THE VOLATILE PAET OF PLANTS. 17 



ble under ordinary circumstances. Hence stakes and 

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

 much longer than when this treatment is neglected. 



The porous varieties of carbon, especially wood char- 

 coal and bone-black, have a remarkable power of absorb- 

 ing gases and coloring matters, which is taken advantage 

 of in the refining of sugar. They also destroy noisome 

 odors, and are used for purposes of disinfection. 



Carbon is the characteristic ingredient of all organic 

 compounds. There is no single substance that 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. 



n Oxygen. — Carbon is a solid, and is recognized by our 

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

 is an air or gas, invisible, odorless, tasteless, and not dis- 

 tinguishable in any way from ordinary air by the unas- 

 sisted senses. 



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 near the temperature at 

 which it boils, it is slowly converted into a red powder 

 (red precipitate, red oxide of mercury, or mercuric ox- 

 ide), which on being more strongly heated is decomposed, 

 yielding metallic mercury and gaseous oxygen in a pure 

 state. 



The substance usually employed as the most convenient 

 source of oxygen gas is the white salt called potassium 

 2 



