The effects of ultraviolet radiation on microorganisms have been studied frequently. The lethal action of sunlight on certain bacteria was demonstrated by Downes and Blunt as early as 1877. Roux (1887) showed that spores as well as bacteria are destroyed by these radiations and in 1903 Barnard and Morgan reported that the bactericidal action of radiant energy is limited to wavelengths shorter than 3000k. The susceptibility of microorganisms to ultraviolet radiation under varying conditions has also received considerable attention. Wells and Wells (1936), Whiser (1940), and Koller (1939) claim that airborne bacteria are ten times as resistant to radiation at high relative humidity as when floating in air at low humidity. Koller's results indicate that floating bacteria are less resistant to ultraviolet than similar bacteria on the agar surface of a Petri plate and Wells (1940) states that airborne bacteria are about twenty times less resistant than those floating in a liquid suspension. Rentschler and Nagy (1940) report the same sensitivity for air borne and for identical bacteria on the surface of agar and no effect due to humidity. The Bunsen-Roscoe reciprocity law for the bactericidal action of ultraviolet has been tested by Coblentz and Fulton (1924), by Gates (1929) and by Koller (1939) with varying results. Conflicting theories have been advanced to explain the nature of the bactericidal action of ultraviolet radiation. In recent years a rapid and simple method for measuring ultraviolet radiation has been made available by one of us (Rent-745
A general survey of contemporary work on the preparation of zirconium is presented. Experimental equipment for the production of rare metals in general is described and applied to zirconium by a detailed study of the reaction: ZrO~ + 2Ca = Zr + 2CaO. The relation of excess reducing agent and addition agents to residual impurities in the metal product is evaluated and the effect of these impurities on sintered and melted compacts is shown. By the control of variables referred to above, the preparation of relatively soft easily machinable zirconium compacts is described and some physical properties of the metal are determined and recorded. I NTRODU CTIONThe metallurgy of the metals of Group IV of the Periodic Table which includes titanium, zirconium, and thorium is not new, although it is only in the past twenty-five years that these metals have been produced in high purity and their properties evalulated. The oxides of the metals in this group are not reduced by hydrogen or carbon to the pure metallic state and, therefore, methods commonly used for iron, nickel or copper are excluded. Because of their high melting points and reaction with ordinary refractories, they are not easily melted or cast. ZIRCONIUM Review of Early WorkMethods of producing zirconium are, with few exceptions, very old, and modern methods are improvements resulting from purer starting materials and better technics. Old procedures and contemporary methods include: (1) reduction of zirconium halides with the alkali and alkaline earth metals; (2) reduction of oxides with calcium or magnesium; (3) alumino-thermic reductions; (4) thermal dissociation.Thermal dissociation of zirconium iodide on a heated filament, classically represented by the work of de Boer, Van Arkel and Fast x t Manuscript
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