_Not in Solar Spectrum_. Antimony, arsenic, bismuth, boron, nitrogen, cæsium, gold, indium, mercury, phosphorus, rubidium, selenium, sulphur, thallium, praseodymium. With respect to these tables, Prof. Rowland adds: "The substances under the head of 'Not in the Solar Spectrum' are often placed there because the elements have few strong lines or none at all in the limit of the solar spectrum when the arc spectrum, which I have used, is employed. Thus, boron has only two strong lines at 2497. Again, the lines of bismuth are all compound, and so too diffuse to appear in the solar spectrum. Indeed, some good reason generally appears for their absence from the solar spectrum. Of course, there is but little evidence of their absence from the sun itself; were the whole earth heated to the temperature of the sun, its spectrum would probably resemble that of the sun very closely." The powerful instrument used at Baltimore for photographing spectra, and the measuring engine constructed to fit the photographs so that its readings give the wave lengths of lines directly within 1/100 of a division on Angström's scale, give the foregoing results a weight superior to many others published. * * * * * ALLOTROPIC FORMS OF METALS. Writing on some curious properties of metals and alloys, Mr. W.C. Roberts-Austen, says the _Engineer_, remarks that the importance of the isomeric and allotropic states has been much neglected in the case of metals. Joule and Lyon Playfair showed, in 1846, that metals in different allotropic states possess different atomic volumes, and Matthiessen, in 1860, was led to the view that in certain cases where metals are alloyed they pass into allotropic states, probably the most important generalization which has yet been made in connection with the molecular constitution of alloys. Instances of allotropy in pure metals are: Bolley's lead, which oxidizes readily in air; Schutzenberger's copper; Fritsche' tin, which falls to powder when exposed to exceptionally cold winter; Gore's antimony; Graham's palladium and allotropic nickel. Joule has also proved that, when iron is released from its amalgam by distilling away the mercury, the metallic iron takes fire on exposure to air, and is therefore clearly different from ordinary iron. * * * * * A New Catalogue of Valuable Papers Contained in SCIENTIFIC AMERICAN SUPPLEMENT during the past ten years, sent _free of charge_ to any address. MUNN & CO., 361 Broadway, New York. * * * * * THE SCIENTIFIC AMERICAN Architects and Builders Edition. $2.50 a Year. Single Copies, 25 cts.