CARBON AND THE OXIDES OF CARBON 335 



In the former case the quicklime (CaO), which is formed at the 

 same time, is a valuable product also. 



4. Carbon dioxide is formed in fermentation, and so is collected 

 from the vats in which beer is brewed (see p. 417). 



Physical Properties. Carbon dioxide is a colorless, odorless, 

 almost tasteless gas. As the molecular weight (CO2 = 44) shows, 

 it is one-half heavier than air. Its greater specific gravity may 

 easily be shown by pouring it from one jar into another, or into a 

 beaker placed on one pan of a balance with an equipoise of shot 

 on the other pan. It is much more soluble in water than is air. 

 One volume of water at 15 dissolves an equal volume of the gas. 

 At two atmospheres pressure, two volumes are dissolved, at three 

 atmospheres, three volumes. Pure water, charged at 3 or 4 

 atmospheres pressure is known as soda water. Effervescent 

 waters, such as Selters and Vichy, contain dissolved salts in addi- 

 tion. 



The gas can be liquefied at any temperature below 31.35 (see p. 

 91). At 20 the pressure required is 60 atmospheres and this is 

 therefore the pressure in a cylinder of liquid carbon dioxide at that 

 temperature. To withstand the pressure very massive cylinders 

 are required, and they weigh, when empty, about twice as much as 

 does the liquid they will hold when full. 



When liquid carbon dioxide is allowed to run from a cylinder 

 nto a cloth bag (non-conductor of heat), the rapid evapora- 

 tion of a part of the liquid consumes so much heat that the rest of 

 the liquid freezes to a snow-like mass of solid carbon dioxide. In 

 the laboratory this solid is used as a cooling agent, being mixed 

 with ether or alcohol to secure closer contact with the object to 

 be cooled. 



Chemical Properties. Stability. 1. Carbon dioxide is 

 very stable (only 7.5 per cent dissociated at 2000), and so, although 

 it contains much oxygen, it will not support combustion. Mag- 



