June 28, 1888] 



NA TURB 



215 



the month of May. — Mr. H. E. Dresser exhibited a specimen of 

 a new Shrike from the Transcaspian district of Central Asia, 

 which he proposed to name Lanitts raddei, after Dr. Radde, of 

 Tiflis, its discoverer. — Mr. Sclater, on the part of Mr. F. M. 

 Campbell, exhibited a pair of Pallas's Sand-Grouse {Syrrhaples 

 paradoxus), shot in Hertfordshire in May last, and made 

 remarks on the recent immigration of this Central Asiatic bird 

 into Western Europe. — The Secretary exhibited, on behalf of 

 Prof. R. Collett, a nest, eggs, and two young ones in down of 

 the Ivory Gull (Lams eburneus), belonging to the Tromso 

 Museum, which had been obtained in Spitzbergen in August 

 1887. — Mr. Warren communicated a paper on Lepidoptera 

 collected by Major Yerbury in Western India in 1886-87, form- 

 ing a continuation and completion of two previous papers by Mr. 

 A. G. Butler on Lepidoptera collected by the same gentleman in 

 similar localities. The present collection contained examples of 

 over 200 species of Heterocera, of which about one-fourth were 

 described as new. Mr. Warren remarked upon the abnormal 

 development of separate organs, such as the antennae and palpi, 

 in tropical insects, as being rather specific aberrations from a 

 generic type, than as warranting the erection of new genera. — A 

 communication was read from Mr. Martin Jacoby, containing 

 descriptions of some new species of Phytophagous Coleoptera 

 from Kiukiang, China.— Mr. F. E. Beddard read some notes on 

 the structure cf a peculiar sternal gland found in Didelphys dimi- 

 diata. — Mr. G. A. Boulenger read a paper on the scaling of the 

 reproduced tail in Lizards, and pointed out that the scaling of 

 the renewed tails of Lizards may, in some cases, afford a clue to 

 the affinities of genera or species to one another. — Mr. F. E. 

 Beddard gave a preliminaiy notice of an apparently new form of 

 Gregarine, found parasitic on an earthworm o the genus 

 Perickcefa from New Zealand. 



Cambridge. 



Philosophical Society, May 21. — Mr. J. W. Clark, 

 President, in the chaT. — On solution and crystallization, 

 by Prof. Liveing. When a substance passes from a state 

 of solution into the solid state, the new arrangement of the 

 matter must be such that the entropy of the system is a maximum ; 

 and, other things being the same, the surface energy of the 

 newly formed solid must be a minimum. If the surface tension 

 be positive, that is tend to contract the surface, the surface 

 energy will be a minimum when the approximation of the mole- 

 cules of the surface is a maximum. The essential difference 

 between a solid and a fluid is that the molecules of the former 

 maintain approximately the same relative places, whereas the 

 molecules of a fluid are subject to diffusion. Further, crystal- 

 loids in assuming the solid form assume a regular arrangement 

 of their molecules throughout their mass, which we can usually 

 recognize by the optical properties of the crystal, and by the 

 cleavage. If we suppose space to be divided into equal cubes 

 by three sets of parallel planes, each set at right angles to the 

 other two, and suppose a molecule to be placed in every point 

 where three planes intersect, we shall have an arrangement which 

 corresponds with the isotropic character of a crystal of the cubic 

 system. But of all the surfaces which can be drawn through 

 the system the planes bounding the cubes meet the greatest 

 number of molecules, those parallel to the faces of the dodeca- 

 hedron meet the next greatest number of molecules, and those 

 parallel to the faces of the octahedron meet the next greatest 

 number. Also if we take an angular point of one of the cubes 

 as origin, and three edges of the cube as axes, and the length of 

 an edge qf the cube as the unit of length, every plane which 

 cuts the rnree axes at distances /, q, r respectively from the 

 origin, where/;, q and r are whole numbers, will be a surface of 

 maximum concentration of molecules, but the concentration will 

 be less as /, q and r are greater. Hence forms which are 

 bounded by these planes, which follow the law of indices of 

 crystal?, will be forms of minimum surface energy and therefore 

 of equilibrium. The tendency in general will be for substances 

 with such a structure as is here supposed to take the form of 

 cubes, since the cube will have the greatest concentration of 

 molecules per unit of surface. But the total surface energy will 

 depend on the total surface as well as on the energy per unit of 

 surface, and for a given volume the surface will be diminished 

 if the edges and angles of the cube are truncated by faces of the 

 dodecahedron and octahedron, or by more complicated forms. 

 When a solid is broken, two new surfaces are formed each with 

 its own surface energy, and the solid must be more easily 

 fractured when the new surfaces have the minimum energy. 



Hence substances with the structure supposed must break most 

 easily in directions parallel to the sides of the cube, dodeca- 

 hedron and octahedron ; and these are the cleavages observed in 

 this system. If we suppose the molecules placed at the 

 centres of the faces of the cubes, instead of at the angles, the 

 arrangement will still be isotropic, but the octahedron will 

 be bounded by the surfaces of greatest condensation, and the 

 cube will come next to it. It is probable that substances 

 which cleave most readily into cubes, such as rock-salt and galena, 

 have the former structure, while those which have the octahedral 

 cleavage may have the latter arrangement of their molecules. 

 For the pyramidal and prismatic systems we may suppose space 

 divided not into cubes but into rectangular parallelopipeds with 

 edges equal severally to the axes of the crystals, and molecules 

 placed as before. For the rhombohedral system we may suppose 

 space divided into rhombohedra, or in crystals of the hexagonal 

 type into right prisms with triangular bases, and for the other 

 systems into parallelopipeds with edges parallel and equal to the 

 axes. In each case if the molecules be disposed at points of 

 intersection of three dividing planes we shall have such an 

 arrangement as satifies the optical conditions, and planes which 

 follow the law of indices are surfaces of maximum condensation. 

 Calculations show that whenever a crystal has an easily obtained 

 cleavage the direction of cleavage corresponds to the surface of 

 greatest condensation, and that the most common forms of 

 crystals correspond in general to forms of minimum surface 

 energy. The surface tension of a plane surface will have no 

 resultant out of that plane, but where two plane surfaces meet 

 in an edge, or angle, the tensions will have a resultant of 

 sensible magnitude in some direction falling within the angle. 

 Whenever all the faces of a crystallographic form are developed, 

 every such resultant will be met by an equal and opposite 

 resultant, and the form will be one of equilibrium. If one edge, 

 or angle, be modified, the opposite edge, or angle, must either 

 be similarly modified, or the resultant arising from the modifica- 

 tion must be equilibrated by some internal forces produced by 

 displacement of the molecules. In general, equilibrium is 

 attained by similar modifications of similar edges and angles, 

 but when only some of the edges or angles of a crystal are 

 modified, while other similar edges or angles are not modified, we 

 usually have evidence of the consequent internal strain. Thus 

 cubes of sodium chlorate, which have half the angles truncated 

 by faces of a tetrahedron, rotate the plane of polarized light, 

 hemihedral tourmalines are pyro-electric, and so on. This theory 

 therefore accounts for the plane faces of crystals, the law of 

 indices, the most common combinations, and the cleavages. 

 The same theory accounts for the development of plane faces 

 when a crystalline solid of any shape is slowly acted on by a 

 solvent. Solution will proceed so long as the entropy of the 

 system is increased by the change, but when the solution is nearly 

 saturated there will be an increase of entropy from the solution 

 of a surface which has more than the minimum surface energy, 

 while there will be no increase from the solution of a surface 

 which has only the minimum energy. — On the effect of an 

 electric current on saturated solutions, by Mr. C. Chree, M.A. 

 This paper contains an account of experiments whose aim was 

 to determine what effect, if any, an electric current may have on 

 the quantity of salt required to form a saturated solution. Strong 

 currents and a rapidly reversing commutator were employed. 

 Certain chlorides were dealt with, and in no case did the exist- 

 ence of a current produce any sensible immediate effect. When 

 heating was allowed to take place, the action of the current 

 appeared to check the solution that would naturally have 

 followed. This view was further supported by experiments on 

 the effects of simple heating. These experiments showed, how- 

 ever, that an originally saturated solution when slowly heated 

 can dissolve salt only with extreme slowness. 



Paris. 

 Academy of Sciences, June 18. — M.Janssen, President, in 

 the chair. — Lagrange's hypothesis on the origin of comets and 

 meteorites, by M. H. Faye. According to the author's calcula- 

 tions, this hypothesis, first submitted to the Bureau of Longitudes 

 in 18 1 2, does not hold good for the comets whose orbits do not 

 quite approach any of the planetary orbits. But it would seem 

 capable of being applied to the meteorites, whose fragmentary 

 character, minute size, chemical and mineralogical identity with 

 the constituent elements of the earth, combined with their great 

 abundance, would seem to be absolutely incompatible with an 

 extra-planetary origin. The earth alone with its satellite best 



