THE VOLATILE PART 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 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. 
Oxygen.—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 metallic 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- 
