VI.] THE ATMOSPHERE. 8r 



with which it is associated. This nitrogen is remarkable 

 for its inertness ; it extinguishes flame and it does not 

 support hfe : yet it kills, not by being absolutely poisonous, 

 but simply by excluding oxygen. A fresh supply of air 

 is therefore constantly required by a living animal, not 

 because the nitrogen is deadly, but because the needful 

 oxygen is absent. 



But although nitrogen is not a dangerous gas, there are 

 other gaseous bodies always present in the atmosphere which 

 in a pure state are active poisons. Let a saucer of clear 

 lime-water be exposed to the air, and in a few hours the 

 surface of the liquid will be covered with a thin pellicle ot 

 whitish matter ; this is produced by something absorbed 

 from the atmosphere, yet neither oxygen nor nitrogen 

 produces the effect. It is due, indeed, to the presence of 

 the gaseous substance to which reference has already been 

 made under the name of carbonic acid gas ; this gas, acting 

 on the lime-water, forms a solid carbonate of calcium, or, as 

 it is more commonly termed, carbonate of lime ; and it is 

 this white solid substance which forms the thin skin on the 

 surface of the water. Carbonic acid gas, which is thus 

 proved to exist in the atmosphere, is a compound of two 

 distinct substances — carbon and oxygen. The oxygen has 

 been already described : the carbon is a solid body abun- 

 dantly distributed through nature, though rarely occurring in 

 a state. of purity. In its purest native form, it crystallises as 

 the diamond ; in a less pure condition it constitutes graphite 

 or " black-lead ; " and, in chemical combination with other 

 substances, it forms a large proportion of coal and of all 

 other ordinary' forms of fuel. It enters largely, too, into the 

 constitution of all living matter, whether animal or vege- 

 table, and is left in a tolerably pure state when these sub- 

 stances are charred or imperfectly burnt, as seen in coke, 



G 



