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also speak of some that have undergone a greater or less amount 

 of artificial treatment either to produce them or to obtain them 

 separated from other substances with which, in nature, they may 

 be mixed up. 



There are at least three varieties or, as we say, allotropic 

 modifications of carbon existing as a free substance. These are 

 known as diamond, graphite and amorphous carbon. Of the last 

 charcoal is a familiar example. The diamond, on account of its 

 extreme hardness and brilliant lustre, has been valued for many 

 ages, and these properties, together with its rarity, placed it in, 

 or near, the front rank of gems long before anything was known 

 of its chemical nature. It was, indeed, thought to be a special 

 variety of rock crystal — i.e., of what we should now call crystal- 

 lised silica. Sir Isaac Newton appears to have been the first 

 to suspect that it was of an altogether different nature. 

 The great lustre of the diamond is due to its great power of 

 bending or refracting light. Newton compared it with turpentine, 

 camphor, amber and other combustible substances and found that 

 it exceeded these in refractive power in much the same proportion 

 that it exceeded them in density and concluded that it was 

 a fatty or unctuous substance congealed. Later on numer- 

 ous experiments were performed to ascertain the behaviour 

 of diamond when strongly heated. It was found that a diamond 

 completely disappeared when placed in the focus of a powerful 

 burning glass and, later still, it was shown that it undergoes a 

 true combustion when powerfully heated in air or oxygen and 

 that the product of its combustion is identical with that of char- 

 coal. Thus the chemical identity of two substances, apparently 

 so unlike each other, was established. The proof was made quite 

 complete by quantitative experiments towards the end of the 

 1 8th Century. By these it was shown that when equal weights of 

 diamond, graphite and charcoal are separately burned in oxygen 

 equal weights of carbonic acid gas are produced in all three cases. 

 Diamond is the hardest of all natural substances ; it scratches 

 every other substance and is itself not scratched by any. It is a 

 poor conductor of heat and electricity, and has a specific gravity 

 of about 3J. Thus, if a piece of rock crystal were cut to resemble 

 a diamond it might be distinguished from it, not only by its 

 inferior hardness but by its less weight. If both were the same 

 size and if the diamond weighed 35 grains the rock crystal would 

 only weigh 25 grains. Fresh eggs may be distinguished from 

 stale ones by preparing a strong solution of salt in which a fresh 

 egg will just sink, whereas a stale one will float. In like manner 

 a very heavy solution may be made in which a true diamond will 

 just sink, but an imitation one will float. The natural occurrence 

 of the diamond I leave to the geological lecturer, but it will be 

 fitting to add a word as to its artificial production. This was first 

 effected in 1893 by the French chemist, Henri Moissan. His 

 method consisted in impregnating iron with charcoal by heating 

 them together in the electric furnace and then very suddenly cool- 



