August 5. i897] V^li^^» NATURE 



325 



lishes, previous to his official communication, a brief abstract of 

 the results at which he has arrived. The discussion, he tells 

 us, was beset with difficulties, the most troublesome being the 

 parallactic motion of the stars arising from the solar motion. 

 Another difficulty was the fact that the proper motions of the 

 stars did not follow the normal or exponential law of error on 

 which the method of least squares and the practice of taking 

 them are based. Prof. Newcomb divided the work into four 

 parts, employing what he calls the statistical method, the method 

 of individual stars, the method of zones and regions, and a 

 method of which the parallactic motion is eliminated. The 

 results may be briefly summed up in the following table. 



Struve-Peters. 



General precession 5025-24 



Luni-solar „ ... 5038 '23 



Value of 100 m. ... 460765 



,, icon. ... 2005-64 



New. 



Diff. 



502453 ... - 071 



5036-84 ... - 1-39 



4607-11 ... - 0-54 



2005-11 ... -■0-53 



DIAMONDS} 



T T seems but the other day I saw London in a blaze of illu- 

 -*• mination to celebrate Iler Majesty's happy accession to 

 the throne. As in a few days the whole empire will be cele- 

 brating the Diamond Jubilee of our Queen, who will then have 

 reigned over her multitudinous subjects for sixty years, what 

 more suitable topic can I bring before you than that of 

 diamonds! One often hears the question asked: "Why 

 Z)/awi7W Jubilee?" I suppose it is a symbol intended to give 

 a faint notion of the pure brilliancy and durability of the Queen's 

 reign ; and in thus associating Her Majesty with the precious 

 •diamond, to convey an idea of those noble qualities, public 

 and private, which have earned for her the love, fealty, and 

 reverence of her subjects. 



From the earliest times the diamond has occupied men's 

 minds. It has been a perennial puzzle — one of the riddles of 

 creation. The philosopher Steffans is accredited with the 

 dictum that, " Diamond is quartz which has arrived at self- 

 consciousness ! " and an eminent geologist has parodied this 

 metaphysical definition, saying, "Quartz is diamond which has 

 become insane ! " 



Prof. Maskelyne, in a lecture " On Diamonds," thirty-seven 

 years ago, in this very theatre, said : " The diamond is a 

 substance which transcends all others in certain properties to 

 which it is indebted for its usefulness in the arts and its beauty 

 as an ornament. Thus, on the one hand, it is the hardest sub- 

 stance found in nature or fashioned by art. Its reflecting 

 power and refractive energy, on the other hand, exceed those of 

 all other colourless bodies, while it yields to none in the per- 

 fection of its pellucidity" ; but he was constrained to add, " The 

 formation of the diamond is an unsolved problem." 



Recently the subject has attracted many men of science. The 

 development of electricity, with the introduction of the electric 

 furnace, has facilitated research, and I think I am justified in 

 saying that if the diamond problem is not actually solved, it is 

 certainly no longer insoluble. 



Graphite. 



Intermediate between soft carbon and diamond come the 

 graphites. The name graphite is given to a variety of carbon, 

 generally crystalline, which in an oxidising mixture of chlorate 

 of potassium and nitric acid forms graphitic acid easy to recog- 

 nise. Graphites are of varying densities, from 2-0 to 30, and 

 generally of crystalline aspect. Graphite and diamond pass in- 

 sensibly into one another. Hard graphite and soft diamond are 

 near the same specific gravity. The difference appears to be 

 one of pressure at the time of formation. 



Some forms of graphite exhibit a remarkable property, by 

 which it is possible to ascertain approximately the temperature 

 at which graphites were formed, or to which they have subse- 

 quently been exposed. Ciraphites are divided into "sprouting " 

 and "non-sprouting." When obtained by simple elevation of 

 temperature in the arc or the electric furnace they do not 

 sprout ; but when they are formed by dissolving carbon in a 

 metal at a high temperature, and then allowing the graphite to 

 separate out on cooling, the sprouting variety is formed. One 



1 A lecture delivered at the Royal Institution, June 11, by William 

 Crookes, F.R.S. 



NO. 1449, VOL. 56] 



of the best varieties is that which can be separated from plati- 

 num in ebullition in a carbon crucible. The phenomenon of 

 sprouting is ea.sily shown. Place a few grains in a test-tube and 

 heat it to about 170° C., when it increases enormously in bulk 

 and fills the tube with a light form of amorphous carbon. 



The resistance of graphite to oxidising agents is greater the 

 higher the temperature to which it has previously been exposed, 

 Graphites, which are easily attacked by a mixture of fuming 

 nitric acid and potassium chlorate, are rendered more resistant 

 by strong heat in the electric furnace. 



I will now briefly survey the chief chemical and physical 

 characteristics of the diamond, showing you by the way a few 

 experiments that bear upon the subject. 



Combustion of the Diamond. 

 When heated in air or oxygen to a temperature varying from 

 760° to 875° C., according to its hardness, the diamond burns 

 with production of carbonic acid. It leaves an extremely light 

 ash, sometimes retaining the shape of the crystal, consisting of 

 iron, lime, magnesia, silica, and titanium. In boart and car- 

 bonado the amount of ash sometimes rises to 4 per cent., but in 

 clear crystallised diamonds it is seldom higher than 0-05 per 

 cent. By far the largest constituent of the ash is iron. 



The following table shows the temperatures of combustion 

 in oxygen of different kinds of carbon : — 



°C. 



Condensed vapour of carbon 650 



Carbon from sugar, heated in an electrical 

 furnace ... ... ... ... ... ... 660 



Artificial graphites, generally 660 



Graphite from ordinary cast-iron 670 



Carbon from blue ground, of an ochrey colour ... 690 

 ,, ,, ,, very hard and black . . 710 



Diamond, soft Brazilian 760 



,, hard Kimberley 780 



Boart from Brazil 790 



,, from Kimberley 790 



, , very hard, impossible to cut ... ... 900 



At the risk of repeating an experiment shown so well at 

 this table by Prof. Dewar, I will heat a diamond to a high 

 temperature in the oxyhydrogen blowpipe and then .suddenly 

 throw it in a vessel of liquid oxygen. Notice the brilliant 

 light of its combustion. I want you more especially to ob- 

 serve the white opaque deposit forming in the liquid oxygen. 

 This deposit is solid carbonic acid produced by the combus- 

 tion 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 per- 

 form this experiment quantitatively, leading the carbonic acid 

 and oxygen, as they 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 car- 

 bonic acid — the theoretical proportion for pure carbon. 



Some crystals of diamonds have their surfaces beautifully 

 marked with equilateral triangles, interlaced and of varying 

 sizes. Under the microscope these markings appear as shal- 

 low depressions sharply cut out of the surrounding surface, 

 and these depressions were supposed by Gustav Rose to in- 

 dicate 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 experiment 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 verj' 

 different in character from those naturally inscribed on crystals. 

 The artificial striae are cubical and closer massed, looking as 

 if the diamond during combustion had been dissected into 

 rectangular flakes, 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 flaky crystalline appearance strangely resembles the surface 

 of a diamond whose internal structure has been partially dis- 

 sected and bared by combustion. It looks as if this piece of 

 graphite was ready to separate out of its solvent as diamond, 

 but owing to some insufficient factor it retained its graphitic 

 form. 



