PROPERTIES OF GASES 95 



formerly prepared from soda, but there is really no soda in 

 soda water. The tanks containing soda water are made of 

 metal and are very strong. They are partially filled with 

 water, and carbon dioxide is then forced into the water with 

 great pressure. As long as the tank is closed, the gas re- 

 mains dissolved in the water. If the valve is opened, the 

 gas escapes with violence, bringing with it a little water in 

 the form of a spray. 



There are many manufactured liquids, called by the names 

 of celebrated medicinal springs, which contain carbon dioxide. 

 Minerals are dissolved in the water and then carbon dioxide 

 is added. The waters are much more agreeable to the 

 taste after they are " charged" with the gas. Such waters 

 are called "carbonated" and are close imitations of natural 

 waters. 



A great number of "soft drinks" owe their effervescence 

 to compressed carbon dioxide. 



103. Diffusion. Gases are soluble in other gases. This 

 dissolving of one gas in another is usually spoken of as mix- 

 ing or diffusion. If ammonia gas escapes from a bottle of 

 ammonia solution or from ammonia smelling salts, its odor 

 is soon evident to a person some distance away. The gas 

 has been dissolved in the air and has spread quickly. Some 

 gases diffuse more rapidly than others; diffusion is assisted 

 by movement of the air. 



Diffusion of gases, and also of liquids, occurs even when 

 they are separated by a thin membrane of plant or animal 

 tissue. Diffusion through a membrane is called osmosis. 

 It plays a very important part in the distribution of gases 

 and liquids in living bodies. Oxygen from the air we inhale 

 passes, by osmosis, through the membranes of the lungs into 

 the blood vessels, to be carried to all parts of the body. 



104. The Pressure of the Air. The atmosphere, 

 which envelops the whole earth and moves with it as it 

 rotates, is held in place, like other movable bodies, by 



