Every crystal can be imagined to have its faces and angles arranged in a definite way around one of these systems of axes. A cube, for instance, is referred to Plan 1, an axis ending in the center of each face; while in a regular octohedron an axis ends in each solid angle. These forms are shown in Fig. 46. It will be seen that both of these figures belong to the same system, though they are very different in appearance. In the same way, many geometric forms may be derived from each of the systems, and the light lines about the axes in the drawings show two of the simplest forms of each of the systems. In general a given substance always crystallizes in the same system, and two corresponding faces of each crystal of it always make the same angle with each other. A few substances, of which sulphur is an example, crystallize in two different systems, and the crystals differ in such physical properties as melting point and density. Such substances are said to be _dimorphous_. EXERCISES 1. (a) Would the same amount of heat be generated by the combustion of 1 g. of each of the allotropic modifications of sulphur? (b) Would the same amount of sulphur dioxide be formed in each case? 2. Is the equation for the preparation of hydrosulphuric acid a reversible one? As ordinarily carried out, does the reaction complete itself? 3. Suppose that hydrosulphuric acid were a liquid, would it be necessary to modify the method of preparation? 4. Can sulphuric acid be used to dry hydrosulphuric acid? Give reason for answer. 5. Does dry hydrosulphuric acid react with litmus paper? State reason for answer. 6. How many grams of iron sulphide are necessary to prepare 100 l. of hydrosulphuric acid when the laboratory conditions are 17 deg. and 740 mm. pressure? 7. Suppose that the hydrogen in 1 l. of hydrosulphuric acid were liberated; what volume would it occupy, the gases being measured under the same conditions? 8. Write the equations representing the reaction between hydrosulphuric acid and sodium hydroxide and ammonium hydroxide respectively. 9. Show that the preparation of sulphur dioxide from a sulphite is similar in principle to the preparation of hydrogen sulphide. 10. (a) Does dry sulphur dioxide react with litmus paper? (b) How can it be shown that a solution of sulphur dioxide in water acts like an acid? 11. (a) Calculate the percentage composition of sulphurous anhydride and sulphuric anhydride. (b) Show how these two substances are in harmony with the law of multiple proportion. 12. How many pounds of sulphur would be necessary in the preparation of 100 lb. of 98% sulphuric acid? 13. What weight of sulphur dioxide is necessary in the preparation of 1 kg. of sodium sulphite? 14. What weight of copper sulphate crystals can be obtained by dissolving 1 kg. of copper in sulphuric acid and crystallizing the product from water? 15. Write the names and formulas of the oxides and oxygen acids of selenium and tellurium.