In that part of the Gangpur State immediately to the north of the Bengal-Nagpur Railway, between Biara and Garpos, lies a tract of dolomite- and limestone-bearing country which has been referred, on lithological grounds, to the Cuddapah (? Algoukian or Palaeozoic) formation. From this area, all the dolomite and most of the limestone requirements of the Bengal iron and steel industry are at present derived, and the close proximity of the enormous haematite ore reserves of Singhbhum indicates a still greater economic importance for these materials in the future.

Mercuric sulphide, HgS, occurs in nature as two well-known modifications, cinnabar and metacinnabarite. The latter is some what rare and occurs as black cubic crystals possessing the symmetry of the zinc-bleude, i.e. the ditesseral polar-class. It is considered to be identical with chemically precipitated black mercuric sulphide, the density of both being 7.81. Cinnabar is of more common occurrence and is found as well-defined crystals and in the massive state. The crystals are assigned to the trigonal holoaxial class. The rhombohedral angle is 92°30' r correspouding to an axial ratio c/a of 1.1453. The crystals possess a very good cleavage parallel to the prism (21̄1̄)=(101̄0). The rotation of the plane of polarization, for red light traversing the crystal in a direction parallel to the optic axis, is 315° per millimetre, a value roughly eighteen times that of a similar thickness of quartz.

The isomorphous relationships for organic compounds are not so simple as they sometimes are for inorganic. Very often a series of chemically similar orgallic substances does not show any apparent trace of crystallographic similarity. It appears that the introduction of further groups into the molecule may cause the crystal-system to change in a manner which cannot be predicted. Thus, glyoxalinc-4(or 5)- sulphonic acid is tetragonal, whilst 4(or 5)-bromoglyoxaline-5(or 4)- sulphonic acid is monoclinic; the introduction of a methyl group into the 2 position gives rise to an orthorhombic crystal, whilst the introduction of the same group into the 1 position leaves the crystal-system unchanged. In some cases it has been found that evell though the crystal-system is changed, there still remains a very close similarity between corresponding angles in the different substances.

As it is not always easy to obtain for the use of students suitable specimens of crystals which exhibit inclined or horizontal dispersion of the mean lines to a marked extent, a ready method of artificially producing optic pictures which show the appearances characteristic of these dispersions may be found of service by teachers.

It is ohvious that, if the optic picture of an ordinary biaxial crystal is viewed or projected through a prism whose refracting edge is either parallel or perpendicular to the axial plane of the crystal, the dispersion produced by the prism will displace the components of the optic picture due to the various colours of the spectrum through distances increasing from red to violet, and will thus reproduce the phenomena of horizontal and inclined dispersion respectively.

We are unable to observe crystalline structure directly, and can only infer it by observation of the physical and chemical properties possessed by a crystal. It is quite reasonable to suppose that every vectorial property possessed by a crystal is dependent upon its leptonic structure. The X-ray method, which has produced such a revolutionary change in our Knowledge of crystal-structure, is not sufficient in some cases for the determination of leptonic structure. The X-ray spectral analysis gives only the spacings between the atomic planes of a crystal. The application of the method of intensities, which was so successful in the case of simple cubic crystals, may not be so in the case of crystals of a Inore complex chemical composition and of a lower symmetry. It is thus desirable to combine several methods and so approach the problem from different directions.

The name chlorophaeite was given by John Macceulloch in 1819 to a mineral exhibiting striking colour change on exposure which he found at Craig nan Stàrdean (=Sgùrr Mòr) in the island of Rum. Sixty years later Professor Heddle visited the type locality and redescribed the mineral, giving for the first time chemical analyses. He also noted an occurrence of the mineral in Canna. In 1908 A. Harker showed that the rock which contained the chlorophaeite belongs to the mugearites.

Records of chlorophaeite arc not always trustworthy since the term has been used somewhat loosely. Professor Heddle (loc. cit., p. 88) confirmed the occurrence at the Giant's Causeway, Co. Antrim, first recorded by Greg and Lettsom in their Manual of Mineralogy.

The widespread use of methods based oil total-reflection for finding the refractive indices of crystals either directly, by the Abbe-Czapski crystal reflectometer or by the Kohlrausch method, or indirectly by means of liquid mixture of known refractive intex which require to be verified and standardized from time to time, renders desirable a rapid graphical method of ascertaining with some accuracy the index of the substance under test, direct from the instrumental data without calculation.

An attempt in this direction was made by the author some years ago. The equation to be solved is n' = n sin θ, where n' is the index to be found, n that of the glass hemisphere, prism, or highly refracting liquid employed in the total-reflectometer, and θ the critical angle of total-reflection observed.