A very delicate test for sulphur in whatever combination it may be found in a substance, and which may be performed with great ease, is to mix the finely powdered assay with four parts, Na_{2}CO_{3}, and fuse in an ignition tube. When thoroughly fused the tube is broken, and the fused mass is placed on a bright silver coin, and a drop of water is added. If the substance contains sulphur, a black spot will be observed on the coin where the fused mass was placed. Before going on to the next chapter, the student should assure himself of his familiarity with the reactions just given, and he should practise with various substances, the nature of which is unknown to him. CHAPTER IV BEHAVIOR OF SOME OF THE PRINCIPAL ORES BEFORE THE BLOWPIPE For the sake of practice, and as a fitting introduction to "Determinative Mineralogy," this chapter is appended. It is not intended to give a detailed account of the minerals, but rather to set before the student the most marked characters, such as hardness, specific gravity, color, lustre, etc. To determine the hardness of a mineral, we try to scratch it with the minerals forming an arbitrary "scale of hardness," proceeding successively from the softest to the hardest. When we say that a certain mineral has hardness = 4, we mean that the mineral is scratched by 4 on the scale, and that 4 on the scale is scratched by the mineral. The scale of hardness chiefly in use is the Mohs-Breithaupt scale, which is as follows:-- 1. Talc, common laminated light green variety. 2. Gypsum, crystallized. 3. Calcareous spar, transparent variety. 4. Fluor spar, crystalline. 5. Apatite, transparent. 6. Orthoclase, white cleavable variety. 7. Quartz, transparent. 8. Topaz, transparent. 9. Sapphire, cleavable variety. 10. Diamond. It seldom happens in determining the hardness of a mineral that its hardness exactly conforms to that of some one member of the scale. In such cases we generally estimate the hardness. For example, suppose a mineral was harder than 4, but softer than 5, and that it was nearer 5 than 4, then we would call its hardness 4-3/4. In order to preserve the scale some operators use a three-cornered file, first cutting the mineral and then the scale until a number is found, which is abraded to about the same depth as the mineral under examination. Since a set of minerals forming a scale of hardness is not always at hand, the following scale given by Chapman is appended:-- 1. Yields easily to the nail. 2. Yields with difficulty to the nail or just receives an impression from it. Does not scratch a copper coin. 3. Scratches a copper coin but is also scratched by it, being of about the same degree of hardness. 4. Not scratched by a copper coin. Does not scratch glass. 5. Scratches glass with difficulty, leaving its powder on it. Yields readily to the knife.