arsenic which would be oxidised by ahout o'5 grm. of potassium dichromate. It was found that o 2500 grm. of arsenious oxide could be successfully oxidised by this amount of that reagent. In experiments (6) and (7) some water and less bydrochloric acid was used without interfering with the oxidation. In experiment (8) neither arsenic nor germanium were present, the object being to determine the possible interfering action of chlorine evolved upon the test with hydrogen sulphide. The result was satisfactory. In experiments (9) and (10) small amounts of germanium oxide were used and a very satisfactory result was obtained when only 0.0005 grm. was present. Finally in experiments (11) and (12) a mixture of the two oxides was treated according to the method, and it was found that 0·0005 grm. of germanium oxide could be readily detected when present with o'r grm. arsenious oxide. | was mounted on a platform in a central brass chamber from the two opposite sides of which projected a brass collimator tube and a brass tube terminated by a plate holder. This latter consisted of a shallow rectangular box carrying a frame which could be slightly rotated when adjustment was necessary. The box was provided with an opening at one end through which the photographic plate could be slipped into position. When the plate was in position and ready for an exposure, the opening was closed with a small brass cap and the joint made air-tight by means of a rubber washer. The operation of inserting the plate and of closing the opening in the plate holder was, of course, carried out in the dark. The photographic plates used were narrow strips cut from Schumann plates recently prepared and put on the market by the Adam Hilger Co. In the course of the experiments it was found necessary to remove all water vapour from the spectrograph in order to secure the best photographic results in the region of shorter wave-lengths. For this purpose a large cylindrical brass drying chamber, shown in Fig. 1, was attached to the collimating tube. Through a narrow opening in one end of this chamber it was possible to insert or remove when desired a shallow tray filled with phosphorus pentoxide. The plate P, Fig. 1, provided with a rubber washer enabled one to close the opening quite satisfactorily. The collimator tube was provided with two closely fitting tubes, A and B, concentric with it. One of these, A, carries a disc in which was inserted the condensing fluorite lens, and the other, B, carried a plate to which the slit jaws were attached. The focussing fluorite lens was VACUUM ARC SPECTRA OF VARIOUS ELEMENTS IN THE EXTREME ULTRA-VIOLET.* By Prof. J, C. MCLENNAN, F.R.S.; D. S. AINSLIE, M.A.; and D. S. FULLER, M.A., University of Toronto. 1. Introduction. IN the prosecution of the work on ionisation potentials recently set in train by one of us, it becomes of import ance to extend the development to include an examination of the series spectra of certain of the elements farther down in the ultra-violet region than was accessible with an optical system of quartz. In order to carry out this extension of the work a small 60° prism and two bi-convex lenses of about 15 cm. focal length for the visible range of wave lengths, all of good quality fluorite, were made for us by the Adam Hilger Co., and these optical parts were used in the constraction of a spectrograph made up in the laboratory workshop of the University of Toronto after our own design. Great difficulty was experienced in developing the instrument to a stage which would enable one to make rapid and satisfactory progress with it. The working out of a satisfactory arrangement of devices for establishing and maintaining at will an arc in vacuo was found especially trying, but eventually the main difficulties were overcome. The most rapid progress could be made, it was found, by cutting out all more or less complicated arrangements and using only the simplest possible operating attachments. carried on a disc borne by a sliding tube, E, similar to A and B. The edges of the discs which carried the lenses were pierced with a number of holes so that the air bad free access through them to all parts of the instrument. The air was removed from the apparatus by means of a set of oil-sealed mechanical Trimount pumps, the connections being made with lead piping at the points shown in the figure. Before the instrument was used with a vacuum arc the slit, lenses, and prisms were all carefully adjusted so that with an aluminium spark in air as a source the lines in the neighbourhood of x 1858 A, as well as those up to 2200 A, could be seen clearly focussed and well up on the right-hand side of the plate. Various forms of "lamp" or "lantern" were tried out without success, but the simple arrangement shown in Fig. I was finally adopted as giving the best and at the same time quite satisfactory results. The arrangement consisted of a T.tube, D, attached to the collimator by means of a shoulder at c. One electrode was led in from below and the other through the end of the "lantern." When soft metals were used the lantern was turned through an angle of approximately 45°, so that both the electrodes were at an angle with the vertical. Flexible joints with oil seals were used with each of the electrodes, so that they could be readily adjusted without allowing any air to enter the spectrograph. The electrodes were originally insulated with ebonite bushings, but in later work, where high temperature arcs were used, the ebonite was replaced by porcelain bushings. The lantern was provided with a window of glass through which the arc could be viewed. Aug. 8, 1919 A short glass Geissler tube, not shown in the diagram, was connected with the spectrograph. The terminals of a small 4-volt induction coil were attached to this tube, and when the discharge from the coil was sent through it the appearance of the discharge served as a guide to estimate the degree of exhaustion attained within the instrument. The Trimout pumps were found to be exceedingly efficient. When all leaks were stopped and the pumps put into operation, the cathode ray vacuum was obtained in a few minutes. Initially, the apparatus was exhausted several times and washed out with hydrogen before steps were taken to make an exposure. After the last washing the pumps were kept running for a quarter of an hour before an exposure was made. At this stage the Geissler tube showed a faint greenish yellow fluorescence similar to that exhibited by a "hard" X-ray tube. In the later experiments it was found that washing out with hydrogen was not necessary when plenty of phosphorus pentoxide was used. The arc used in the experiments was produced by joining the electrodes to the 220 volt D.C. mains with a suitable resistance in series to regulate the current. The arc was established by simply bringing the ends of the electrodes together and then drawing them slightly apart. With substances having a high fusion point such as copper, the temperature of the arc was not sufficient to melt the metal and the arc was of an intermittent nature giving an intense flash on each break when a current of 8 to 15 amperes was used on closed circuit. For metals of low melting-point quartz tubes were originally placed around the metal, leaving a small end projecting so as to permit electrical contact to be made. When the arc was once started the metal at the top of the electrode melted and a continuous intense source of light was secured with a current of from 4 to 6 amperes. In the later experiments with soft metals it was found to be more convenient to place the metal in hollows made in the ends of two iron rods used as electrodes. The time of exposure was equivalent to about five minutes continuous arcing. By one or other of the methods indicated ahove, arc spectra for copper, aluminium, zinc, carbon, iron, tin, thallium, lead, nickel, cobalt, and cadmium were obtained. In la er work, a hydrogen discharge tube of the end-on type was constructed so as to fit in the socket at the end of the collimator, which was also used for attaching the vacuum arc lamp. A fluorite window was sealed in the end of the discharge tube through which the light passed to the slit. The discharge was obtained by attaching the tube to a small induction coil without additional capacity in the circuit. In addition to the spectrum of hydrogen, several lines were obtained corresponding to the main lines of the carbon monoxide spectrum as given by Lyman. This, no doubt, was due to an impurity in the hydrogen used in the discharge tube. In order to compare the metal spectrum with that of hydrogen, an adjustable cover was provided for the slit, so that one half of the slit could be exposed for the metal arc and the other half for the gas discharge. By means of this slit cover the metal and gas spectra were photo graphed on the same plate one above the other. In working with metal arcs the secondary hydrogen spectrum was always present along with the metal spectrum. The lines due to this gas effect were, however, easily picked out by having the metal and hydrogen spectrum photographed together on the same plate. When an intermittent arc with a small current was used the time of exposure was longer, and the secondary spectrum came up strong, while with higher currents the arc remained much steadier, and the secondary spectrum was relatively much weaker. 3. Method of Determining Wave-lengths. In calculating the wave lengths of the lines in the different spectra certain lines previously measured were used as standards. From these the wave-lengths were determined by means of graphical interpolation. In NEW BOOKS.-Hodder and Stoughton have in the press "The New Teaching Series" of text-books. The volumes already in hand include: "Chemistry from the Industrial Standpoint" and "Chemistry and Bacteriology of Agriculture." It is claimed that the series will represent a new departure in text-books. OWING to the death of Sr Boverton Redwood, Bart., Mr. Arthur W. Eastlake, and Mr. William Sutton, who have been associated with him for the past 28 and 23 years respectively, in inspecting and reporting on petroleum properties in all parts of the world, have arranged to enter into partnership for continuing inspection and advice on all matters in connection with the petroleum industry in a similar manner to that undertaken by Sir Boverton. Robert Redwood has kindly consented to co-operate with them, to give them the benefit of his experience, and to undertake the examination of oil samples, &c. They will also be assisted by Mr. W. H. Dalton in geological matters, and by other gentlemen pieviously associated with the late Sir Boverton Redwood. Mr. Works Chemist required immediately. Salary about £350. Must have Works experience and sound Scientific training, especially Organic Chemistry. Responsible position. Good prospects. Address, Box C 309, care of Dawson's Advert. Service, 121, Cannon Street, E.C. 4. BIRKBECK COLLEGE. SENIOR LECTURER IN CHEMISTRY. The Governing Body invite applications for the above Appointment. Commencing salery £320-350 (according to experience and qualifications), with regular increments. Applicants specially qualified in Organic Chemistry preferred.-Appli- BIRMINGHAM MUNICIPAL TECHNICAL cations by SEPTEBER 10 to the PRINCIPAL CLERK, Birkbeck College, Breams Buildings, E.C. 4, from whom further particulars may be obtained. SCHOOL. invited for the Post of LECTURER IN METALLURGY. Salary £250 to £450. In addition to the salary indicated, there is at the present time a War allowance of £78 per annum. The commencing salary will be dependent upon the experience of the applicant. Full particulars may be obtained on application to the SECRETARY, Municipal Technical School, Suffolk Street, Birmingham. UNIVERSITY OF BIRMINGHAM. SCIENCE, Faculties. MEDICINE, COMMERCE. SPECIAL SCHOOLS OF LANGUAGES. DEPARTMENT FOR TRAINING OF TEACHERS. F. E. BECKER & CO.'S SHORT BEAM Sensitive to 110th mgrm. Agate knife edges and planes throughout; richly gilt beam graduated along its entire length with rider apparatus of new construction; extra wide pans; double hooks and wooden bridge for specific gravity work. Highest quality workman. ship and finish, in elegant polished case with front counter poised sliding door and two side doors; all mounted on heavy thick black plate-glass base provided with levelling screws and spirit level. 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