sure. PROPORTION OF OXYGEN IN AIR. 179 When freshly made, liquid air is about half oxygen, but the proportion of oxygen increases by the evaporation of the nitrogen (liquid nitrogen boils about 10° C. below liquid oxygen) until the liquid is over 90% oxygen. Liquid air is preserved in open, double-walled vessels called Dewar bulbs (Fig. 43). The space between the walls of the bulbs is exhausted of air to secure nonconductivity of heat. Tin or wooden boxes having double walls filled with silk may also be used. Alcohol, liquid carbon dioxide, mercury, etc., solidify when placed in liquid air, and steel burns in it like tinder; yet the hand may be held in it for a short time without injury, because protected by a non-conducting film of air in the gaseous state. FIG. 43. 199. Determination of the Proportion of Oxygen in Air. The amount of oxygen in a given volume of air is usually determined either (1) by absorbing the oxygen, or (2) by exploding the air with a known volume of hydrogen. The phosphorus absorption method, a crude form of which was described under nitrogen (cf. § 187), is carried out more accurately as follows:- A gas analysis tube, partly full of air, is inverted in a hydrometer jar (Fig. 5), and the gas volume is carefully measured. The FIG. 44. temperature and pressure are recorded. A thin stick of phos-phorus is introduced into the tube by means of a bent wire, and is left there twenty-four hours. The phosphorus is then removed, and the residual gas is measured. When the necessary corrections have been made, the volume of oxygen absorbed and its per cent of the original air are readily calculated. The oxygen may also be absorbed by potassium pyrogallate (cf. § 549) in a Hempel pipette (Fig. 44). The second general method, viz., the explosion of a mixture of known volumes of air and hydrogen, may be illustrated as follows: 10 cc. H 10 cc. Air In the straight eudiometer tube shown in Fig. 45, a known volume of air is mixed with an excess of hydrogen, and the electric spark is passed through the mixture. All of the oxygen of the air taken will thus unite with hydrogen to form water. The volume of steam formed will be equal to that of the hydrogen used up, but the volume of liquid water produced by the condensation of the steam will be FIG. 45. THE AIR A PHYSICAL MIXTURE. 181 insignificant. Hence there will be a contraction of One-third of this contraction will be the volume. volume of oxygen present in the original air. To take a concrete example: If 10 c.c. of each of the gases, air and hydrogen, are taken, the volume remaining after the explosion will be about 13.7 c.c., i. e., the contraction will be 6.3 c.c. Of this contraction 2.1 c.c. (=3 of 6.3) are oxygen; hence 10 c.c. of air contain 2.1 c.c. (=21%) of oxygen. The proportions of the chief constant constituents of the atmosphere have been determined in many different places, and have been found everywhere practically the same. This is a remarkable fact when we remember that air is only a physical mixture, yet it is a necessary consequence of the circulation of the air and the rapid diffusion of its gases. 200. The Air a Physical Mixture. That air is not a compound of nitrogen and oxygen is proved by several facts, of which the following are illustrations: (1) When nitrogen and oxygen are mixed, there is no evidence of union, such as evolution or absorption of heat, change of volume, etc. (2) If air were a chemical compound, like the nitrogen oxides, it ought to have the same composition in the liquid state as in the gaseous. This, however, is not the case, as was stated in the description of liquid air. (3) There is no reason why air, if a compound, should have its composition changed by solution in water; yet this is the case. Air that has been expelled from solution in water contains about 35%, by weight, of oxygen, instead of 23%, owing to the fact that oxygen is much more soluble than nitrogen. 201. Exercises. 1. Calculate the percentage composition of a mixture of gases, if 76 c.c. of the mixture contain 48 c.c. oxygen, 12.5 c.c. hydrogen, and the remainder nitrogen. 2. Why is not all the carbon dioxide at the earth's surface? If it were, how deep a layer, in feet, would there be? (Cf. §§ 193 and 278.) 3. It has been calculated that 900 grams of nitric acid fall every year upon an acre of ground. Can you suggest how it was formed? 4. 16 c.c. of a mixture of nitrogen and oxygen were put with 25 c.c. of pure hydrogen, and exploded. The residual gas had a volume of 23 c.c. How much of the original mixture was oxygen? 5. How many liters of air are there in a room 3 m. high, and 5 m. square? How much does it weigh at 20° C. and 740 mm.? How much of it is oxygen? How much of it is argon? How much is carbon dioxide? 6. Explain why liquid air becomes more and more blue on standing. 7. 20 liters of air at 15° C. and 730 mm. were passed through lime-water, and the calcium carbonate (CaCO3) formed weighed 0.0256 gram. How many c.c. of CO2 were there in the 20 liters of air? CHAPTER XVII. AMMONIA. 202. Occurrence. Ammonia is formed in nature by the decay of organic compounds containing nitrogen, and by electric discharges in moist air (cf. § 188), consequently ammonia and ammonium compounds occur in the atmosphere, in rain water, and in the soil. Small amounts of ammonium compounds are found in plants and in animal secretions. 203. Laboratory Method of Preparation. In the laboratory, ammonia is made, either (1) by heating an ammonium salt with a base, or (2) by warming concentrated ammonium hydroxide solution. The ammonium salt in method (1) is usually the chloride or sulphate. The base is generally slaked lime [Ca(OH)2], or powdered sodium hydroxide. "Soda-lime," a mixture of sodium hydroxide and quicklime, is also used. The equations for the double decomposition are: 2 NH,Cl+Ca(OH)2 →→→→ CaCl2+2 NH4OH; Ammonium hydroxide is unstable, and breaks up readily into ammonia and water: |