upon a plane surface of glass and the system is illuminated by an obliquely incident beam of monochromatic light, as, for example, by a sodium-flame, the well-known phenomenon of Newton's rings is observed with remarkable distinctness and perfection of definition. The symmetry of the rings will depend, in part, on the perfection of figure of the lens, in part on that of the plane surface. An extremely minute deviation from a perfect plane will produce a marked distortion of the circular figure of the ring nearest the centre. That this distortion is or is not due to the lens may be determined by rotating the lens round its optical axis normal to the plane. No change of figure will be seen if the lens is perfect in form and the inequality is in the plane surface only. Different parts of the plane surface may of course be tested in succession, by moving the lens from point to point; and, if necessary, the rings may be observed with a telescope. Prof. Rood, of New York, has suggested for the observation of Newton's rings a method which permits of the employment of a lens of comparatively small radius of curvature and a microscope. In his arrangement the lens and plate of glass are placed upon the stage of the microscope, the light from beneath being cut off; and monochromatic light is then thrown down upon the system by means of a plate of glass with parallel surfaces inclined to the axis of the microscope at a convenient angle, and placed between the objective and the plano-convex lens. In this manner the rings are seen with great distinctness and beauty, and the arrangement is particularly compact and convenient. The interference bands of Talbot afford a method not merely of observing with great precision the inequalities of surface and want of parallelism of the faces of plates of glass, but also of photographing these defects and obtaining a permanent chart of the glass which may be of material assistance in correcting its figure. It is only necessary for this purpose to place the glass to be examined near to the object-glass of the collimator and perpendicular to its axis, so as to intercept that half of the bundle of parallel rays which falls upon the first surface of the first prism nearest its refracting edge. If the plate has perfectly plane and parallel surfaces, the interference bands will be sharply defined and parallel in the whole field of view. The slightest inequality of surface or inclination of the faces will produce curvature or distortion of the bands; and if the eyepiece of the observing-telescope be removed, the image may be received on a sensitive plate and photographed. The number of prisms to be employed in a particular case will depend upon the thickness of the plate of glass examined, and, in general terms, upon its dispersive power. For a piece of French plate glass four millimetres in thickness, two bisulphide-of-carbon prisms of 60° must be used to produce a sufficient separation of the interference bands to enable them to be seen distinctly. More prisms must be used for thicker plates; and in this way a limit is soon reached at which the method ceases to be applicable.-Silliman's American Journal, July 1870. Tectures on Mineralogy applied to Geology and the Arts are given by Professor TENNANT, F.G.S., at King's College, London, on Wednesday and Friday Mornings, from 9 to 10 o'clock, and on Thursday Evenings from 8 to 9, from October 7th to Christmas, to which the public are admitted on paying the College Fees, namely, Two Guineas to the Morning Course, and One Guinea to the Evening. The Course commences with a description of the Physical and Chemical characters of Minerals in general, and includes a minute description of all the substances entering into the composition of Rocks, and of those Minerals which are also used in the Arts; illustrated by an extensive collection of characteristic specimens, and diagrams of the principal crystalline forms, &c. The Students are accompanied by the Professor to the Museum of Practical Geology, the British Museum, and other public institutions, and also on excursions into the country. MR. TENNANT also gives private instruction in Mineralogy and Geology at his residence, 149 Strand, London, W.C. TO GEOLOGISTS. Professor TENNANT has received instructions to dispose of, by private contract, the extensive and valuable Collection of Tertiary and other Fossils the property of N. T. WETHERELL, Esq., F.G.S., of Highgate. This Collection (well known as the North London) consists principally of a very large series of London-Clay Fossils and Minerals from Highgate and its vicinity, as also from Bognor, Bracklesham Bay, Hampshire, and Sheppey. The fossils are for the most part carefully arranged and named. Many of the specimens are the originals which have been figured in various works, while others are quite new to science. In addition to the above-named, there is an extensive series of Fossils and Minerals from the Boulder Clay of Muswell Hill and Finchley. Also numerous Fossils from the Chalk, the Greensand, Oolite, Lias, &c. Apply to Professor TENNANT, 149 Strand, London, W.C. MR. TENNANT, 149 Strand, London, W.C., has for sale two Cabinets, measuring 9 feet 3 inches long, 2 feet 4 inches wide, and 3 feet 10 inches high; each containing 45 drawers, with a Glass Case on the top of each Cabinet, 4 feet 11 inches high, and 15 inches from back to front. One Cabinet is filled with 2600 Minerals, the other with 3400 Fossils. The Collection consists of six thousand specimens, many very select. The first Gold Nugget received from Australia, which was exhibited in the Exhibition of 1851, is in the Collection; it cost £37, and contains about 8 ounces of gold: also a fine series of Diamonds, illustrating crystalline form and colour. The specimens have been used to illustrate the Lectures on Mineralogy and Geology at King's College, London, and at the Royal Military Academy, Woolwich. Price THREE THOUSAND GUINEAS. Any person wishing to become practically acquainted with the interesting and important study of Mineralogy and Geology will find this a good opportunity to obtain an instructive and valuable Museum, the specimens having been collected with care and at great expense during the last thirty years. Elementary Collections at 2, 5, 10, 20, 50, to 100 guineas each, and every requisite to assist those commencing the study of these interesting branches of Science, a knowledge of which affords so much pleasure to the traveller in all parts of the world. A collection for Five Guineas, to illustrate the recent works on Geology, by Ansted, Buckland, Lyell, Mantell, Murchison, Page, Phillips, and others, contains 200 specimens, in a plain Mahogany Cabinet, with five trays, comprising the following specimens. viz. :— MINERALS which are either the components of Rocks, or occasionally imbedded in them-Quartz, Agate, Chalcedony, Jasper, Garnet, Zeolite, Hornblende, Augite, Asbestos, Felspar, Mica, Talc, Tourmaline, Calcite, Fluor, Selenite, Baryta, Strontia, Salt, Sulphur, Plumbago, Bitumen, &c. NATIVE METALS, or METALLIFEROUS MINERALS; these are found in masses or beds, in veins, and occasionally in the beds of rivers. Specimens of the following Metallic Ores are put in the Cabinet :-Iron, Manganese, Lead, Tin, Zinc, Copper, Antimony, Silver, Gold, Platina, &c. Rocks: Granite, Gneiss, Mica-slate, Clay-slate, Porphyry, Serpentine, Sandstones, Limestones, Basalt, Lavas, &c. PALEOZOIC FOSSILS from the Cambrian, Silurian, Devonian, Carboniferous, and Permian Rocks. SECONDARY FOSSILS from the Rhætic, Lias, Oolite, Wealden, and Cretaceous Groups. TERTIARY FOSSILS from the Plastic Clay, London Clay, Crag, &c. In the more expensive collections some of the specimens are rare, and all more select. JAMES TENNANT, Mineralogist (by appointment) to Her Majesty,! 149 Strand, London, W.C. October 1870. XXIX. On Ocean-currents.-Part III. On the Physical Cause of Ocean-currents. By JAMES CROLL, of the Geological Survey of Scotland..... page 233 XXX. On Statical and Dynamical Ideas in Chemistry.—Part II. Chemical Substance and Chemical Functions. By EDMUND J. MILLS, D.Sc... XXXI. On the Magnetism of Electrodynamic Spirals. By GEORGE GORE, F.R.S. ... XXXII. Memoir on Internal Work in Gases. By M. ACHILLE CAZIN XXXIII. On the Thermodynamic Acceleration and Retardation of Streams. By W. J. MACQUORN RANKINE, C.E., LL.D., F.R.SS. Lond. and Edinb..... 259 264 268 288 XXXIV. On Thermodynamics. By W. J. MACQUORN RANKINE, C.E., LL.D., F.R.SS.L. & E................... 291 XXXV. On an Object-glass Spectral apparatus. By SIGMUND MERZ. (With a Plate.) ..... 186 XXXVI. On the Action of Low Temperatures on Supersaturated Saline Solutions. By CHARLES TOMLINSON, F.R.S. .... XXXVII. Addendum to a Theory of Nebulæ and Comets. By A. S. DAVIS, B.A., Mathematical Master, Leeds Grammar School XXXVIII. Notices respecting New Books :-Researches on Diamagnetism and Magnecrystallic Action, including the question of Diamagnetic Polarity. By JOHN TYNDALL, LL.D., F.R.Ś. XXXIX. Proceedings of Learned Societies: ROYAL SOCIETY:-Dr. W. HUGGINS on the Spectra of Erbia and 294 295 300 301 302 309-310 XL. Intelligence and Miscellaneous Articles :Experimental Research on the Influence of Heat on Electromotive Force, by Dr. L. Bleekrode.. On Tests for the Perfection and Parallelism of Plane Surfaces of Glass, by Wolcott Gibbs, M.D., Rumford Professor in Harvard University 310 311 It is requested that all Communications for this Work may be addressed, post-paid, to the Care of Messrs. Taylor and Francis, Printing Office, Red Lion Court, Fleet Street, London. Vol. 40. ここ NOVEMBER 1870. No. 268. Published the First Day of every Month.-Price 2s. 6d. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE, AND JOURNAL OF SCIENCE. Being a Continuation of Tilloch's Philosophical Magazine,' CONDUCTED BY SIR ROBERT KANE, LL.D. F.R.S. M.R.I.A. F.C.S. AND WILLIAM FRANCIS, PH.D. F.L.S. F.R.A.S. F.C.S. FOURTH QERIES. N° 268.-NOVEMBER 1870. WITH TWO PLATES, Illustrative of Professor ZÖLLNER'S Paper on the Temperature and Physical LONDON: PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET, Printers and Publishers to the University of London. Sold by Longmans, Green, Reader and Dyer; Kent and Co.; Simpkin, Marshall and Nearly ready. By PROFESSOR MORRIS, F.G.S., and PROFESSOR RUPERT JONES, F.G.S., Series 1.-Heads of Eight Courses of Lectures on Geology and Mineralogy, with Synopses and Tables: for the use of Students and Teachers. Series 2.-A Handbook of Geology and Mineralogy. In the press. John Van Voorst, 1 Paternoster Row. In One Volume, post 8vo, price 5s. cloth, THE FORCES OF THE UNIVERSE. By GEORGE BERWICK, M.D. London: Longmans, Green, and Co., Paternoster Row. BOOKS PRINTED FOR THE USE OF THE CHARTERHOUSE SCHOOL. Rudiments of the Latin Language, 3s. cloth. Rudiments of the Greek Language, 3s. cloth. A Greek-English Vocabulary of Words contained in the Four Gospels and the Acts, 4s. cloth. 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