April 2 1, 1881] 



NA TURM, 



595 



— The following communications were made : — On the geodesic 

 curvature of a curve on a surface, Prof. Cayley, F.R.S. — On 

 operative symbols in the differential calculus, Prof. Crofton, 

 F.R.S. — Note on the resolution in factors of numbers differing 

 but slightly from ir, Mr. J. W. L. Glaisher, F.R.S.— On the 

 nature of the quadric represented by the general equation of the 

 second degree in tetrahedral co-ordinates, and on the five focal 

 quadrics of a cyclide, Mr. H. Hart, M..\.— The discrimination 

 of the maximum or minimum path of a ray of light reflected at 

 a given curve, Mr. H. M. Taylor, M.A. — On certain tetrahedra 

 specially related to four spheres meeting in a point, and his- 

 torical note on Dr. Graves's theorem on confocal conies, the 

 President. 



Physical Society, April 9. — Prof. \V. G. Adams in the 

 chair. — Prof. Helmholtz was elected an honorary member of the 

 Society, and Dr. James Moser an ordinary niembei'. — Dr. 

 J. H. Gladstone read a note on thermal electrolysis, by himself 

 iind Mr. Alfred Tribe. The authors found that when sheet- 

 silver was plunged into fused silver chloride or iodide of silver 

 crystals of silver formed on the sheet. When copper was im- 

 mersed in fused cuprous chloride co]iper crystals deposited on it, 

 and when zinc wa^ pl.nced in melted zinc chloride or iron in melted 

 ferrous chloride, these two metals crystallised on the plates. They 

 found this to be due not to a difference in the physical condition of 

 the rolled metals, but to the unequal heating of the different parts 

 of the immersed metals. By the contact theory of voltaism 

 there will lie a difference of potential between the metal and the 

 liquid chloride in contact with it, and this difference of potential 

 will [vary with temperature. Since all parts of the immersed 

 metal cannot be supposed at the same temperature always, there is 

 the possibility of a current being set up and consequent electrolysis 

 of the salt. This view was corroborated by heating the metal un- 

 equally, when a crop of crystals appeared in the cooler part of 

 the liquid. Again, two silver r ids connected together were 

 plunged, the one in a hotter, the other in a cooler part of fused 

 silver chloride, and at the end of fifteen minutes the latter W'as 

 studded \\ith cr)-stals of silver, whilst the former was clean. A 

 galvanometer showed a strijnger current between the rods, the 

 greater the difference of teaiperature between the parts of the 

 fluid in which they were placed, and transposing the rods reversed 

 this current. These experiments bear on the nature of voltaic 

 action, and form a lecture illustration of the conversion of heat 

 into electricity and chemical force. Mr. W. H. Walenn stated that 

 he had found when zinc is immersed in an electro-brassing solution, 

 crystals of brass (i.e., zinc and copper) were deposited on it. — 

 Capt. Abney exhibited a number of photographic negatives 

 taken by himself and Col. Festing, by radiation through thin 

 sheets of ebonite. The light from the positive pole of an electric 

 lamp was sent through a thin sheet of ebonite ^\ inch thick, 

 and photographs taken showed the radiation to have a low wave- ' 

 length, from 8000 to 14,000. The carbon points of the lamp 

 could be phot'igraphed through the sheet, and Col. Festing 

 observed the sun'- disk throui^h it. The ebonite showed a grained 

 structure, and different samples of ebonite gave different results, 

 but all gave some result in course of time at least ; old ebonite, 

 like that used in some of Mr. Preece's experiments, scattering the 

 light more than new ebonite. — Dr. Moser exhibited the passage 

 of the rays through the ebonite to the audience by means of a 

 galvanometer. Prof. Guthrie observed that Capt. Abney had 

 proved that light as well as heat traversed the ebonite, and 

 Dr. Coffin stated that the C'lmposition of ebonite apparently the 

 same might vary consitierabiy, and hence its radiant transparency 

 might vary too. — Prof. Helmholtz addressed the meeting on 

 the localisation of objects by the eyes. We estimate distance 

 with one eye by the outlines of the. more distant objects being 

 covered by the nearer ones where they meet, and by the shadows 

 thrown by the anterior objects. These conditions are very rarely 

 overpowered by others, as, for instance, binocular vision. This 

 is shown by Dove's pseudoscope and the fact that closing or 

 blinding one eye makes little difference to the power of judging 

 distance, e pecially when not very close to the eye. The relative 

 shifting ol object-, as the eye is moved from side to side, or to 

 and fro, or up and do" n, which may be called the parallax of 

 motion of the head, is also a strong factor in estimating distance. 

 The authwr had concluded from a study of the stereoscope that the 

 perception - f the absolute convergence of our eyes is very indistinct, 

 and that 'iily differences of c invergence related to apparently 

 near or disaiit olijects produce the stereoscopic effect. Recent 

 observati ns of his show that the incongruity between the degree 

 of conver'Jeiice and the parallax of motion is perceived with great 



accuracy. Dr. Stone remarked that a person suddenly blinded 

 in one eye acquires a lew judgment of distance by moving the 

 head (a habit seen in nocturnal birds), and in taking certain 

 French stereoscopic pictures the camera is shifted to another 

 point, so that the combined images produce an impression of 

 smallness in the object. These facts corroborated Prof. Helm- 

 holtz's view ; and jlr. Lewis Wright pointed out that santonin, 

 which changes the sense of colour, also appears to change the 

 sense of distance, perhaps by relaxing the muscular sense. 



Geological Society, April 6.— J. W. Hulke, F.R.S,, vice- 

 president, in the chair. — Edward F. Boyd, Lieut. Herbert de 

 Haga Haig, R.E. ; ]. C. Margetson, Edward David Price, and 

 James Tonge were elected Fellows of the .Society. — The fol- 

 lowing communications w*ere read : — The microscopic characters 

 of the vitreous rocks of MontJina, U..S., by F. Ruiley, F.G.S., 

 with an appendix by James Eccles, F.G.S. — On the microscopic 

 structure of devitrified rocks from Beddgelert, .Snowdon, and 

 Skomer Island, by F. Rutley, F.G.S. —The date of the last 

 change of level in Lancashire, by T. Mellard Reade, C.E., 

 F.G.S. The author described some observations made liy him 

 at Blundelhands, on the coast of Lancashire, near Liverpool, 

 according to which, judging from the position of high-water 

 mark, the land had gained considerably upon the sea between 

 1866 and 1874. The author adduced evidence in support of his 

 view, and concluded that if the last change of level in South- 

 West Lancashire was a downward one it could not have taken 

 place within 2500 years. 



Institution of Civil Engineers, April J. — Mr. Brunlees, 

 F.R.S E., vice-president, in the chair. — The paper read was on 

 the actual lateral pressure of earthwork, by Mr. B. Baker, 

 M.Inst.C.E. 



Edinburgh 

 Royal Society, April 4. — Prof. Balfour in the chair. — Prof. 

 Tait communicated the results of his experiments on the pressure 

 errors of the Challenger thermometers, the correction for which, 

 as originally furnished to the expedition, was o°'5 F. per mile of 

 depth. The mode of experimenting was to subject the thermo- 

 meters to considerable pressure in a hydraulic press, which was 

 essentially a strong steel cylinder that was warranted to stand 

 a pressure of 25 tons weight on the square inch. It was 

 supported ijt an upright position upon a strong tripod stand. 

 Water was filled in from above ; and into the upper end of the 

 cylinder there was lowered a tight-fitting plug wdiich was fixed 

 in position by a transverse steel bolt. The lower end of the 

 cylinder was connected through a narrow copper tube to a 

 hydraulic pump, which, by pumping in water to the cylinder, 

 raised the pressure to the required amount. At three tons pres- 

 sure an average effect of i°'5 F. was produced upon the inclosed 

 thermometers. Before drawing any conclusions as to the cor- 

 rection to be applied in deep-sea sounding, it was necessary to 

 consider how far this effect could be explained as resulting 

 from the peculiar condiiions under which the experiments were 

 made. From the known compressibility of glass it was calcu- 

 lated that the volume of the bore of a thermometer tube, closed 

 at both ends, would be diminished by only one-thousandth part 

 for an increase of pressure of one ton weight on the square 

 inch ; and from a direct experiment made w ith a metre-long 

 tube this was proved to represent very approximately the real 

 effect. Hence it was quite out of the question that this could 

 have any appreciable effect on such comparatively short thermo- 

 meters as those of the Challenger, which were besides subject 

 to much graver errors, such as those arising from the shifting of 

 the indices during the ascent from the depths, or even from the 

 effect of parallax when taking the reading. The direct action of 

 pressure may then be disregarded, and the effect produced upon 

 the thermometers in the compression apparatus must be due to 

 secondary effects of pressure, such as evolution of heat. The 

 various sources of heat were four: I. Heating of the water by 

 compression. This depends greatly on the original temperature 

 of the water, being ni! at the point of maximum density (40'' F.), 

 and larger for higher temperatures. One-fourth of the total 

 eflect is due to this. 2. Heating of the water due to pumping 

 in through the narrow tube. This accounts for three-twentieths 

 of the effect. 3. Heating of the vulcanite frame by compression. 

 This explains another fifth. 4 Heating due to the effect upon 

 the protecting bulb. This probably explains the remaining two- 

 fifths of the effect. In this last case however there is not only 

 compression, but distortion ; and of the thermal effects of such a 

 strain no one yet knows anything. These four sources of error 



