the four outer circles show by their imperfection the development of the circle from the two points that are formed by a double-refracting crystal. Authorities. HOOKE.-Micrographia. 1664. GRIMALDI. Physico-mathesis de lumine, coloribus, et iride. Bologna, 1665. FRAUNHOFER.-Neue Modifikation des Lichtes, &c. Munich, 1823. SCHWERD.-Die Beugungserscheinungen. Mannheim, 1835. VI. On some Experiments with Carbon Dioxide in the Solid State. By Dr. L. BLEEKRODE*. WHEN working of late with very low temperatures produced by evaporating solid carbon dioxide I met with some facts which are perhaps not generally known, and may I think prove useful for lecture experiments. Í obtained the solid substance in the usual way from its liquid state, now so easily to be had in large quantities in the iron bottles prepared for industrial purposes. If a cloth bag (first used by Bianchi in 1870) is tied to the deliveryvalve, the solid may be readily collected inside, and this method is as effective and more simple than the use of ebonite or metal boxes, which are liable to have their parts frozen together and then cannot be quickly emptied. Compressed Carbon Dioxide.-This substance on being removed from the bag presents itself in a very divided state and therefore evaporates comparatively quickly; but on submitting it in suitable moulds of wood or metal to high pressure, either by hammering, or still better with screw and lever (as for instance the arrangement for regelation experiments with ice), a most compact substance is obtained, in the form of cylinders, disks, lenses, cups, &c. that can much longer endure the heating-effect of surrounding air. Such compressed cylinders were described by Prof. Landolt in 1884†; he obtained them by hammering the mass together; the specific gravity of the carbon dioxide in that condition was determined, first by calculating the volume of the cylinder from the geometrical dimensions, and * Communicated by the Author. † Landolt, Chemische Berichte, 1884, xvii. p. 309. Phil. Mag. S. 5. Vol. 38. No. 230. July 1894. G then by ascertaining its weight with the balance, taking care to collect the gas, which evaporated during the process, in a potash solution, connected with a delivery-tube to the vessel containing the solid substance, the whole arrangement of course standing on one of the scales of the balance. In this way the specific gravity was found to be 1-2 (in the liquid state, at 12.5 C. it is 0.88), and accordingly the larger pieces of the dioxide sink in water; the smaller ones remaining suspended on the surface owing to the evolving gas. I myself found in different experiments the number 1-3, and even 16; this greatly depending on the pressure exerted, which, with my apparatus, could be made very high; at the latter density, the cylinders looked semi-transparent. The great advantage now gained lies in the very slow evaporation. Landolt mentions that he was able to keep a cylinder with dimensions of 25 mm. and 26-8 mm., having a weight of 15.63 grammes, during one hour and a half; I could maintain one, at a temperature of the atmosphere of 15° C., having a weight of 41 grammes, during nearly the same time; but he was even able to keep a cylinder of 41 mm. diameter and 53 mm. high during five hours. The surface, as is to be expected, soon becomes covered with ice needles that are easily nipped off, but Landolt is of opinion that a hydrate is also formed. * Production of Electricity.-It has been remarked by more than one observer that a strong jet of carbon dioxide, obtained from its liquid, on expanding in the atmosphere, may produce an electric charge of marked intensity. Riess in his wellknown book on frictional electricity mentions that, as long ago as 1852, Joly saw electric sparks given off by an iron bottle which was filled with liquefied gas; and also Ducretet at Paris made a communication in 1884, on the appearance of sparks, when he caused the gas to escape from its liquefied state in a box of ebonite, in order to collect the solid substance. More recently Hausknecht published the same observation, but adds the important remark that it is necessary, when powerful effects are aimed at, to use a gas absolutely free from air, and therefore that prepared by chemical means is better suited for these experiments than that obtained from natural sources, as manufactured on the Rhine. The cloth collectingbags show an intense electric charge, emitting a violet light inside, and he got sparks of even 20 centim.; but they do not appear before a dense layer of crust of solid matter has been deposited. I found this confirmed, and after I had insulated the iron bottle on paraffin supports, I applied a delicate gold-leaf electroscope, and ascertained, when gas was strongly issuing, *Chemische Berichte, 1890, xxiv. p. 1032. * that in general it was negatively electrified. The gas jet, escaping through the bag, charged the electroscope with positive electricity when very near to the cloth, but frequently, when at some distance, the instrument indicated a negative charge, when struck by the gas-current. When the bag was inserted in a cylinder of narrow copper gauze I got sparks of more than 2 centim. when approaching a metallic conductor. A year ago Wesendonk published an extensive research bearing on the question whether electricity is actually generated by friction of gases against metals, already treated by Faraday †, and he came to the conclusion that the gaseous state does not develop electricity, and if indeed a charge manifests itself in the electroscope, it is due to the fact that the gas contains moisture or particles of dust. Experiments were made with air, oxygen, and carbon dioxide under high pressure, and he stated that the latter gas is very apt to develop electricity, because aqueous vapour so easily condenses from the surrounding air about the expanding current of gas which is at very low temperature. It seems to me that great importance is to be ascribed to the minute particles of solid carbon dioxide that certainly are present in the escaping gas, when the liquid substance comes in contact with the aperture and issue-valve, as this is opened. I found, what I have not seen mentioned anywhere else, that the solid carbon dioxide itself is a substance having a great propensity to become electric, as the following experiments show. When the snow-like solid matter, after it has been taken from the receiving-bag, is put directly on the plate of the electroscope, a strong charge of negative electricity is at once apparent; on placing this instrument under the receiver of an air-pump, the divergence of the leaves rather diminishes when the air is rarefied, and it increases again on admitting air. A disk, formed by strongly compressing carbon dioxide as indicated before, when rubbed with the hands or pressed against the skin becomes negatively electrified and attracts an electric pendulum in a very marked way, even when the * Wiedemann's Annalen, 1892, xlvii. p. 529. + Faraday examined air and vapour of water mixed with different substances, and expressed the opinion that when pure they fail to produce electricity; otherwise they communicate a positive charge to the electroscope when it is near to the exit valve, the gas then striking violently against the knob of the instrument. At some distance below he states that this becomes charged negatively, because it acts only as a receiver to the gas already electrified on escaping, whilst in the former case the knob of the electroscope is itself directly electrified by the collision of the gas particles. friction has been feeble. Rubbing against a zinc or a copper plate acts in the same way, these metals showing positive electrification, and the solid carbon dioxide gets negatively electrified; this is also the case when rubbed with cloth. The readiness with which this substance acquires an electric charge is the more remarkable, as the surface of the disk or cylinder, from the very low temperature, is soon covered with moisture turning to ice particles, but these disappear on moving the hands over the surface. A more complicated case of electrification ensues when the compact disk is simply laid down on the plate of the electroscope. When freshly prepared immediately after the contact, the leaves diverge to an extent corresponding to a difference of potential of at least 200 volts, and more as compared with that effected by a zinc-water-copper battery. It is E, and when the disk of carbon dioxide is now removed without touching the metal, the leaves first collapse, and diverge again directly, and now indicate + E. Sometimes, when the substance has already been exposed to the atmosphere for a time, no charge is immediately produced on touching the electroscope, but afterwards, when it is taken from the instrument, this still shows a positive charge. It is not at once evident how to account for the production of the two opposite electricities in this case*, as several causes may have been at work. It appears to me that the chief part is played by the carbon dioxide escaping in the state of gas between the disk and the spot where it is in contact with the plate of the electroscope. At this place it is lifted up and down somewhat rapidly by the gas evolved, and it gets a negative charge, for the same reason as when rubbed with the hand as before stated, and no strong motion is required, because of its high electric properties, as I have remarked. The E that is free is carried away with the current of gas, and the negative charge of the disk may induce an opposite one in the metallic plate, on which it is lying, and recombination is prevented, because a fresh layer of gas is present and prevents actual contact; the positive charge of course becomes apparent in the electroscope as soon as its inducing electricity is removed with the disk of carbon dioxide. That the action after a certain time is much lessened may perhaps be due to the fact that the copper plate of the electroscope after prolonged contact has become too cold to provoke a rapid evolution of gas, or perhaps the surface of the carbon dioxide, by atmospheric influence, has lost, to some extent, its electric * It is necessary, to avoid errors, to make the disk of carbon dioxide neutral after each experiment: this may easily be done by moving it rapidly to and fro in a gas-flame. property. And the effect in general may be augmented by the friction of the current of gas against the surrounding air of low temperature and saturated therefore with moisture. Production of Sound.-The experiment just described, demonstrating that a piece of strongly compressed carbon dioxide on mere contact with a copper plate may acquire a sufficient electric charge to attract an electric pendulum, and that this should be originated by little impacts from the metal, seems to obtain fresh evidence from another curious fact I often observed on putting this substance in contact with metals, and which consists in a loud sound produced for some time. On further investigation it was soon ascertained that this is due to the thermal conductivity of the metal, acting by increasing the rapid evaporation of the solid matter in different points where the contact was best, and the gas evolved, more or less prevented from easily escaping between the surfaces, was alternately compressed and expanded, producing a vibratory condition of the current of gas. A convenient way to perform this experiment is to take a disk or cylinder of carbon dioxide, very highly compressed, and to lay down on it a small brass sphere with a diameter of 1 cm. ; if the whole system is placed on the resonance box of a tuning-fork (for instance that of Ut answers very well), a loud sound of very high pitch makes itself at once audible throughout the whole room; if the metal is pressed against the solid substance, it may even become so loud as to be painful to the ear. It slowly dies out as the metal grows too cold to accelerate the evaporation in an effective way, but the sound is again restored when the metal is heated for a moment. Things may be reversed : that means, a cylinder or, still better, a lens of plano-convex form may be set down on a metal surface, and if this consists of a thin copper disk of 8 cm. diameter fixed in its centre, this emits a loud sound, resembling the ringing of a bell. It is easily perceived that the tone is produced at the place of contact where the metallic sphere touches the extremely cold carbon dioxide; a little cavity is formed (as the metal is comparatively very hot), and this is somewhat closed up by the sphere sinking slowly down; the occluded gas gets more tension till it can lift the metal, when for a very brief moment it again sinks down, and so on in rapid succession. Of course the same state of things nearly occurs as with a steam-jet escaping downwards from a conical aperture, closed up more or less by a little sphere. It is well known, when high pressure is used, it is not blown away, but remains in front of the opening in a very rapid oscillating condition, and there is also a dull sound often audible. |
