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taining up to about 26 mol% Fe,O, were studied. Pronounced changes in properties were observed a t compositions containing about 6, 10, and 13 mol% FezO,. The X-ray diffraction spectra of devitrified (heat-treated) samples showed new compounds near these compositions. Electron spin resonance and optical studies confirmed the presence of Fe3+ and Fez+ in both 4 and 6-coordination. An increase in total iron in these samples was associated with a decrease in the ratios Fez+ 4-coordinated/FeZ+ 6-coordinated and Fe'+ 4-coordinated/Fe3+ &coordinated up to about 2.0 mol% Fez03, as shown by the intensity of the optical absorption bands at about 2.0 and 1.0 2 o,5 a respectively. Samples containing up to 4.3 mol% FezO, showed u F 0.4 an increase in Fex+ concentration and a decrease in Fez+ cona 0 0 . 3 centration after gamma irradiation. The electrical conductivity and activation energy decreased sharply with increasing Fe,O, content. 0.9 0.8 0.7 > I-V, 0.6 pm and by the intensity of the ESR lines at ge4.2 and 2.0, -I 0.2

The changes in gamma-ray induced optical absorption in phosphate glasses resulting from changes in composition, conditions during melting, and additions of small amounts of some oxides a r e discussed. A resolution of the induced spectra showed that the observed absorption is due to the superposition of three bands at 2.3, 2.9, and 5.5 e.v. (540, 425, and 225 mp) a n d to a fourth band whose absorption peak is beyond 6 e.v. The ultraviolet induced absorption increases, whereas the visible absorption decreases in glasses melted under reducing conditions as opposed to those melted under normal conditions in air and on replacement of I(+ by Na+ or Li+. A similar effect is produced on replacement of Ba++ by P b f + and on the addition of TlzO to a CaO.PzOs glass or the addition of Asz03 to a CaO.PzOs.GeOz glass. Additions of large amounts of GeOz are accompanied by a decrease in the number of nonbridging oxygens and a decrease in the visible induced absorption. Replacement of C a + + by Ba++ ions showed an over-all decrease in the induced absorption. The addition of small amounts of the oxides of germanium, titanium, iron, thallium, niobium, and arsenic showed a n appreciable effect in inhibiting the visible induced coloration.

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The gamma‐ray‐induced optical absorption in a series of cabal (calcium‐boron‐aluminum) glasses was studied and is interpreted, wherever possible, in terms of structural concepts. A resolution of the observed absorption spectra showed that three Gaussian‐shaped bands were induced with their maxima at about 2.3, 3.5, and 5.0 e.v. (550, 350, and 250 mμ). The 2.3‐e.v. band decreased in intensity with increasing CaO content, reaching a minimum intensity at a composition corresponding to the four‐coordination of about 20% of the boron. Further increase in CaO content was associated with an increase in the intensity of this band. The intensity of the 3.5‐e.v. band decreased gradually with increased mole per cent of CaO and increased with increased Al2O3. The 5.0‐e.v. band showed an abrupt increase in intensity which corresponded to the appearance of non‐bridging oxygens in the network. Replacing Ca2+ by Mg2+, Sr2+, or Ba2+ or replacing Li+ by Na+ or K+ showed that glasses containing large ions of low field strength give less induced absorption than glasses containing small ions of high field strength. A potassium alumina borate glass melted under reducing conditions gave a considerably higher ultraviolet transmission, before irradiation, as compared with the same glass melted under normal conditions. The gamma‐induced absorption of these two glasses showed that reducing conditions resulted in a decrease in the intensity of the 2.3‐ and 3.5‐e.v. bands, whereas it caused an increase in the far‐ultraviolet‐induced absorption. The effect of additions of arsenic, thallium, titanium, germanium, and some rare‐earth oxides is discussed.