z6o THE POPULAR SCIENCE MONTHLY. 



Every body possesses its own distinct crystalline form ; every crys- 

 tal is a geometrical figure, usually bounded by plane surfaces having 

 angles of constant value, and the science of crystallography teaches us 

 to distinguish substances by the measurement of these angles. It is 

 invariably found that artificial crystals which have been deposited 

 slowly and quietly surpass in size, regularity, and beauty those of more 

 rapid formation ; hence it is conjectured that natural minerals owe 

 their great perfection to very gradual deposition in the rocks within 

 which they are found. 



Under different conditions the same substance sometimes assumes 

 two crystalline forms, of which somewhat uncommon phenomenon car- 

 bon furnishes an example by crystallizing now into diamond, and now 

 into graphite, or plumbago. 



Although found in every quarter of the globe, the diamond is the 

 rarest as it is the hardest known mineral. It occurs exclusively among 

 gold-bearing rocks, or sands derived from gold-bearing rocks, and 

 among strata which, though originally soft, shaly deposits of sand or 

 mud, have been " metamorphosed," as it is called, into hard crystalline 

 schists. It was once supposed by geologists that the metamorphic 

 rocks were deposited in their existing crystalline form from a boiling 

 ocean enveloping the still heated globe ; but it is now known that 

 these formations were originally deposited as mud or sand, and have 

 been transmuted into schists by the influence of subterranean heat 

 acting under great pressure, through lengthened periods of time, and 

 aided by thermal water or steam permeating the porous rocks and giv- 

 ing rise to various chemical decompositions and new combinations 

 within them. The diamond probably originates, like coal or mineral 

 oil, from the gradual decomposition of vegetable or animal matter ; 

 we may therefore regard the brilliants which we prize in the drawing- 

 room as having been slowly elaborated from carbonaceous matter fur- 

 nished by some dead fish, or rotting plant, originally buried in the mud 

 of an inconceivably ancient palaeozoic shore. 



It will now be seen that, in order to produce the diamond artifi- 

 cially, some means must first be devised whereby the element carbon, 

 which will dissolve in no liquid and vaporize in no flame, can be ren- 

 dered soluble or gaseous, from either of which conditions it might then 

 probably be recovered in a crystalline form, as happens in the case of 

 other bodies. 



Mr. Hannay's attempts to crystallize carbon originated from a re- 

 search of a very different character. Water, as we all know, vaporizes 

 at a heat of 212 Fahr., and in the same way every liquid has its "boil- 

 ing-point," or temperature at which it ceases to be a fluid and becomes 

 a gas. Little is known about the condition of matter immediately 

 beyond the " critical point," as the moment of passage from the liquid 

 to the gaseous state is called ; and while investigating this subject it 

 occurred to Mr. Hannay that some insight might be gained into a state 



