1897.] on Diamonds. 487 



table by Professor Dewar, I will beat a diamond to a bigh tempera- 

 ture in tbe oxyhydrogen blowpipe and tben suddenly throw it in 

 a vessel of liquid oxygen. Notice tbe brilliant light of its combus- 

 tion. I want you more especially to observe the wbite opaque 

 deposit forming in the liquid oxygen. This deposit is solid carbonic 

 acid produced by the combustion of the carbon. I will lead it 

 through baryta water, and you will see a white precipitate of barium 

 carbonate. With a little more care than is possible in a lecture I 

 could perform this experiment quantitatively, leading the carbonic 

 acid and oxygen, as tliey assume the gaseous state, through baryta 

 water, weighing the carbonate so formed, and showing that one 

 gramme of diamond would yield 3*666 grammes of carbonic acid — 

 the theoretical proportion for pure carbon. 



Some crystals of diamonds have their surfaces beautifully marked 

 with equilateral triangles, interlaced and of varying sizes (Fig. 21). 

 Under the microscope these markings appear as shallow depressions 

 sharply cut out of the surrounding surface (Fig. 22), and these 

 depressions were suppcsed by Gustav Kose to indicate the probability 

 that the diamonds at some previous time had been exposed to 

 incipient combustion. Rose also noted that striations appeared on 

 the surfaces of diamonds burnt before the blowpipe. This experi- 

 ment I have repeated on a clear smooth diamond, and have satisfied 

 myself that during combustion in the field of a microscope, before 

 the blowpipe, the surface becomes etched with markings very 

 ditferent in character from those naturally inscribed on crystals. 

 O'he artificial striae are cubical and closer massed, looking as if 

 the diamond d^uring combustion had been dissected into rectangular 

 flakes (Fig. 23), while the markings natural to crystals appear as if 

 produced by the crystallising force as they were being built up. 



I exhibit on a diagram a form of graphite from the Kimberley 

 blue ground (reproduced from M. Moissan's work) which in its 

 crystalline appearance strangely resembles the surface of a diamond 

 whose internal structure has been partially dissected and barred by 

 combustion. It looks as if this piece of graphite was ready to 

 separate out of its solvent as diamond, but owing to some insufiicient 

 factor it retained its graphitic form. 



The specific gravity of the diamond is from 3 '514 to 3*518. 

 For comparison, I give in tabular form the specific gravities of the 

 different varieties of carbon : — 



Amorphous carbon 1*45 to !• 70 



Graphite 2-11 „ 3-0 



Hard gas coke 2-356 



Boart 3-47 . 3-49 



Carlonado 3- 50 



Diamond 3 '514 „ 3*518 



The diamond belongs to the isometric system of crystallography. 

 It frequently occurs with curved faces and edges (Fig. 24). Twin 



2 K 2 



