THE ORIGIN AND FORMATION OF THE DIAMOND. I7I 



and therefore the diamonds containing it, was never exposed 

 to a very high temperature, otherwise the graphite would have been 

 changed from the soft to the hard kind. As diamond ignites 

 in oxygen gas at from 750° to 800° C. the soft variety of graphite 

 mentioned above must ignite at a temperature not exceeding 

 600° C. 



Most diamonds when burnt leave a small amount of ash, which 

 varies from a very minute quantity in perfectly clear diamonds 

 to as much as 4% in boart and carbonado. This ash is fownd 

 to consist of oxide of iron, silica, magnesia, lime, and some- 

 times alumina and oxide of titanium. That these oxides exist 

 as such in the diamond before it is burnt there can, I think. 

 be httle doubt, as sometimes they can be seen as coloured specks 

 inside the diamond. 



In describing the Cullinan Diamond Sir William Crookes 

 speaks of brown specks of what may be oxide of iron, and he 

 talks of diamonds being coloured by the same or similar oxides. 

 Whether the colouring matter in man}^ brown diamonds is dise 

 to oxide of iron may be a question. I have, however, certain 

 proof of the existence of both proto- and peroxide of iron in some 

 pieces of carbonado which I examined some years ago. Some 

 small splinters were carefully cleaned and powdered in a mortar 

 and the powder thrown into pure boiling hydrochloric acid 

 which was carefully covered up from the air. After boiling fo-r 

 half an hour the liquid became quite yellow and it was then 

 found to contain iron both as protochloride and as perchloride, 

 proving that the iron existed in both conditions in the carbonado, 

 probably as the black magnetic oxide. That being so. it is. 

 speaking chemically, impossible to suppose these oxides ot iron 

 embedded in this carbonaceous substance, and exposed to, say, 

 no more than a red heat, and yet to remain unaltered. The 

 peroxide would have been reduced to the protoxide with the 

 formation of carbon monoxide, which latter would have reduced 

 it farther to the metal. This conclusion, I must add, is on the 

 assumption that water, or water vapour, is absent. In the 

 presence of water or steam, the iron, especially if finely divided, 

 would, at a red heat, or even at a much lower temperature, be 

 oxidised to the magnetic oxide. That, however, would not 

 prevent the carbonaceous material being acted on and oxidised 

 if the temperature were high enough : and it is probable that, 

 under certain conditions, oxide of iron, in presence of carbon- 

 aceous materials and water, or steam, might act as a carrier of 

 oxygen to the carbon. In this connection I have thought that 

 the geologists are sometimes not sufficiently careful in drawii-ig 

 inferences in matters chemical. As I wish later on to make reference 

 to the production of graphite from fossils in ca.ses of local and 

 regional metamorphism, I will here just mention an instance. 

 Sir Archibald Geikie in his Geology, Second Edition, in treating 

 of regional metamorphism in the Ardennes says : — 



" Renard, however, points out that the eruptive rocks are really absent, 

 and that the association of minerals proves that the metamorphosed 

 rocks could not have been softened by a high temperature, as supposed 



