Using a new, non-destructive method of gem testing, magnetic susceptibility, the authors show how the major endmember composition of any garnet may be confi dently predicted by plotting RI against measured susceptibility. On this diagram, eight end-member garnets are plotted, so that any measured garnet can be placed in an appropriate ternary area. This method shows how previous methods of identifying garnetsby their colour, RI and spectrumare insuffi cient to accurately identify chemistry in the garnet group. Furthermore, it can be done with inexpensive equipment available to most gemmologists.

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We examine residual properties of word-hyperbolic groups, adapting a method introduced by Darren Long to study the residual properties of Kleinian groups.

Structural and optical measurements have been made on a series of amorphous thin films of Si1-xTix with 0

Structural and optical measurements have been made on a series of amorphous thin films of Si1-xNix with 00.5. The optical data show that a metal-insulator transition occurs at about 10-15% Ni content. We suggest that this transition should be interpreted in terms of a percolation theory involving regions of conducting amorphous NiSi in an amorphous Si matrix.