2/6' IRON. ternal lustre, and the transparency ; hut as it advances, it also changes even the structure, hardness, solidity, and weight of the mineral. The oxidation of the iron and manganese destroys the weak combination of these me tals w:th the carbonic acid, and there is formed a hydrate of iron, sometimes also an oxide of iron, and hydrate of manganese. The whole mass is disintegrated bv the escape of the carbonic acid, this acid combining with per colating w r ater dissolves the small portion of lime in the ore, and also portions ot the still undecomposed carbonate of iron and oxide of manganese. A knowledge of these changes enables us to understand the very different re sults obtained in the analysis of specimens more or less weathered or decomposed, and also throws some light on fhe different results obtained in the smelting of sparry ironstone more or less decomposed. 4. I lie analysis'of Hielm, published under the sanc tion of Bergmann, is the earliest we possess of this iron stone : it gives as the constituent parts, 22.38 Oxide of Iron : 24.28 Oxide of Manganese : 29.43 Carbonate of Lime; and 6.9 Water. The errors of this analysis have been pointed out and corrected by the labours of Drappier, Descotils, Berthier, Klaproth, and Buchoiz. 13. Clay Ironstone. Thoneisenstein, Werner. This species is divided into the following subspecies, viz. Keddle, Columnar Clay-Ironstone, Lenticular Clay- Ironstone, Jaspery Clay-Ironstone, Common Clay-Iron stone, ileniform or Kidney-shaped Clay-Ironstone, and Pea-ore or Pisiform Clay-Ironstone. first