CHEMICAL PARADOXES. 



tion with another gas, hydrogen, the liquid 

 which we drink as water ; and, combined with 

 silicon, it makes the solid quartz, flint, or sand 

 beneath our feet. 



An element, however, can sometimes alter 

 its appearance and behaviour without combin- 

 ing with other elements. Thus oxygen, by 

 simply having its atoms grouped in little masses 

 or molecules of three each instead of two 

 each, appears as a different gas, ozone. So 

 sulphur can appear as yellow crystals, as a 

 yellow amorphous solid, or as a soft tenacious 

 mass. Phosphorus may appear as a soft yellow 

 material so combustible as to burn spontane- 

 ously, or with a dark colour and not inflammable 

 except at a high temperature. 



The most remarkable instance of allotropic 

 forms is given by carbon, which appears as char- 

 coal, graphite, or diamond. In each of these 

 forms it is exactly the same element as in the 

 others ; and on burning with oxygen the same 

 weight of each of them produces exactly the 

 same weight of carbonic acid gas. So that 

 coals are quite literally and strictly entitled 

 to their name of black diamonds. 



What we have seen to be true of elements is 

 equally true of compounds, a fact designated 

 by the term isomerism. Thus the gas acetylene 

 and the liquid benzene are composed of exactly 

 the same elements in the same proportions. 

 Their different properties depend on the fact 



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