HE structure of U2Na has been described by Rundle et al.' as body-centered cubic (Mn20a type) with a lattice parameter of 10 6 i 8 A. Vaughan,2 on the other hand, found a hexagonal structure (La203 type) with a = 3.69 A and c = 5.83 A He concluded that the body-centered cubic and the hexagonal forms are allotropic modifications of the same compound, UzN3.The significant difference in the two investigations that yielded these results was in the methods by which the compounds were prcpared The U2N3 examined by Rundle and co-workers was prepared as a powder (by treating U N with excess nitrogen). That investigated by Vaughan occurred as an intermediate layer between UN2 and UN formed on the surface of a massive uranium bar by reaction with nitrogen.I t is suggested that the hexagonal form of U2N3 is in fact what may be termed a "coherency structure," akin to epitaxial growth, dictated by the structure of the outermost nitride layer, UN2. .4ltliougli no orientation relations have (to the writer's knowledge) been rcported for a layered sequence of the nitrides UNZ, U Z N~, and UN there would be a very good match if (0001) and [2110] T 305 of the hexagonal U2N3 were parallel to (111) and [lTO], respectively, of the UNZ. Rundle et al. measured the lattice parameter of UN2 and found it to be 5.310 A. The corresponding spacing along [lT0] is therefore 3.755 A. The value of a (3.69 A) quoted by Vaughan thus represents a mismatch of only -1.7% in terms of the UN2 [170] spacing, along [ZiTO] in U2N3. Received March 13, 1962. The writer is senior lecturer in metallurgy,

In this study, the relative crystal growth rate (Vcr) of calcium oxalate (Caox) and a number of other parameters were determined in 17-h daily (d) and 7-h nocturnal fractions (n) of whole urine from 20 recurrent Caox stone formers (SF) and 29 age-matched male normal controls (NC). Vcr, which was determined by the gel crystallization method (GCM), showed the largest difference between SF and NC among all parameters under investigation. Mean values (+/- SD) obtained for Vcr were: 0.73 +/- 0.58 (SF-d)/0.21 +/- 0.22 (NC-d; P less than 0.001) and 0.63+/- 0.58 (SF-n)/0.24 +/- 0.25 (NC-n; P less than 0.01). Significantly higher concentrations of Ca and lower concentrations of thermodynamic and kinetic effectors of Caox crystal growth were responsible for the higher crystal growth rates observed in SF as compared with NC, i.e., they should be partially causative in Caox urolithiasis. However, other properties of urine or the urinary tract (potentially, crystal agglomeration and adhesion) must be accounted for in the genesis of Caox stones.

No abstract

NOWLEDGE of thc apparent density of solid materials is often useful for determining the porosity of many products and K raw materials. In the usual applied methods' the porous core sample is immersed in mercury with the assumption that mercury, because of its high surface tension, does not penetrate the capillaries. These methods give good results when the sample is large enough to be pushed into the mercury with prongs. This method, however, cannot be used in the case of granular or powdered samples. The use of standard pycnometer bottles causes serious errors because (1 ) many materials have densities less than that of the mercury and will float on its surface, (2) air bubbles are formed between the particles and the pycnometer plug, and (3) a thin layer of air on the surface of the solids may prevent direct contact between the mercury and the solid surface. The pycnometer presented here (Fig. 1) is designed on the same principle as that previously designed by the writer2 but with several improvements. I t avoids the sources of error mentioned and is simple to operate. The weighed sample is placed in the pycnometer and evacuated. The shape of the pycnometer permits the mercury, introduced after evacuation, to squeeze the particles to the convex side of the pycnometer and contact of particles with the pycnometer plug is avoided. The sloping bottom keeps the particles in the lowest part of the pycnometer before the mercury is introduced. As mercury usually contains a small quantity of included air, it is also evacuated before intro-

No abstract