From what I know of the apparatus and its performance I am sure that no regular and abrupt retardation or acceleration amounting to as much as the hundredth part of an average wave-length could have escaped observation in the experiments. It will be remembered also that the jump of the fringes at high potential extended to four-fifths of the fringe width. With reference, therefore, to the dielectric CS2, and the two principal vibrations parallel and perpendicular to the line of force, it appears that the regular effect of the electric strain upon one of the vibrations is a positive retardation, while upon the other vibration there is very probably no effect whatever, and certainly no effect as large as the eightieth part of the former. Second Positive Dielectric: A paraffin oil, specific gravity 0.845. This liquid was far inferior to CS, electrically, and also as an optical medium. The method of experiment finally adopted as the best was a little different from that with CL2. The prime conductor had its capacity enlarged by connexion with a Leyden jar; the machine was kept working at a constant rate, and the prime conductor was partially discharged, at short and regular intervals, by sparks upon the knob of the first internal conductor, which was of course discharged in each interval. The phenomenon looked for was a quick motion of the fringes at the instant of the spark; that is, at the instant of electric charging of the liquid. (1.) Plane of polarisation of the pencil BF (through the electric field) vertical: Rise of fringes indicates relative retardation of this pencil. At the instant of the spark there was a quick upward jump of the fringes through something like one-fifth of the fringe width, generally followed by a set of large and comparatively slow disturbance-movements. In most cases also, immediately after the spark, the observer was able to detect the contrary jump quite clearly by laying his finger on the knob of the first conductor. Through a long set of observations, taken at different potentials, the upward jump of the fringes at the instant of charging was obtained with perfect regularity; and-amplitude excepted-the effect was not inferior to that in CS2. (2.) Plane of polarisation of the pencil BF horizontal: Rise of fringes indicates relative retardation of BF. Many observations were taken at different potentials, high and low. There were sluggish and irregular disturbance movements, great and small, but no trace of a regular jump of the fringes in one direction or the other at the instant of the spark. There could be no doubt as to the true meaning of these results. In this positive dielectric, as in CS2, the vibration along the line of force is retarded by electric strain, and the perpendicular vibration is unaffected. First Negative Dielectric: Oil of Colza.-This liquid also was far inferior to CS2, especially as an optical medium. The method of experiment followed with paraffin was retained here as the best; the first internal conductor was charged by spark from the prime conductor at regular intervals, and was put to earth for a moment in each interval. (1.) Plane of polarisation of the pencil BF vertical: Rise of fringes indicates relative retardation of BF. The fringes were generally curved and very imperfect at the beginning of an experiment, but a few successive charges brought them, after some disturbance, into permanently good form, and then there was a quick downward jump, seen always at the instant of the spark. And, as in the contrary case of paraffin, this jump was a thing as distinct as possible from the sluggish and irregular disturbance-movements by which it was generally followed. When the spark was taken at every 10th turn of the plate, the potential was about as high as the liquid could bear, and the extent of the jump was fully one-fifth of the fringe-width. In the course of a long set of observations this downward jump of the fringes at the instant of charging was seen with perfect regularity, and always distinctly. In this case, therefore, the regular optical effect of electric strain was an acceleration. (2.) Plane of polarisation of the pencil BF horizontal: Rise of fringes indicates relative retardation of BF. When the fringes were imperfect at starting, the effects of a few successive charges were the same as in the first case, irregular displacements and changes of inclination, the fringes generally rising and falling in their lower and higher parts till they came into permanently good form. Afterwards there were smaller disturbances always present in this case as in the former; but neither there nor here were they such as to interfere ultimately with exact observation. The experiment was carried on for some time till the liquid was well mixed and the fringes good. Many observations were then taken, some of them at highest potential, but there was no trace of a jump ever seen at the instant of the spark. In this liquid, therefore, as in carbon disulphide and paraffin, the only one of the two principal vibrations which is affected by electric strain is that along the line of force; but, as the present dielectric is of the negative class, the retardation produced is negative. Second Negative Dielectric: Seal Oil.-From want of homogeneity this liquid was very defective optically, the image of the slit L being much deformed and sometimes broken by streaks. The defect was remedied in a good degree by strong charges given to the liquid on both sides of the second conductor. The method of experiment was the same as with oil of colza. (1.) Plane of polarisation of the pencil BF vertical: Rise of fringes indicates relative retardation of BF. At first, the electricity produced very large displacements and deformations of the fringes, in the midst of which there was no regular effect to be seen; but as the experiment went on, and the medium improved, the expected effect came out distinctly: a quick downward jump of the fringes at or immediately after the instant of the spark. Under good optical conditions, and at potentials high and low, the effect was perfectly regular, and was distinct and pure as that in oil of colza, though apparently not quite so large. (2.) Plane of polarisation of the pencil BF horizontal: Rise of fringes indicates relative retardation of BF. The disturbance of the fringes was greatly reduced as the experiment went on, till at last there was nothing left but a set of slow movements, very irregular and very small, sometimes invisible. In the midst of these, as in their absence, and in a long set of observations, taken at different potentials, from low to highest, there was no trace ever seen of a jump of the fringes at the instant of the spark. It appears, therefore, that in this negative dielectric, as in oil of colza, the total optical effect of electric strain is an acceleration of the vibration which is directed along the line of force. The conclusion to be drawn from the preceding experiments has been stated already by anticipation; but I repeat it finally in other terms as follows:: If light pass through an electrostatically-strained medium at right angles to the lines of force, and be represented by two component lights whose planes of polarisation are respectively parallel to the lines of force and perpendicular, then the proper and immediate optical effect of the electric strain is a change of velocity of the latter component.* The use of the words proper and immediate in this statement may be thought objectionable; but some such words are required for the purpose here chiefly intended, which is to exclude those undoubtedly remote effects of electric action that appeared as disturbances in all the experiments. IV. "On the Liquation of Silver-Copper Alloys." By EDWARD MATTHEY, F.C.S., Assoc. Roy. Sch. Mines. Communicated by Sir G. G. STOKES, F.R.S. Received February 16, 1894. It is a well-known fact that during the solidification of certain alloys groups of the constituent metals fall out of solution, giving rise to the phenomenon called "liquation." The molecular arrangement which results from this behaviour of alloys has been investigated by many experimenters, notably by Devol, Roberts-Austen, and Guthrie. The author has also studied the behaviour of a large The change of velocity in the case of any positive dielectric is of course a decrease. series of the alloys of the precious metals and metals of the platinum group, and the results have been published in the 'Philosophical Transactions' for 1892, and in other papers to which reference may be made. It is, however, in the case of alloys of silver and copper that liquation is most marked, and gives rise to results of much interest and industrial importance. It is, for instance, often a matter of great importance to obtain a plate of standard silver (925 parts of silver in 1000) of uniform standard. The great difficulty of effecting this has been shown by Roberts-Austent, and, as the results of an elaborate series of experiments, he was led to the conclusion that slow and uniform cooling of the mass was most effective in obtaining uniformity of standard. He informs me of a fact of which I was not aware until the present experiments were concluded, viz., that he also tried the effect of the rapid cooling of a thin casting in a large mould which was no less than 45.7 cm. long. He found, however, that castings made in this mould were comparative failures as regards uniformity of standard, and that, as in the case of other published results given. by castings in thicker moulds, it was not possible, either by rapid or by slow cooling, to obtain masses of alloys which did not give points of local richness. During the last few years I have returned to the investigation of the silver-copper alloys, and as the results of a series of some hundreds of experiments, only the final ones of which must be referred to here, I find that it is preferable to cast the alloy very thin, and to promote the uniformity of cooling. A bar of this alloy was cast into a "skillet" mould to produce a casting 30 cm. in length, 13 cm. in width, and 6 mm. in thickness, weighing 5 kilos. Punchings were taken through its thickness at the points marked, and the assays which were made of these punchings showed the composition at the respective points to be as follows: * Phil. Trans.,' 1892, A, pp. 629-652, and 'Roy. Soc. Proc., vol. 47, 1890, pp. 180-186. + 'Roy. Soc. Proc.,' vol. 23, 1875, p. 481, and 'Chem. Soc. Journ.,' vol. 27, 1874, p. 197. |