~Selenium and tellurium.~ These two very uncommon elements are still more closely related to sulphur than is oxygen. They occur in comparatively small quantities and are usually found associated with sulphur and sulphides, either as the free elements or more commonly in combination with metals. They form compounds with hydrogen of the formulas H_{2}Se and H_{2}Te; these bodies are gases with properties very similar to those of H_{2}S. They also form oxides and oxygen acids which resemble the corresponding sulphur compounds. The elements even have allotropic forms corresponding very closely to those of sulphur. Tellurium is sometimes found in combination with gold and copper, and occasions some difficulties in the refining of these metals. The elements have very few practical applications. ~Crystallography.~ In order to understand the difference between the two kinds of sulphur crystals, it is necessary to know something about crystals in general and the forms which they may assume. An examination of a large number of crystals has shown that although they may differ much in geometric form, they can all be considered as modifications of a few simple plans. The best way to understand the relation of one crystal to another is to look upon every crystal as having its faces and angles arranged in definite fashion about certain imaginary lines drawn through the crystal. These lines are called axes, and bear much the same relation to a crystal as do the axis and parallels of latitude and longitude to the earth and a geographical study of it. All crystals can be referred to one of six simple plans or systems, which have their axes as shown in the following drawings. The names and characteristics of these systems are as follows: 1. Isometric or regular system (Fig. 46). Three equal axes, all at right angles. [Illustration: Fig. 46] 2. Tetragonal system (Fig. 47). Two equal axes and one of different length, all at right angles to each other. [Illustration: Fig. 47] 3. Orthorhombic system (Fig. 48). Three unequal axes, all at right angles to each other. [Illustration: Fig. 48] 4. Monoclinic system (Fig. 49). Two axes at right angles, and a third at right angles to one of these, but inclined to the other. [Illustration: Fig. 49] 5. Triclinic system (Fig. 50). Three axes, all inclined to each other. [Illustration: Fig. 50] 6. Hexagonal system (Fig. 51). Three equal axes in the same plane intersecting at angles of 60 deg., and a fourth at right angles to all of these. [Illustration: Fig. 51]