq., Vice-President and Treas., John Wesley Williams, and James Yates, Esqrs., were severally elected Fellows of the Society. A paper was read, entitled, "Some account of the Art of Photogenic Drawing, or the Process by which Natural Objects may be made to delineate themselves without the aid of the Artist's Pencil." By H. F. Talbot, Esq., F.R.S. In this communication the author states, that during the last four or five years he has invented and brought to a considerable degree of perfection, a process for copying the forms of natural objects by means of solar light, which is received upon paper previously prepared in a particular manner. He observes, that a prior attempt of this kind is recorded in the Journal of the Royal Institution for 1802; by which it appears that the idea was originally suggested by Mr. Wedgwood, and afterwards experimented on by Sir Humphry Davy. These philosophers found, that their principle, though theoretically true, yet failed in practice, on account of certain difficulties; the two principal of which were: first, that the paper could not be rendered sufficiently sensible to receive any impression whatever from the feeble light of a camera obscura; and secondly, that the pictures which were formed by the solar rays could not be preserved, owing to their still continuing to be acted upon by the light. The author states that his experiments were begun without his being aware of this prior attempt; and that in the course of them he discovered methods of overcoming the two difficulties above related. With respect to the latter, he says, that he has found it possible by a subsequent process, so to fix the images or shadows formed by the solar rays, that they become insensible to light, and consequently admit of being preserved during any length of time: as an example of which, he mentions, that he has exposed some of his pictures to the sunshine for the space of an hour, without injury. With respect to the other point, he states that he has succeeded in discovering a method of preparing the paper which renders it much more sensitive to light than any which had been used previously; and by means of which he finds, that there is no difficulty in fixing the pictures given by the camera obscura and by the solar microscope. He states that in the summer of 1835 he made a great number of portraits of a house in the country of ancient architecture, several of which were this evening exhibited to the Society. After some speculations on the possibility of discovering a yet more sensitive paper, the author mentions, that the kind employed by him may be rendered so much so, as to become visibly affected by the full light of the sun, in the space of half a second. The rest of this paper contains an account of various other ways in which this method may be employed in practice, according to the kind of object which it is required to copy: also, a brief mention of the great variety of effects resulting from comparatively small differences in the mode of preparation of the paper: and, of certain anomalies which occur in the process, the cause of which has not hitherto been rendered distinctly manifest. In conclusion, the author designates this as "a new process, which he offers to the lovers of science and nature." |