The temperature coefficient of capacitance (TCC) of most insulators is dominated by five components. The theoretical analysis of these components is outlined and it is shown that, for given ranges of permittivity, the analysis can be greatly simplified. This simplified treatment is shown to apply to widely different types of compound. Since TCC depends on permittivity, the permittivity of a variety of materials has been rationalized by reference to their `I' factor, which is defined as mean atomic number per cubic ångström.

Summary. The compositions of more than eighty synthetic members of the apatite group with the general formula Alo(XO4}6Z ~ have been used as a basis for classifying the apatites into three groups. The ratios of the mean sizes of the 'A' ions to those of the 'X' ions for all these compositions lie between the limits of 1.89 and 4.43. A discontinuity occurs between ratios of 2.50 and 2.60 and probably another between 3-25 and 3.35, and these discontinuities provide a structural basis for the division into the three groups. The groups are named after well-known mineral species occurring in each group, i.e. the vanadinite-svabites with an A :X ratio less than 2.5, the apatite-mimetites between 2.60 and 3.25, and the pyromorphites with A : X greater than 3"25.

Summary.A number of new alkaline-earth-rare-earth silicate and germanate compounds with the apatite-type structure have been synthesized including a new calcium lanthanum silicate, Ca4La6(SiO4)6(OH)2 , which has a 9'63 -~, c 7.12, and c/a 0"740. This has a random distribution of Ca 2+ and La a+ over the (4f) and (6h) sites in the lattice. X-ray data are given for Ca4La6(SiOa)6(0H)2 together with cell dimensions for other synthetic silicate and germanate apatites. Silicate apatites with partially occupied or vacant halogen (2a) sites analogous to those of the pyromorphites have been prepared, and a classification for silicate apatites based on synthetic end-members is proposed.T HE large number of naturally occurring and synthetic compounds known that have the apatite-type structure (hexagonal, space group P63/m ) form a group of unique interest and importance. Mineral members occur as common minor constituents of many igenous, metamorphic, and sedimentary rocks, and occasionally form deposits of considerable economic importance. In the biosphere, the calcium phosphate apatites are important constituents of teeth, bones, and urinary calculi, while their synthesis throughout human life during the progressive mineralization of the artery walls affects the lives of us all.During the course of a study of synthetic compounds for possible use as low-dielectric-loss ceramic dielectrics stable under reducing conditions, a number of new apatite-type compounds were prepared and their electrical properties determined. The extensive ionic substitutions of simple or complex type that may be made without destroying the apatite-type structure, coupled with the ease with which most members may be synthesized, makes the apatites an attractive group in which to seek a range of controlled dielectric properties. In addition, the relationships of some naturally occurring members to each other have been clarified and this has enabled a classification of alkaline-earth-rare-earth silicate apatites based on synthetic end members to be proposed.

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