128 PRACTICAL LESSONS IN SCIENCE. 



sole product, as in the case of the diamond, is carbon dioxide. 

 Graphite is insoluble and unaffected by moisture or air, and 

 forms a valuable coating for protecting iron from the effect of the 

 weather. Its chief use is, perhaps, the manufacture of pencils. 

 For this purpose it is reduced to a powder and subjected to great 

 pressure. It is then sawed into little bars which are cased with 

 wood. A mass of graphite seems soft and friable, but the par- 

 ticles are extremely hard, and soon wear out the steel saws used 

 in cutting it into shape. 



We know nothing about the formation of diamonds, but 

 geologists consider graphite to be the residue of some of the 

 forms of life occupying the earth when the materials of the older 

 rocks were being deposited . 



Another form of nearly pure carbon is charcoal. It is formed 

 when any organic substance, as wood, sugar, flesh, bone or blood 

 is burned in a limited supply of air. It is a black, brittle, tasteless, 

 solid. It is not easily affected by the action of air or water at 

 ordinary temperatures; charred cloths, grains, fruits, and other 

 remains of the Lake Dwellers of Switzerland, are as perfect to-day 

 as when entombed in the mud and waters of the lakes thousands 

 of years ago. The charcoal of commerce is formed from wood 

 by burning it in a limited supply of air. Wood is made up 

 mainly of carbon, oxygen and hydrogen, and when burned in the 

 open air the oxygen combines with the hydrogen forming water, 

 and with the carbon forming carbon dioxide until only a few 

 ashes are left ; but if the supply of oxygen is limited the hydrogen 

 and part of the carbon are driven off, but the main part of the 

 carbon remains as charcoal. Charcoal is used as a fuel, burning, 

 when dry, without flame, yielding a high degree of heat. Burned 

 in oxygen the sole product is carbon dioxide, as in the case of 

 diamond and graphite. 



Charcoal is very porous, and possesses in a remarkable degree 

 the power of absorbing gases, some specimens absorbing 90 

 times their bulk of ammonia, 35 times their bulk of carbon diox- 

 ide. It is said that freshly burned charcoal will absorb moisture 

 from the air so as to increase at least 10 per cent, in weight. 



