In the account of an investigation of the optical properties of gypsum published by one of us (A. E. H. T.) in 1908 it was stated, as the result of direct measurement in sodium-light of the optic axial angle of a section-plate perpendicular to the first median line, which was heated in an air-bath, that the substance became uniaxial at 105.2° C. Three other section-plates examined subsequently gave somewhat higher values, the results ranging from 109.5° to 114.2° C. These figures were arrived at after a considerable correction, amounting to as much as 7°, had been applied for conduction along the crystal-holder, the temperatures actually observed being greater by this amount.

The mineral schwartzembergito was first described by Domeyko in 1864 as an oxychloroiodide of lead from the Desert of Atacama. The specimens, which are said to have been collected about ten leagues from Paposo in the Desert of Atacama, at a mine worked for argentiferous galena, were brought to his notice by Dr. Schwartzemberg, an assayer at Copiapo, who had recognized the material as being an iodide of lead. Domeyko made a chemical analysis (see p. 82) of the mineral, a yellowish crust, 1 to 2 mm. in thickness, upon galena, and obtained the formula, 2Pb(CI,I)2.3PbO, the specific gravity being 5.7; no crystal measurements were obtainable, neither were any optical observations made.

Analyses of Indian manganese-ores generally show the presence of small amounts of arsenic. The form in which this constituent occurs was discovered by Dr. L. Leigh Fermor in the course of his investigations of these well-known deposits. At two different localities crystalline arsenates were found. One of these occurrences is at the Sitapar deposit in the Chhindwara district, Central Provinces. The pinkish-white to white arsenate found there is the new mineral to which we have given the name fermorite. The other locality at which an arsenate was found is Kajlidongri, Jhabua State, Central India. The pale-green arsenate, somewhat resembling apatite in appearance, which occurs at this locality has been found by us to be identical with the tilasite from Sweden described by Sjögren.

For the first of the three minerals to be described in the following pages we are indebted to Mr. W. G. Fearnsides, who collected it in North Wales in the autumn of 1908, and we desire to express our best thanks to him for the readiness with which he placed at our disposal enough suitable material for a complete analysis. The other two minerals were collected by ourselves.

The interesting results which have followed the investigation by one of us of the isomorphous series, of double sulphates and selenates of the general formuh, R′2M′′[(S, Se)O4]2.6H2O have led us to extend the investigation to the corresponding double chromates in which M is represented by magnesium.

This paper deals with various specimens received at the Imperial Institute from Ceylon. Some of these specimens were obtained by Dr. A. K. Coomaraswamy at Wa]aweduwa, in the Bambarabotuwa district of the Sabaragamuwa province, during the course of the Ceylon Mineral Survey. Walaweduwa is a little to the north of the Bambarabotuwa locality where thorianite has been found in conslderable abundance. Dr. Coomaraswamy also found specimens in the upper part of the Alupola oya, a stream in the same district. A chemical examination of these specimens showed that titanium dioxide, zireonia, thoria, lime, and iron oxide were the chief constituents.

In the course of an investigation of the atomic weight of nitrogen Mr. F. P. Burt, of University College, London, prepared by the following process crystals of nitrogen sulphide, N4S4, surpassing in purity any that had been previously obtained. The nitrogen sulphide was recrystallized from benzene, washed with carbon disulphide to remove as much sulphur as possible, and finally washed with a little cold benzene. The product was powdered on a porous tile, and kept in a desiccator with calcium chloride and cocoa-nut charcoal until it no longer smelt of benzene. It was then sublimed in vacuo over silver gauze heated to 100° C. in a steam-jacket, by which means the absence of liquid or gaseous inclusions and of free sulphur in the sublimate was assured.

The main portion of this memoir as far as it relates to wiltshireite was in the Editor's hands in January, 1911 ; but before it was sent by him to press I learnt that Mr. R. H. Solly had, at the Anniversary Meeting of the Mineralogical Society in 1903, described an unnamed crystal, which, from the statement made to the Society on March 21, 1911, and published in the ‘Mineralogical Magazine’ (1911, vol. xvi, p. 121), is undoubtedly the same substance as that named by me in August, 1910, wiltshireite. In the statement of March, 1911, Mr. Solly pointed out the striking relation of the crystals of wiltshireite to those of rathite ; and suggested that the latter is ohlique and not orthorhombic, as described in this Magazine (1901, vol. xiii, pp. 77-85).

Early in the year 1912 extensive modifications were introduced in the apparatus for slitting rocks, and for grinding and polishing thin-sections in the Mineral Department of the British Museum, the whole of the alterations and additions being carried out to my designs by Messrs. Plume and Parry, 47 Old Compton Street, London, W. The changes made were chiefly in the means for holding the specimen or section during the operations; much of the actual machinery remained unaltered, and some of it dates back to the initial installation, upwards of forty years ago. It may not, therefore, be without interest if a brief description of the history of the installation be included with the description of the present apparatus.

An occurrence of the rare mineral dundasite (hydrous carbonate of lead and aluminium) at the Welsh Foxdale mine, Trefriw, Carnarvonshire, was described by Dr. G. T. Prior in 1906. Since then the mineral has been found by the writer at two additional localities in the United Kingdom. Certain specimens collected in 1910 at the well-known Mill Close mine, Wensley, Derbyshire, show a mineral which examination has proved to be dundasite. It occurs here as minute white or yellow spheres, not exceeding ¾ mm. in diameter, the exterior smooth and waxy, the interior radiating and silky.

In this paper an attempt is made to state and explain, in a simple manner, the essential features of the phenomena which are seen when plates of pleochroie minerals are examined in ordinary light.

The pleochroie effects exhibited by minerals are usually studied in light of practically normal incidence. For the observation of facial colours, transparent crystals, or, better still, specially orientated cube-shaped pieces of minerals, are examined in ordinary light. In general, by this mode of observation, only facial colours are seen when ordinary light is used, and for the observation of axial colours the incident light must be polarized.

Nine years ago (in 1904) I described a diagram—the moriogram, as I called it—which served for the graphical determin.stion of angles and indices in the case of zones in which the principal poles included a right angle. It was, therefore, restricted in its application to zones in the cubic, tetragonal, orthorhombic, hexagonal, and rhombohedral systems, and to zones in the monoclinic system passing through the pole of symmetry. Although the moriogram may be used with sufficient accuracy for practical purposes in the case of many substances with triclinic symmetry, in which the angles between the principal poles differ little from right angles, there necessarily remain many instances in which it cannot be applied with any approximation to accuracy.

According to the accounts which have been published by members of the Egyptian Geological Survey, a number of meteoric stones fell to earth on June 28, 1911, at about 9 in the morning, in the hamlets to the west and north of the village of El Nakhla El Baharia (31° 19′ N., 30° 21′ E.) in the district of Abu Hommos, about twenty-four miles east of Alexandria, Egypt.

In my previous papers I described and illustrated several examples of the concentrically banded and radiated varieties of cassiterite known as ‘wood-tin’, and stated that one very constant character of this variety was its rather remarkable opacity as compared with ordinary cassiterite, which is indeed somewhat exceptionally transparent considering its (often) deep coloration. I also noted the frequent occurrence of specimens of wood-tin encrusted with more or less distinct crystals of cassiterite, or traversed by fissures filled with such crystals, as indicating successive formation.

Crystals of Inosite, C6H6(OH)6.2H2O , were first measured by Cloёtta, and somewhat later by Zepharovieh, and by Lewis; they are monoclinic, and the elements adopted are those of Zepharovieh, viz., a : b : c = 1.0872 : 1 : 1.5602 ; β = 111° 39′. The observed forms on the crystals were {010}, {110}, {100}, {001}, , , {210}, and {410}, and it is seen that the form-development is of a somewhat anomalous character, the indices of the last three forms being remarkable for their complexity.

The instrument to be described in the following pages is a goniometer of the suspended type. It is intended primarily for tlle examination of small crystals, and by its aid all the usual crystallographic and optical determinations can be readily carried out. In its design the attempt has been made to combine efficiency with simplicity and strength of construction, together with adaptability to very various purposes. The instrument was completed in the spring of 1899, and has been in continuous use in the Mineralogical Laboratory at Cambridge ever since. Other instruments of the same general type were in existence at the time, or have been described during the past twelve years, and several features which were novelties when it was first constructed can no longer lay claim to originality.