~The blast lamp.~ A similar form of apparatus is commonly used in the laboratory as a source of heat under the name _blast lamp_ (Fig. 17). This differs from the oxyhydrogen blowpipe only in the size of the tubes. In place of the hydrogen and oxygen the more accessible coal gas and air are respectively used. The former is composed largely of a mixture of free hydrogen and gaseous compounds of carbon and hydrogen. While the temperature of the flame is not so high as that of the oxyhydrogen blowpipe, it nevertheless suffices for most chemical operations carried out in the laboratory. ~Uses of hydrogen.~ On account of its cost, hydrogen is but little used for commercial purposes. It is sometimes used as a material for the inflation of balloons, but usually the much cheaper coal gas is substituted for it. Even hot air is often used when the duration of ascension is very short. It has been used also as a source of heat and light in the oxyhydrogen blowpipe. Where the electric current is available, however, this form of apparatus has been displaced almost entirely by the electric light and electric furnace, which are much more economical and more powerful sources of light and heat. EXERCISES 1. Will a definite weight of iron decompose an unlimited weight of steam? 2. Why is oxygen passed through the inner tube of the oxyhydrogen blowpipe rather than the outer? 3. In Fig. 14, will the flame remain at the mouth of the tube? 4. From Fig. 15, suggest a way for determining experimentally the quantity of water formed in the reaction. 5. Distinguish clearly between the following terms: oxidation, reduction, combustion, and kindling temperature. 6. Is oxidation always accompanied by reduction? 7. What is the source of heat in the lime light? What is the exact use of lime in this instrument? 8. In Fig. 12, why is it necessary to dry the hydrogen by means of the calcium chloride in the tube X? 9. At what pressure would the weight of 1 l. of hydrogen be equal to that of oxygen under standard conditions? 10. (a) What weight of hydrogen can be obtained from 150 g. of sulphuric acid? (b) What volume would this occupy under standard conditions? (c) The density of sulphuric acid is 1.84. What volume would the 150 g. of the acid occupy? 11. How many liters of hydrogen can be obtained from 50 cc. of sulphuric acid having a density of 1.84? 12. Suppose you wish to fill five liter bottles with hydrogen, the gas to be collected over water in your laboratory, how many cubic centimeters of sulphuric acid would be required? CHAPTER IV COMPOUNDS OF HYDROGEN AND OXYGEN; WATER AND HYDROGEN DIOXIDE WATER