it would not be suspected that the two gases hydrogen and chlorine will unite instantly in the sunlight, but will stand mixed in the dark a long time without change. It will therefore be necessary to pay much attention to the details of the conditions under which a given reaction occurs, as well as to the expression of the reaction in the form of an equation. EXERCISES 1. Calculate the percentage composition of the following substances: (a) mercuric oxide; (b) potassium chlorate; (c) hydrochloric acid; (d) sulphuric acid. Compare the results obtained with the compositions as given in Chapters II and III. 2. Determine the percentage of copper, sulphur, oxygen, and water in copper sulphate crystals. What weight of water can be obtained from 150 g. of this substance? 3. What weight of zinc can be dissolved in 10 g. of sulphuric acid? How much zinc sulphate will be formed? 4. How many liters of hydrogen measured under standard conditions can be obtained from the action of 8 g. of iron on 10 g. of sulphuric acid? How much iron sulphate (FeSO_{4}) will be formed? 5. 10 g. of zinc were used in the preparation of hydrogen; what weight of iron will be required to prepare an equal volume? 6. How many grams of barium dioxide will be required to prepare 1 kg. of common hydrogen dioxide solution? What weight of barium sulphate will be formed at the same time? 7. What weight of the compound Mn_{3}O_{4} will be formed by strongly heating 25 g. of manganese dioxide? What volume of oxygen will be given off at the same time, measured under standard conditions? 8. (a) What is the weight of 100 l. of hydrogen measured in a laboratory in which the temperature is 20 deg. and pressure 750 mm.? (b) What weight of sulphuric acid is necessary to prepare this amount of hydrogen? (c) The density of sulphuric acid is 1.84. Express the acid required in (b) in cubic centimeters. 9. What weight of potassium chlorate is necessary to furnish sufficient oxygen to fill four 200 cc. bottles in your laboratory (the gas to be collected over water)? CHAPTER VII NITROGEN AND THE RARE ELEMENTS: ARGON, HELIUM, NEON, KRYPTON, XENON ~Historical.~ Nitrogen was discovered by the English chemist Rutherford in 1772. A little later Scheele showed it to be a constituent of air, and Lavoisier gave it the name _azote_, signifying that it would not support life. The name _nitrogen_ was afterwards given it because of its presence in saltpeter or niter. The term azote and symbol Az are still retained by the French chemists. ~Occurrence.~ Air is composed principally of oxygen and nitrogen in the free state, about 78 parts by volume out of every 100 parts being nitrogen. Nitrogen also occurs in nature in the form of potassium nitrate (KNO_{3})--commonly called saltpeter or niter--as well as in sodium nitrate (NaNO_{3}). Nitrogen is also an essential constituent of all living organisms; for example, the human body contains about 2.4% of nitrogen.