June 30, 1904] 



NA TURE 



209 



like a gnu or horse, and not one with a terminally tufted 

 tail of the ox type. Again, the general form of the animal 

 is much more like that of a gnu than of a bull. 



Accordingly, there appears a very strong presumption 

 that this sculpture represents the hunting of a species of 

 gnu, and if this be really the case, it would be a fact of 

 very considerable interest in connection with animal dis- 

 tribution. The two living species of gnu are now confined 

 to Africa, but their near relatives, the hartebeests, range 

 into Syria, while fossil species of that group, as w-ell as of 

 other antelopes of an African type, occur in the Upper 

 Tertiary strata of northern India and China. Nothing is 

 therefore more likelv than that gnus should have formerly 



Fig. 3. — A gnu (".') hunt, from Nimroud. 



had a more extensive range. If this be so, it would be one 

 more argument in favour of the old view that the present 

 antelope fauna of Ethiopian .\frica immigrated into the 

 country from the north, and against the modern theory of 

 its autochthonous origin in Africa itself. For It is surely 

 much more probable that animals should have died 

 out in their ancient habitat and flourished in the country 

 in which there are comparatively new arrivals rather than 

 the converse. 



.A more extensive and detailed study of the old Assyrian 

 and Babylonian sculptures and of the Egyptian frescoes would 

 doubtless lead to the identification of species of animals 

 other than those mentioned above ; but such identifications 

 as I have been able to make are sufficient to demonstrate 

 that the subject has a definite bearing on the past dis- 

 tributional history of mammals, and that it ought not to 

 be neglected by students of that branch of zoology. 



R. LVDEKKER. 



THE ACTION OF RADIUM EMANATIONS 

 ON DIAMONDS 



\X7HEN diamonds are e.xposed to the impact of radiant 

 matter in a high vacuum they phosphoresce of 

 different hues, and assume a dark colour, becoming almost 

 black when the bombardment is long continued {Phil. 

 Trans., 1879, part ii., p. 658, par. 625). 



Some diamonds blacken in the course of a few minutes, 

 while others require an hour or more to discolour." This 

 blackening is only superficial, and although no ordinary 

 means of cleaning will remove the discoloration, it goes at 

 once when the stone is polished with diamond powder. The 

 fact that the black stain is not affected by ordinary o.Kidising 

 reagents would seem to show that it is not due to a layer 

 of amorphous carbon ; but it might be graphite, which is 

 much more resistant to oxidation. Becquerel has shown 

 that graphite is converted into graphitic oxide by long 

 digestion in a warm mixture of potassium chlorate and 

 strong nitric acid, while diamond — even in a very finely 

 powdered state — is absolutely unaffected by the mixture 

 (Ann. de Chim. et de Phys., [4], vol. xix. p. 392). 



Some forms of graphite dissolve in strong nitric acid ; 

 others require a mixture of highly concentrated nitric and 

 potassium chlorate to dissolve them, and even with this 



I Read before the Royal Society on June t6 by Sir William Crookes, 

 F.R.S. 



- At a lecture before the Royal In!;tilution on June 11, 1S07, I exposed a 

 flat made crystal of diamond to radiant matter bombardment before the 

 audience for about five minutes, a strip of metal covering part of the stone. 

 On removing the diamond from the vacuum tube and projecting its image 

 on the screen with the electric lantern, the image of the d.irkening was 

 very apparent. 



intense oxidising agent some graphites resist longer than 

 others. M. Moissan has shown that the power of resist- 

 ance to nitric acid and potassium chlorate is in proportion 

 to the temperature at which the graphite has been formed, 

 and with reasonable certainty we can estimate this tempera- 

 ture by the resistance of the graphite to this reagent. 



Judging from the long time required to remove the super- 

 ficial darkening from diamond, the graphite is as resistant 

 as that formed at the temperature of the electric arc. 



On one occasion when I had blackened the surfaces of 

 diamonds by molecular bombardment in vacuo M. Moissan 

 was present, and took some away with him for further 

 examination. He subsequently reported the results in the 

 Comptes rendus, vol. cxxiv.. No. 13. He heated the 

 diamond to 60° in an oxidising inixture of potassium 

 chlorate and fuming nitric acid prepared from mono- 

 hydrated sulphuric acid and potassium nitrate fused and 

 quite free from moisture. The action on the black layer 

 is very slow. There is produced graphitic oxide, which at 

 an increased temperature yields pyrographitic acid, which 

 is easily destroyed by nitric acid. Hence the variety of 

 carbon which coated the diamond was graphite. The trans- 

 formation of diamond into graphite requires the high 

 temperature of the electric arc. The higher the temperature 

 to which graphite is raised the greater is its resistance to 

 oxidation. M. Moissan concludes that the temperature 

 reached by the surface of the diamond in my radiant matter 

 tubes is probably about 3600°. 



The /3-rays from radiuin having like properties to the 

 kathode stream in a radiant matter tube, it was of interest 

 to ascertain if they would exert a like difference on diamond. 

 Two Bingara diamonds, A and B, weighing respectively 

 0-960 and 1020 grains, were selected as near as the eye 

 could judge of the same size and colour — very pale yellow, 

 technically known as " off colour." Diamond A was put 

 in a drawer far removed from radium or any radio-active 

 body. Diamond B was kept close to a quartz tube con- 

 taining about 15 milligrams of pure radium bromide sealed 

 in vacuo. It phosphoresced brightly and continued to glow 

 the whole time of the experiment. 



After a fortnight the two diamonds were put side by side 

 and compared. I could see no appreciable difference in 

 colour between them. Diamond B was now replaced close 

 to the quartz tube of radium, and they were kept in contact 

 for six weeks. At the end of that time examination again 

 showed scarcely any difference between the two. The one 

 which had been near the radium might be a little the 

 darker of the two, but the difference was too slight to 

 enable me to speak positively. 



Diamond B was now put inside a tube with radium 

 bromide, the salt touching it on all sides, as it was thought 

 possible that a screen of quartz inight interfere with the 

 passage of emanations which would act on the diamond. 

 The comparison diamond was kept removed from the eman- 

 ations as before. The experiment was continued for seventy- 

 eight days, when the two diamonds were again examined. 

 There was now a decided difference in colour between them : 

 diamond A was of its original pale yellow " off colour," and 

 diamond B was of a darker appearance and of a bluish 

 tint, with no yellow colour apparent. 



It thus appears that the property which radium eman- 

 ations possess of darkening transparent bodies which they 

 impinge upon — a property very marked in the case of glass, 

 and less with quartz — also holds good in the case of 

 diamond. 



Diamond B was now heated to 50° C. in a mixture of 

 strongest nitric acid and potassium chlorate for ten days, 

 the mixture being renewed each day. At the end of this 

 time the diamond had lost its dull surface colour, and was 

 as bright and transparent as the other stone, but its tint 

 had changed from yellow to a pale blue-green. 



The radium emanations have therefore a double action on 

 the diamond. The g-rays (electrons) effect a superficial 

 darkening, converting the surface into graphite in a manner 

 similar to, but less strongly than, the more intense electrons 

 in the kathode stream. But the alteration of the body 

 colour of the stone by emanations which are obstructed by 

 the thinnest film of solid matter, even by a piece of thin 

 paper, is not so easy to understand. A superficial action 

 might be expected, but not one penetrating through the 

 whole thickness of the diamond. I believe the alteration 



NO. 1809, VOL. 70] 



