with lime, of composition varying in different instances between that indicated by the respective formulæ CaO. MnO, and Ca0.2MnO2, a small portion of the lime CaO being usually replaced by its equivalent of protoxide of manganese MnO. Some idea of the rapidity of oxidation effected under favourable circumstances may be gathered from the following result. Of the entire quantity of oxygen contained in 175,000 cubic feet of air blown into the sludge within five hours, 14.8 per cent., equal to rather more than four hundredweights, was absorbed in the production of twenty-two hundredweights of manganese peroxide MnO2. It is obvious that where Weldon's process is carried out, what happens is this:-The original peroxide of manganese first oxidizes a quantity of muriatic acid, and sets free its chlorine. Then the spent manganese is caused to acquire a fresh dose of oxygen from the air, and is used to oxidize a fresh quantity of muriatic acid, and so on continuously. Thus, except during the first use of the manganese, the muriatic acid is really oxidized by the oxygen of the air, the manganese serving only as a carrier. Latterly, Mr. H. Deacon, of the Widnes Alkali Works, has conceived the idea of dispensing with the carrier-that is the manganese— altogether. It has been known for some time that when muriatic acid vapour mixed with air is passed through heated tubes, the oxygen of the air effects a partial oxidation of the muriatic acid, setting free its chlorine; but the reaction is not sufficiently complete to render the process commercially available. Mr. Deacon however has ascertained that when a mixture of air and muriatic acid vapour, heated to about 700° F., is passed over a mass of brickwork that has been steeped in solution of sulphate of copper and afterwards dried, an almost complete decomposition of the muriatic acid is effected. The copper salt certainly takes part in the reaction as an intermediary, but in the end is quite unaffected, the sole obvious result being that the chlorine of the muriatic acid is set free by the oxygen of the air, with a rapidity which seems to leave nothing to be desired. The nature of the intermediary action exerted by the copper-salt, so ingeniously made serviceable by Mr. Deacon, has not been clearly ascertained. It would appear, however, that by two simultaneous reactions taking place continuously throughout the process, chloride of copper is ever being alternately formed and decomposed; it being remembered that chloride of copper gives off a portion of its chlorine with much facility, even by heat alone. The extreme result would probably be somewhat as follows: Although this new process has not yet been worked on a manufacturing scale, considerable experience has been acquired with regard to it, and the difficulties to which it at first seemed liable have been successfully provided against. One inevitable difficulty arising from the diluted state of the produced chlorine, has proved to be less serious than was anticipated. This state of dilution is at all times considerable; for the chlorine being generated by the action of atmospheric oxygen on muriatic acid, every volume of chlorine produced even in an absolutely perfect working of the process, must be necessarily diluted with twice its volume of atmospheric nitrogen. Nevertheless, according to Mr. Deacon's experience, the diluted state in which the chlorine is always obtained, has not been found to interfere with its application to the manufacture either of chlorate of potash or bleaching powder. The requisite plant for carrying out the invention on a large scale is now being erected at Messrs. Gaskell and Deacon's works. For experimental purposes the new process of continuous chlorine production may be performed with oxygen instead of air. It then manifests a striking reverse relationship to a process of continuous oxygen production, in which oxide of cobalt acts as a permanent intermediary. In the one experiment the gas goes in oxygen and comes out chlorine, muriatic acid being decomposed and water formed. In the other experiment the gas goes in chlorine and comes out oxygen, water being decomposed and muriatic acid formed. In the presence of slaked lime to arrest the muriatic acid, this last reaction takes place readily and completely below the boiling point of water. [W. O.] WEEKLY EVENING MEETING, SIR HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. President, W. SPOTTISWOODE, Esq. M.A. Treas. R.S. and R.I. On some Experiments on Successive Polarization of Light made by Sir C. Wheatstone. THE experiments which formed the subject of this discourse were made by Sir Charles Wheatstone some years ago, but the pressure of other avocations delayed their publication. The term "Successive polarization" was applied by Biot to denote the effects produced when a ray of polarized light is transmitted through a plate of rock crystal cut perpendicularly to the axis. The plane of polarization is found to be changed on emergence and through a different angle for each homogeneous ray. The introduction of instrumental means for converting the plane polarization of the ordinary apparatus into successive, or, as it is more commonly called, circular polarization, and the explanation of the phenomena thence arising, constitute the main purpose of the communication. Polarized light is distinguished from common light by the presence of certain peculiarities not ordinarily found; but the peculiarities in question cannot be discerned by the unassisted eye, and require special instrumental appliances for detection. A simple mode of bringing light into the condition in question is by allowing it to pass through a plate of the crystal called tourmalin, cut parallel to the axis; and if the light be then examined by causing it to pass through a second similar plate, held parallel to the former and caused to revolve like a wheel in its own plane, it will be found that the intensity alternately diminishes and revives, being zero for two positions 180° from one another, and a maximum for two positions at 90° from each of the former. Thus combination of tourmalins constitute in fact a polariscope, which in general consists of two parts, counterparts of each other, the first for bringing common light into the condition in question, in other words for polarizing; the second for examining or analyzing the light. The explanation of this fundamental phenomenon is as follows:The vibrations upon which the sensation of light depends, may in ordinary light take place in any direction in a plane perpendicular to the ray. By the process of polarization they are all brought into one direction, still, however, perpendicular to the ray; so that throughout the entire ray they lie in one plane. On this account the polarization here considered is called plane polarization. There are other kinds of polarization, such as circular and elliptic, whose names are derived from the curves, or orbits, described by the vibrating particles. There are also other methods for producing plane polarization beside that above described; e. g. reflexion at particular angles from the surfaces of transparent media, transmission through parallel plates of glass, &c.; but as they all agree in reducing common light to the same condition, it is unnecessary for the present purpose to allude to them more in detail. If a ray of polarized light fall upon a plate of doubly-refracting crystal, it is divided into two, whose vibrations lie in planes perpendicular to one another. These rays traverse the crystal with different velocities, and therefore emerge with a difference of phase. On entering the analyzer the vibrations of both rays are resolved into one plane. If the plane of vibration of the analyzer be parallel to one of those of the crystal, one ray will be cut off, the other will be transmitted without change. In any other position of the analyzer the transmitted portions of the two rays will interfere so as to produce colour; and if the analyzer be then turned through 90°, the portion of the original light cut off in the first position will be transmitted, and vice versa. Of this theory the following are the experimental results:-If a plate of doubly-refracting crystal, e. g. selenite, be placed between the polarizer and analyzer, and turned round in its own plane, it will be found that in certain positions at right angles to one another no effect is produced. These may be called neutral positions. In all other posi tions the field is tinted with colour, which is most brilliant when the plate has been turned through 45° from a neutral position. If the analyzer be turned, the crystal remaining still, the colour will fade, and entirely vanish when the angle of turning amounts to 45°. From this position the complementary colour will begin to appear, and will be brightest when the angle of turning amounts to 90. The colour depends upon the thickness of the crystal, so that by a suitable preparation any arrangement of colours may be produced. So far for plane polarization. The principle of circular or successive polarization, as regards the present purpose, is as follows: If two sets of rectilinear vibrations lying in planes perpendicular to one another meet and combine, the resulting vibration will be curvilinear, whose form and position depends upon the difference of phase of the components. If the second set be in advance or in rear of the first by a quarter of a wave length, the resulting vibration will be circular; but the motion will in one case be direct (like the hands of a watch), in the other it will be reverse. If two sets of circular vibrations in opposite directions meet and combine, the resulting vibrations will be rectilinear, and the position of its plane will depend upon the difference of phase of the components. If the second set advance upon the first, the plane of resultant vibration will undergo direct rotation; if it recede, it will undergo reverse rotation. If in such an experiment white light be used, the vibrations of the different component prismatic colours will (on account of their different wave lengths) undergo different retardation; and consequently the resulting vibrations will lie in different planes, arranged in prismatic order. The order will be from red to violet, or vice versa, in accordance with the law stated above. If a ray of plane polarized light fall upon a metallic reflector, it is divided into two, whose vibrations are respectively parallel and perpendicular to the reflector; and the latter is retarded behind the former by a difference of phase depending upon the angle of incidence. If the plane of vibration of the incident ray be inclined at an angle of 45° to the plane of incidence, the two rays into which it is divided have nearly the same intensity. At an angle, nearly 45°, which varies with the metal employed, but which is perfectly definite, the intensities become accurately equal. And further, if the angle of incidence have a particular value dependent upon the nature of the metal (for silver 72°), the retardation will amount to a quarter of a wave length. These two rays, on leaving the reflector, will recombine, and in accordance with the laws above given, will in the last-mentioned circumstances become a circularly polarized ray. Lastly, the direction of motion in this circular ray will depend upon the side on which the original plane of vibration is inclined to the plane of incidence; if, when it is inclined on one side, the circular ray becomes right-handed, then, when it is inclined on the other, it becomes left-handed. Reverting then to the phenomena of double refraction produced by a plate of crystal cut parallel to the axis on a plane polarized ray; let the crystal be placed in such a position that the planes of vibration of the two resulting rays are inclined at angles of 45° on the two sides respectively of the plane of incidence; and let there be interposed between the crystal and the analyzer a silver plate at an angle of 72° to the direction of these emergent rays. Each of these rays will, in virtue of the principles enunciated above, be converted by reflexion into a circularly polarized ray; but one will be a right-handed and the other a left-handed ray; and the difference of phase produced by the doubly-refracting plate will be undisturbed by the reflexion. This difference of phase depends, as is well known, upon the wave length, in other words, upon the colour of the light. So that the two circular rays will combine to form a plane polarized ray, whose plane of vibration depends upon the difference of phase, i. e. upon its colour. And if, finally, the light be then examined by an analyzer in the usual manner, we shall have all the phenomena of circular or successive polarization. From what has been stated above, it appears that the direction of motion in the two circular rays, and consequently the order of colours produced, depends upon the position (to the right or left of the plane of incidence) of the ray which has been most retarded in the passage through the crystal plate. If, therefore, the plate being in a given position, the colours appear in an ascending order, then on turning the plate through 90° in its own plane, or on turning it over about an axis in the plane of incidence, the swifter and the slower rays will change position, and the order of colours will be reversed. The reversal of the order of colour may be exhibited in another way. Uniaxal crystals are divided into two classes: one, called positive (e. g. quartz), in which the extraordinary ray moves more slowly than the ordinary; the other, called negative (e. g. Iceland spar), in which the ordinary ray is the slowest. If, therefore, a plate of quartz placed with its axis at 45° on one side of the plane of incidence give the colours in one order, a similar plate of Iceland spar similarly placed will give them in the reverse order. The same principles apply to the case of biaxal crystals cut parallel to a plane containing the two optic_axes. A ray of plane polarized light transmitted through such a plate is divided into two, whose vibrations respectively bisect the angles formed by the two axes; the line which bisects the smallest angle is called the intermediate section, and the line perpendicular to it the supplementary section; and the order of the colours depends upon the relative velocity of the two rays. In selenite the ray whose vibrations lie in the supplementary section is the slowest; in mica it is the swiftest. Hence these two crystals will, all other circumstances being alike, give opposite orders of colour, and may be regarded as positive and negative respectively, like quartz and Iceland spar. The phenomena by which these principles may be illustrated are very numerous and varied, but are better seen than described. [W. S.] |