THE CHEMICAL NEWS. VOLUME XXXI. EDITED BY WILLIAM CROOKES, F.R.S., &c. ON No. 788.-FRIDAY, JANUARY 1, 1875. ATTRACTION AND REPULSION RESULTING FROM RADIATION.* By WILLIAM CROOKES, F.R.S., &c. (1). IN a paper "On the Atomic Weight of Thallium," presented to the Royal Society June 18, 1872, after describing a balance with which I was enabled to perform weighings of apparatus, &c., in a vacuum, I noted a peculiarity in relation to the effect of heat in diminishing the apparent weight of bodies. I said, "That a hot body should appear to be lighter than a cold one has been considered as arising from the film of air or aqueous vapour condensed upon or adhering to the surface of the colder body, or from the upward currents of air caused by the expansion of the atmosphere in the vicinity of the heated body. But neither hypothesis can be held when the variation of the force of gravitation occurs in a vacuum as perfect as the mercurial gauge will register, and under other conditions which I am now supplying, and which I purpose embodying in a paper to be submitted to the Royal Society during a subsequent session."+ With the vacuum-balance mentioned above I carried out many experiments, but was unable to obtain results which were at all concordant; and it was soon found necessary to investigate the phenomena with smaller and less complicated apparatus. (2). Most chemical manuals warn beginners against the errors occasioned by weighing substances while hot; and, up to a moderately high degree of exhaustion, I was prepared to find a piece of glass apparatus, when hot, apparently lighter than the weights which should balance it were the whole system at the same temperature. But, instead of the interfering causes diminishing as the rarefaction proceeded, they seemned rather to increase, or at all events to become irregular in their action, sometimes appearing to oppose, and at others to supplement the force of gravity. In such a vacuum as a good air-pump would produce, the actions of the ascending current of air and of the adhering film, it might be presumed, should cease to exert an influence; and I could think of no other disturbing cause except the lengthening of the beam, owing to the heat radiated from the apparatus below it. An increase in the length of the beam should make a mass suspended at its extremity appear heavier; but, whilst I frequently noticed an action which might be due to this cause, I occasionally obtained results which were so anomalous as to convince * From the Philosophical Transactions of the Royal Society of London, vol. clxiv., part 2. + Phil. Trans., 1873, vol. clxiii., p. 287. me that some cause which I had not hitherto recognised was at work (49), and to lead me to hope that perhaps I might succeed in tracing a connection between heat and the force of gravity. (3). Many physicists have worked on the subject of repulsion by heat. I give here a brief resume of the state of knowledge on this subject up to the time of my commencing these experiments, premising, however, that much of this historical information was unknown to me until some of the experiments here recorded were finished and I comI can find is by the Rev. A. Bennet, F.R.S., who in the menced putting my notes together. The earliest mention year 1792 published a paper on “A New Suspension of the Magnetic Needle intended for the Discovery of Minute Quantities of Magnetic Attraction; also an Air-Vane of Great Sensibility; with New Experiments on the Magnetism of Iron Filings and Brass." Mr. Bennet used a spider's thread as a means of suspension. This he found by experiment to be absolutely free from torsion. I quote the following experiments from his paper : "Experiment IV. A bristle was suspended horizontally by a spider's thread, somewhat stronger than the last, and after turning the wheel till it produced 4800 revolutions, it shortened the thread from 3 inches to 1 inch; yet either end of the bristle would move towards any warm substance which was presented to it either with or against the direction of the twist.† "Experiment V.-Several other light substances were suspended by fine spiders' threads and placed in a cylindrical glass about 2 inches in diameter, as the thinnest part of the wing of a dragon-fly, thistle-down, and the down of dandelion; of these the last appeared most sensible to the influence of heat; for when this down was fastened to one end of a fine gold wire, suspended horizontally on to one end of two bits of straw joined together in the form of the letter T inverted, it would turn towards any person who approached it at the distance of 3 feet, and would move so rapidly towards wires heated by my hand, as very much to resemble magnetic attraction. "Experiment VI.-A bottle filled with cold water was brought near the glass cylinder standing in a warm room, and soon after the down of dandelion appeared to be repelled by the bottle by turning away from it. The bottle was removed to the other side, and the dandelion again moved towards the opposite side. "Experiment VII.-A piece of paper was tied over the mouth of a glass jar, about 4 inches in diameter. Two holes were made in the paper opposite to each other, and 2 Attraction and Repulsion Resulting from Radiation. near the edge of the glass. The jar was placed upon a table, and suffered to stand a considerable time to cool in a room without fire. I then sat near it on the side where One of the holes in the paper was in the nearer and the other in the farther end of the diameter. I next filled another glass with smoke, and placed it with its mouth over the two holes in the paper. The smoke was now seen to descend through the farthest hole, and mixing with the air in the lower jar, plainly showed that the air moved slowly towards the side of the glass warmed by the heat of my body. "Experiment X.-To the end of a fine gold wire 3 inches long, and suspended by a spider's thread in a cylindrical glass, was fastened a small circular bit of writing-paper; light was admitted through a small hole, and also the focus of a large lens was thrown upon the paper, with the intention of observing whether it would be moved by the impulse of light; but though these experiments were often repeated, and once with the paper suspended in an exhausted receiver, yet I could not perceive any motion distinguishable from the effects of heat. Perhaps sensible heat and light may not be caused by the influx or rectilinear projection of fine particles, but by the vibrations made in the universally diffused caloric or matter of heat or fluid of light. I think modern discoveries, especially those of electricity, favour the latter hypothesis." (4). In his "Elementary Treatise on Heat,"* Professor Balfour Stewart, F.R.S., cites this experiment of Mr. Bennet's as one of the arguments against the emissive and in favour of the undulatory theory of light and heat. Bearing in mind the overwhelming proofs we now possess that the undulatory theory more nearly expresses the truth than does the emissive theory, it is not likely that the very different results I have succeeded in obtaining (56, 57, 58), by the employment of instruments of a delicacy unattain able eighty years ago, will have any weight in modifying the accepted theories of light and heat. (5). The next mention of the dynamic action of heat is by Laplace, who, in his "Mécanique Céleste,"+ speaks of the " repulsive force of heat" as subsisting among the particles of a fluid, but observes that experiment shows it has no other effect on capillary attraction than what results from its diminishing the density of the fluid. (6). In the year 1824 Librit published some experiments on the movement of translation experienced by a drop of liquid suspended to a metallic wire, one of the ends of which is heated. This he inferred was due to repulsion produced by the heat between the wire and the particles of the liquid. The Rev. Baden Powell, F.R.S., says that trying to repeat Libri's experiment he has never been able to succeed, except in producing a slight apparent motion in the drop, which seems explicable from the mere effect of evaporation on the side next the heat. (7). In the Annales de Chimie et de Physique for 1825§ are two papers by Fresnel, in which he gives an account of an experiment on the repulsion exerted by heat. To the two extremities of a fine magnetic needle, suspended by a cocoon fibre, he attached vertical disks of foil and mica, so as to test with the same apparatus an opaque and a transparent body, The fixed body which was to repel the torsion-balance was another disc of foil. The whole was covered with the receiver of an air-pump, and a vacuum, up to 1 or 2 m.m., was obtained. The whole was then taken into sunshine, and turned so that the needle was kept slightly out of the magnetic meridian by pressure of the fixed disk against one of the movable disks. On concentrating the sun's rays on either of these disks, they instantly separated, sometimes to the extent of a millimetre. O withdrawing the lens, the torsion-balance only gradually returned to its original position. To see if these phenomena were due to the residual gas, air was gradually let in ; and Oxford, at the Clarendon Press, 1866, pp. 161, 352 + Suppl. Livr., x., p. 75, A.D. 1799-1805. * Mem. Accad. Torrino, xxviii. Phil. Trans., 1834, p. 485. § Vol. xxix., pp. 57, 107. CHEMICAL NEWS, on repeating the experiment when the density of the enclosed air was fifteen or twenty times greater than at first, it was found that the repulsion had not sensibly increased, as it should have done had it been due to currents of heated air. Under some conditions, indeed, the movement was not so great as in a vacuum. Sometimes Fresnel observed an action of attraction, the disks adhering when heated, and separating when the lens was removed. With pieces of copper suspended to the magnetic needle the attraction was very apparent; when the movable and fixed disks were near together, they approached on applying heat. Reasons are given why these effects cannot be due to electricity or magnetism, but the author does not seem to have tried any further experiments. (8). M. Saigey in 1827* described an experiment with a needle of lead delicately suspended at different distances from a bar of copper. He found the number of oscillations in a given time decrease with the distance. From his experiments he arrived at the following results :-All bodies exert between themselves a feeble repulsive action under ordinary circumstances. A very marked attraction may be observed between a cold and a heated body, or between two bodies of different temperatures, whether screens be interposed or not. Saigey concludes that in many cases results obtained without the appreciable development of magnetism or electricity have been attributed to these forces. (9). In the year 1834 Professor J. D. Forbest published an elaborate research on the vibrations which Mr. A. Trevelyan had found to take place between metallic masses having different temperatures. The general conclusion at which he arrived is, that there is a repulsive action exercised in the transmission of heat from one body into another which has a less power of conducting it. These repulsions only take place between bodies having a certain amount of conducting-power, below which some metals fall; it must be excitable in a most minute space of time, and is energetic in proportion to the difference of conducting-power of the substances and to their difference of temperature. (10). The Rev. Baden Powell, in the same year,‡ pub"On the Repulsive Power of Heat." lished a paper employed an arrangement somewhat similar to Fresnel's (7), the disks being two small plates of glass with truly plane surfaces. He found that if in the first instance they were pressed together so as to adhere, heat always overcame the attraction, and the movable disks sometimes receded to a sensible distance; but Professor Powell says that this effect (and perhaps also that in Fresnel's experi ment) appeared to him in a great measure due to another cause than repulsion, viz., the slight curvature which will be given to the plate of glass by the greater expansion of the more heated surface producing a convexity towards the heat. He (11). By pressing the disks closely together, the coloured rings formed would give a test of the interval between the disks. Professor Powell found that the tints invariably descended in the scale when heat was applied, showing that the interval between the disks increased, and proving the existence of a repulsive power exerted between heated surfaces at small, though sensible, distances the warping, or change of figure, if any, in the glasses by heat being readily seen to be such as ought to cause the rings to enlarge at the first instant. From experiments made by the contact of a lens with different substances, Professor Powell inferred that whatever tends to increase the rapidity of communication of heat, tends to increase the observed effect. The effect is increased when water, instead of air, is introduced between two lenses. (12). In 1838 Professor Powell gave some additional notes on the same subject, but no new form of experiment was tried. * Bulletin Mathématique, tom. ix., pp. 89, 167, 239; tom. xi., No. 167. Bull. Sci. Nat., viii., p. 287. ↑ Trans. Roy. Soc. Edinburgh, vol. xii., p. 429. Phil. Mag., vol. xii., April, 1838. NEWS (13). Dr. Joule, F.R.S., gave an account, in 1863, of a new and extremely sensitive thermometer. It was based upon the disturbing effect of currents of air upon finelysuspended magnetic needles. By diminishing the directive force of the needle, the instrument was made sufficiently delicate to move to the heat radiated from a small pan containing a pint of water heated to 30°, placed 3 yards off, and also to give evidence of the heat of the moon. I have little doubt that these movements were not so much due to currents of air as to the mechanical effects of radiation described in this paper. (14). It is right that I should mention here that in September, 1871, I received a letter from Mr. J. Reynolds, mentioning that he had constructed a little instrument which would turn to the hand, to a fire, or to any source of heat. It consisted of a thin slip of deal suspended by a filament of spider's web, and enclosed in a thin glass flask. This little instrument was more sensitive than any I had then constructed, as the spider's web was much freer than cocoon-silk from torsion, and Mr. Reynolds kindly allowed me to experiment with it. (15). I cannot do better, in bringing this historical summary to a conclusion, than draw attention to a passage written in 1868 by Professor Guthrie, F.R.S.,† in which he distinctly points out a probable relation between heat and gravity. He says:-"If the ætherial vibrations which are supposed to constitute radiant heat resemble the aërial vibrations which constitute radiant sound, the heat which all bodies possess, and which they are all supposed to radiate in exchange, will cause all bodies to be urged towards one another." (16). Were it such a relation between heat and gravity of which I had been getting glimpses, it was evident that a much more delicate apparatus would be necessary to render it distinct, and I accordingly commenced a series of experiments with the view of ascertaining what form of apparatus would be most sensitive to the action sought. The first requisite was to get rid of the error arising from the expansion of the beam by heat; and since, in working with hot bodies, the metallic masses used as weights would themselves become warm, and since the action I sought to establish was only likely to be due to a difference in temperature between the hot body on one arm of the balance and the cold weights on the other arm, both being under the influence of the same force of gravity, I endeavoured to obtain the desired results by means of a spring-balance, one in which the variations of gravity should be measured, not against gravity itself, but against the tension of a spring. (17). I tried many forms of spring-balance, and obtained with them results which, at the time, I thought sufficiently satisfactory. The sources of error were, however, so numerous, and the manipulation was so difficult, that I ultimately gave up that form of balance in favour of the one usually employed. In order to obtain a very high degree of rarefaction, without the trouble and uncertainty attending the use of an ordinary air-pump, it was necessary to have the balance sufficiently small to enable it to be exhausted by the Sprengel pump. Before proceeding to the forms of apparatus finally adopted, and the experiments made therewith, it will, I think, be useful, for the sake of other experimentalists, if I briefly describe some of the arrangements successively tried and rejected, with the reasons for so doing. (To be continued.) Procedures for Determining Thein in Tea.-M. R. Weyrich has made a comparative investigation of the methods proposed of Mulder, Peligot, Claus, Zoeller, and Liventhal, and gives the preference to the first-mentioned. CHEMICAL NEWS, vol. vii., p. 150. t Proceedings of the Royal Society, vol. xix., p. 35. LECTURES ON THE MORPHOLOGY OF CRYSTALS AT THE CHEMICAL SOCIETY. By NEVIL STORY MASKELYNE, M.A., F.R.S., &c. I. THE lecturer, after defining crystallography as the science dealing with chemical morphology, took in hand a large of three kinds, and that those of each kind were symcrystal of apophyllite, and pointed out that its faces were metrically disposed; the similarity of the faces of each kind being at once recognisable in the nature of its surface and physical features no less than in its form. A group The repeated morphological features of a crystal are its of such similar repeated faces he designated as a form. faces, its edges, and its quoins (or solid angles). It was seen that the faces belonging to either of the three forms on the crystal of apophyllite differed in magnitude; whereas the repetition of an edge implies absolute constancy in the dihedral angle so repeated. This result, of equality in the edge-angles, brings the crystal under the domain of number-makes it, in short, an exact science. repeated features. The properties and external characters that distinguish the faces of one form from those of another were treated as the results of properties in the crystal; such, for instance, as the character of its cohesion perpendicularly to the face. Where faces are parallel to cleavages, the directions of least cohesion are perpendicular to such faces. The first subject for consideration is the mode of measuring an edge, by determining, in one form or other, the angle made by two lines meeting in the edge perpendicularly to it; of which lines, one lies in each of the planes. The kinds of goniometer in use for measuring angles were alluded to in passing; and the lecturer went on to 4 Effect of Heat on Iodide of Silver. take the case of four faces of a crystal of barytes, which he exhibited. These faces had all their edges parallel, and so served to define a zone. Supposing a section to be made through the crystal perpendicularly to all these planes (and so to their edges), he gave a profile view of this section, which he termed the zone-plane of the zone of the four faces, M, L, D, P. The method of employing two fixed lines (in this case perpendicular to each other) as axes to which the faces should be referred, was then explained; and, as the planes M and P were at right angles, their profile lines, or "traces," on the plane of the zone (the plane in which the figure was drawn) were taken for the directions of the axes X or Y; but lines parallel to them, passing through a point, O, inside the crystal, were actually taken for the axes. On the line P, i.e., the axis X, the line L (representing the plane L) forms an angle of 68° 3', the line D making with that line an angle of 51° 8'. To compare these planes, lines parallel to the lines L and D were made to pass through the same point, C, on the axis Y; and then we have point of the iodide. This would give as the specific gravity of the molten iodide 5'406. The mass was then allowed to solidify in the tube, and a large conical cavity appeared at the moment of congelation. This cavity contained 4552 grammes of mercury, and would contain 18149 grammes of iodide. Hence, if the volume of the iodide before fusion be taken as 100, the volume of the resulting fused iodide will be 104'499. Or, again, 100 volumes of molten iodide contract to 95'694 volumes of the solid. If the volume at the point of maximum density (116° C.) be taken as unity, the volume of the molten iodide will be 104499. The principal expansion takes place at the moment of fusion, and the expansion between 116° C. and 450° C. is not considerable. No really satisfactory method has been yet found for determining the coefficient of expansion between 116° C. and 145° C.; but if we assume it to be equal to the mean expansion on the other side of 116° C. (of course omitting the sudden expansion which takes place when the amorphous passes into the crystalline condition,) we find that a volume ro at 116° C. will become a volume of 101455238 at 450° C. just below the melting-point, while in passing from the solid to the liquid condition the volume increases from 101455 to 104499, an expansion =0'03044. When a mass of iodide of silver passes from the amorphous into the crystalline condition the molecular commotion is so considerable that portions of the mass are sometimes jerked off from the ends of a bar, and large fissures appear in the mass. These are sometimes as much as half a millimetre broad and several centimetres long in a cylindrical mass, weighing from 10 to 20 grammes. They penetrate to the centre of the mass, as may be shown by cooling the iodide under mercury, when the whole mass is found to be permeated by the metal. The capacity of these intercrystalline spaces was determined by allowing a known weight of iodide to pass from its amorphous to its crystalline condition beneath the surface of mercury, If we represent OA, OC, OH by a, c, , respectively, and again weighing. 2 Treating the zone as thus represented, merely by lines which are the traces of faces in the zone on a plane, we may say that the ratio of a: c is the parametral ratio, a and c being the parameters of the zone, as referred to M and P as axes; and the numbers by which these parameters have to be divided for each plane are called the indices of the plane-1,1 being the indices of D, 1,2 those of L, and these, bracketed, give symbols (1,1) and (1,2) for symbols of the planes, as referred to two axes parallel to the traces of two planes in the zone. (To be continued). a. 3643 grammes AgI after thus cooling weighed 3.968 grammes. B. 5913 grammes AgI after thus cooling weighed 6:417 grammes. And as we know the specific gravity of mercury and of the iodide, it is easy to deduce from the above that the volume of the cracks is represented respectively by (a) o'1353 gramme and (B) 0.2098 gramme of iodide; hence a. 3.643: O'1353 :: 100 : 37112 B. 15913 02098 :: 100 : 3.5481 which give a mean of 3.6296. Therefore 100 grammes of iodide in the amorphous condition produced, in passing into the crystalline condition, intercrystalline spaces capable of containing 3.6296 grammes of iodide. From an observation which was made on a cylindrical mass of iodide a centimetre diametre, which in undergoing expansion in the passage from the amorphous to the crystalline condition had produced a separation amounting to half a millimetre in a tube which had yielded to the expansion, the expansion of the mass, plus the intercrystalline spaces within it, was found to be o'047619; hence a volume of amorphous icdide represented by unity, becomes a volume of 1047619 in passing into the crystalline condition, plus the intercrystalline spaces; and the volume of these spaces having been determined above, we find that the actual change of volume which takes place simultaneously with the change of molecular condition amounts to 0011323; that is, a volume of iodide at its point of maximum density (116° C.) represented by unity, becomes a volume of 1'011323 in changing to the crystalline condition. Frequent fusion and cooling appear to render a mass of iodide more brittle and crystalline, and to promote the formation of large fissures. The iodide prepared by dissolving silver in hydriodic acid and subsequent fusion, |