Abstract
The zeolites form a group of minerals of considerable interest by reason of their peculiar properties. In particular, their remarkable continuous dehydration curves, in part reversible, and the ready base-exchange of some members have attracted attention. But in spite of much work, many important characters remain quite uncertain. Many of the zeolites show variations in composition which are not recognized or are incorrectly interpreted by the accepted formulae. Optical investigation has all too rarely been made on analysed material, and little is known of the variation of the optical properties of most members with their composition, and practically nothing of the variations consequent on artificial base-exchange reactions. In spite of much work, conflicting views are still held as to the interpretation of the dehydration results. In some instances even the crystal-system is still uncertain.

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