~The Dewar bulb.~ It is not possible to preserve air in the liquid state in a closed vessel, on account of the enormous pressure exerted by it in its tendency to pass into the gaseous state. It may however be preserved for some hours or even days before it will completely evaporate, by simply placing it in an open vessel surrounded by a nonconducting material. The most efficient vessel for this purpose is the _Dewar bulb_ shown in Fig. 29. The air is withdrawn from the space between the two walls, thus making it nonconducting. ~Properties and uses of liquid air.~ When first prepared, liquid air is cloudy because of the presence of particles of solid carbon dioxide. These may be filtered off, leaving a liquid of slightly bluish color. It begins to boil at about -190 deg., the nitrogen passing off first, gradually followed by the oxygen, the last portions being nearly pure oxygen. To a certain extent oxygen is now prepared in this way for commercial purposes. The extremely low temperature of liquid air may be inferred from the fact that mercury when cooled by it is frozen to a mass so hard that it may be used for driving nails. Liquid air is used in the preparation of oxygen and as a cooling agent in the study of the properties of matter at low temperatures. It has thus been found that elements at extremely low temperatures largely lose their chemical activity. EXERCISES 1. When oxygen and nitrogen are mixed in the proportion in which they exist in the atmosphere, heat is neither evolved nor absorbed by the process. What important point does this suggest? 2. What essential constituent of the air is found in larger amount in manufacturing districts than in the open country? 3. Can you suggest any reason why the growth of clover in a field improves the soil? 4. Why are the inner walls of a Dewar bulb sometimes coated with a film of silver? 5. To what is the blue color of liquid air due? Does this color increase in intensity on standing? 6. When ice is placed in a vessel containing liquid air, the latter boils violently. Explain. 7. Taking the volumes of the oxygen and nitrogen in 100 volumes of air as 21 and 78 respectively, calculate the percentages of these elements present by weight. 8. Would combustion be more intense in liquid air than in the gaseous substance? 9. A tube containing calcium chloride was found to weigh 30.1293 g. A volume of air which weighed 15.2134 g. was passed through, after which the weight of the tube was found to be 30.3405 g. What was the percentage amount of moisture present in the air? 10. 10 l. of air measured at 20 deg. and 740 mm. passed through lime water caused the precipitation of 0.0102 g. of CaCO_{3}. Find the number of volumes of carbon dioxide in 10,000 volumes of the air. CHAPTER IX SOLUTIONS