The Atmosphere 105 



the open country, 5 parts is common in towns, and as much 

 as 30 parts of carbonic acid in 10,000 of air may be found 

 in badly-ventilated overcrowded rooms. More than this 

 proportion acts poisonously on animal life. Carbonic acid is 

 the most soluble of the atmospheric gases, water at 60 F. 

 and under ordinary pressure absorbing its own volume. 



155. Mixture of Gases. One consequence of the 

 nature of gases is that when -two or more different kinds 

 are mixed, each one acts as if it alone were present. This 

 is known as Dalton's Law. Thus there is an atmosphere 

 of nitrogen surrounding the globe, and exerting the pres- 

 sure of its weight upon the Earth's surface, and an atmo- 

 sphere of oxygen pressing upon the surface with its weight, 

 which is rather less than one quarter of the pressure exerted 

 by nitrogen, and a very thin atmosphere of carbonic acid 

 exerting a very feeble pressure. There is also an atmo- 

 sphere of water-vapour pressing with its independent weight 

 on the Earth's surface, and all these partial pressures together 

 make up the pressure exerted by the whole atmosphere. 

 The particles of the different gases pass each other freely, 

 without interfering, like crowds moving in different directions 

 across a market-place. Thus it is that the composition of 

 the atmosphere as a whole remains constant so far as 

 regards the three gases, nitrogen, oxygen, carbonic acid, and 

 the proportion of each of them is the same at all heights. 



156. Water -vapour. Next to oxygen, water- vapour 

 is the most important ingredient of the atmosphere. The 

 other gases are a long way above their liquefying point, so 

 that the addition or withdrawal of heat only affects their 

 temperature and their volume. But water-vapour in the 

 atmosphere is near the temperature at which it becomes 

 liquid or solid, and is nearly always in the presence of liquid 

 water, hence changes of temperature greatly affect the 

 amount of vapour present. Let us suppose for a moment 

 that the atmosphere consisted of water -vapour only, and 

 that the hydrosphere covered the Earth uniformly with a 

 liquid layer. The amount of this atmosphere, and con-, 

 sequently its pressure, would depend upon the temperature. 

 Evaporation takes place from cold water, or even ice, but 



