THE VOLATILE PAET OF PLANTS. 33 



unless exposed to an elevated temperature. Hence stakes 

 and fence posts, if charred before setting in the ground, 

 last longer than when this treatment is neglected. 



The porous varieties of carbon, especially wood charcoal 

 and bone-black, have a remarkable power of absorbing 

 gases and coloring matters, which is taken advantage of 

 in the refining of sugar. They also destroy noisome 

 odors, and are therefore used for purposes of disinfection. 



Carbon is the characteristic ingredient of all organic 

 compounds. There is no single substance tliat is the ex- 

 clusive result of vital organization, no ingredient of the 

 animal or vegetable produced by their growth, that does 

 not contain this element. 



Oxygeili — Carbon is a solid, and is recognized by our 



senses of sight and feeling. Oxygen, on the other hand, 



■ is invisible, odorless, tasteless, and not distinguishable 



in any way from ordinary air by the unassisted senses. It 



is an air or gas. 



It exists in the free (uncombined) state in the atmos- 

 phere we breathe, but there is no means of obtaining it 

 pure except from some of its compounds. Many metals 

 unite readily with oxygen, forming compounds (oxides) 

 which by heat separate again into their ingredients, and 

 thus furnish the means of procuring pure oxygen. Iron 

 and copper when strongly heated and exposed to the air 

 acquire oxygen, but from the oxides of these metals 

 (forge cinder, copper scale,) it is not possible to separate 

 pure oxygen. If, however, the metal mercury (quicksil- 

 ver) be kept for a long time at a boiling heat, it is slowly 

 converted into a red powder (red precipitate or oxide of 

 mercury), which on being more strongly heated is decom- 

 posed, yielding metaUic mercury and gaseous oxygen in 

 a pure state. 



The substance usually employed as the most convenient 

 source of oxygen gas is a white salt, the chlorate of pot- 

 2* 



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