Nature Publishing Group 1962 'lo.4ats March 3, 1962 NATURE 867 from 1961. and the tendency seems to be the same. Analysis of single samples during the spring of 1960 indicates that the French low-yield tests of February and March that year produced no significant effect on the monthly fall-out of loug-lived radioactivity in Sweden.Owing to the low activity-level of the fall-out from weapons tests during 1960 and up to September 1961 it has been possible to carry out a quantitative examination of the presence of beryllium-7 {produced by cosmic rays) in ground-level air {Fig. 2). Beryllium-7 is a y-emitter with its main photopeak at 0•48 MeV. and 53 days half-life. There seems to be considerable variation with time. There are, however, some difficulties involved in the analysis of the spectrum', and the result is only preliminary.

Crystal Structure of Yttrium and Other Rare-Earth Borates 253( C ) 1400" F Heat Treatment: Maximum crystallinity was developed during the 1400°F heat treatment as well as maximum resistance to thermal shock. An average AT of 925°F was required to cause failure. The phases lithium titanium silicate, sphene, and albite increased to a maximum whereas rutile remained constant. The structure exhibited no crystal orientation, as in the 1600°F heat treatment, and the crystals were much better developed, as shown in Fig. 11. Fracture characteristics were completely nonconchoidal, as shown in Fig. 12. There were also increases in transcrystalline failure and in secondary fracture planes intersecting the principal fracture surface.(D) 1100°F Heat Treatment: Heat treatment a t 1 100°F Fig. 12. Fracture surface of crystallized glass caused by failure in thermal shock at 925OF.structure is somewhat comparable to a titanium-opacified enamel. Thermal-shock failure occurred a t 550°F with the type of failure common to glassy materials. The conchoidal fracture surface resulting from the ineffective crystallization is shown in Fig. 8. ( B ) 1 600°F Heat Treatment: Thermal-shock resistance following the 1GOO"F heat treatment showed little improvement over the non-heat-treated specimen. The size of the rutile crystals increased slightly and traces of lithium titanium silicate, sphene, and albite developed. The typical structure of the specimen which failed a t 600°F is shown in Fig. 9. The orientation of the crystals in the center of the micrograph was caused by coating flow during heat treatment at too high a temperature. The fracture characteristics were similar to those of the non-heat-treated coating with a slight increase in discontinuities in the predominantly planar fracture (Fig. 10).was below the critical crystallization temperature, resulting in the same structure as the non-heat-treated specimen even though the time was extended to 65 hours. Fracture characteristics and thermal-shock resistance were also the same. The sequence of crystallization for the series started with spontaneous crystallization of rutile during firing. High-temperature heat treatment developed only trace amounts of lithium titanium silicate, sphene, and albite. At 1400°F the maximum development of crystals occurred, resulting in the maximum disturbance of the fracture pattern and increase in thermal-shock resistance. V. SummaryThe growth of crystals and their cumulative effect in developing internal stresses in glass coatings can be conveniently studied with the electron microscope. The information gathered in this manner can be related to the physical properties of the coatings. Heat treatments which developed maximum crystallinity with good distribution of the crystalline phase within the matrix resulted in materials with the best resistance to thermal shock. As resistance to thermal shock increased, the fracture paths became increasingly disrupted owing to the internal stresses developed between crystal and matrix. As i t became necessary for fra...

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