522 Me Coy and Ashman — Urano- Uranic Oxide. 



of radium present was very small : 2X10- 6 of the equilibrium 

 quantity. After conversion into nitrate and two crystalliza- 

 tions of the latter from water, no trace of impurity could be 

 found by chemical tests ; the material formed sample B. 



(C) The third sample was prepared from the uranium 

 extracted from 29 g. of pitchblende from the Wood Mine, 

 Colorado and purified as previously described, {toe. cit.) The 

 process in brief consisted in treating the nitric acid solution of 

 the mineral, after removal of silica, with an excess of ammonium 

 carbonate solution to remove iron, etc., and the filtrate with 

 ammonium sulphide to remove copper, lead, etc. The crude 

 uranyl carbonate, obtained by boiling the last filtrate, contained 

 two percent of the equilibrium quantity of radium. This was 

 removed by three precipitations of barium sulphate in the solu- 

 tion, the first precipitate taking out 95 per cent of the radium 

 then present. The U 3 8 made from this material three years 

 ago seemed to be very pure and had, as then stated, an activity 

 within 0'15 per cent of that of another sample of tl 3 8 , which 

 was supposed to be, and probably was, of great purity. A 

 large portion of this material had been kept in solution as 

 ammonium uranyl carbonate from May, 1905 to November, 

 1907. During this time a very small precipitate had formed. 

 The filtrate from this precipitate was boiled ; the uranyl car- 

 bonate so formed was called sample C. 



By decomposing uranyl nitrate at a temperature below red- 

 ness, the orange oxide, U0 3 , is obtained ; this loses oxygen at 

 a higher temperature, giving U 3 8 . This latter oxide is not 

 perfectly stable, but loses oxygen slowly when very strongly 

 heated, as first observed by Zimmerman.* 



From considerations based upon the phase-rule, for the three- 

 phase system, U 3 8 , U0 2 and 2 , the partial pressure of the 

 oxygen is a function of the temperature ; consequently for a 

 fixed pressure of oxygen (say that in the atmosphere) there 

 must be a definite temperature at which the three phases can 

 exist in equilibrium ; above this temperature U 3 8 will pass 

 into U0 2 ; below it, the dissociation will not take place. Since 

 the temperature of a solid contained in a crucible heated in the 

 flame of a blast-lamp is far from uniform throughout the mass, 

 we have used an electric muffle which gave perfectly definite 

 temperatures, which were accurately measured by means of a 

 platinum-rhodium pyrometer. 



In one experiment Y g. of sample A, purified uranyl nitrate, 

 was converted at a moderate temperature into the orange- 

 colored trioxide. This was placed in a platinum crucible and 

 heated in the electric muffle. The crucible was loosely covered, 

 allowing free access of air. After constant weight had been 

 * Ann. ccxxxii, 276, 1885. 



