Metahalloysite is one of the kaolin group of minerals with a composition approximating to the formuia Al2O3.2SiO2.2H2O, and its structure is generally considered to be built of composite layers of atoms of the kind found in other minerals of this group. It differs from them most strikingly in that it can be produced by the dehydration of the hydrated mineral halloysite, which has the approximate composition Al2O3.2SiO2.4H2O. Both minerals give comparatively poor X-ray diagranm showing relatively few lines attd bands. The lines correspond to reflections from a basal spacing of about 7·2 Å. in metahalloysite and 10·1 Å. in halloysite.

The experiments described below on the dehydration of halloysite were largely carried out in the laboratoire Central des Services Chimiques de l'État, Paris, where we enjoyed not only the excellent facilities of the laboratory but also the advice of Monsieur J. Méring and Mile R. Glaser who had made similar studies of montmorillonite. The use of a Guinier-type focusing camera with strictly monochromatic radiation was especially useful. These experiments were undertaken because little was known about the dehydration process beyond the recognition that it occurs very readily in dry atmospheres and at low temperatures. The main experimental difficulty lies in differentiating between water adsorbed oil external surfaces of the clay particles (adsorbed water) and water internally absorbed between the kaolin layers (interlayer water).

This paper concludes the account of our experiments on halloysite and is to be read in conjunction with the two previous parts, to which frequent reference will be made.

The effect of heat-treatment on metahalloysite was examined with the primary object of ascertaining if a recrystallization or ‘kaolinization’ of metahalloysite could be achieved.

From the beginning of classical times it was well known that the mineral conveniently called loadstone, whether magnetite in the strict sense or some other compound, attracted or, better, adhered to iron ; also that this property was imparted to a piece of iron in contact with a loadstone, so that a chain of disconnected iron rings could be suspended from it. This action aroused such wonder that many superstitions gathered round the stone which was reckoned throughout tlle Middle Ages as a gem. A genuine economic value was given to loadstone when the further discovery was made that a suitably shaped piece of the stone, or of iron touched with it, if free to move, would always aline itself approximately north and south.

This stage-attachment simplifies microscopic comparison of refractive indices of small transparent grains by the immersion-liquid methods of O. Maschke, Schroeder van der Kolk, and other workers. It was designed in 1938 for use in mineralogy and petrology, but has applications in several branches of pure and applied chemistry, patticularly for determining the relative purities of numerous raw materials and refined products of the chemical industry and in checking the constituents of glasses, slags, refractories, and cements. It can be used with advantage, also, in the optical study of soils.

There is general agreement as to the basis on which most major igneous rock-types are defined despite some difference of opinion as to quantitative limits. This is not true of diorites and gabbros (or of their hypabyssal and volcanic equivalents) in distinguishing between which at least four different criteria are in current use.