April 22, 1922] 



NATURE 



517 



and the annotations in a Hebrew MS. " Jad Chazaka " 

 (Poc. 235) of about 1650 are also in lead or a lead 

 alloy. 



The notebook of the Swiss scholar Casaubon, 

 (Casaubon MS. 6i), written about 1613, is particularly 

 interesting. The leaves of the book are of thick 

 horn and are covered on each side with minute 

 writing, which is in a metallic pigment, showing 

 much finer spicules than is usual in the writing 

 done with a metal style. It resembles the pigment 

 used in the drawings of the Stowe MS. " Arms 

 of Ancient Nobilitie " of the early seventeenth century 

 (British Museum). 



A series of almanacs interleaved and containing 

 Anthony Wood's Diary from 1676 to 1685 (Wood's 

 Diaries 20-29, 742), shows ruled lines in a metallic 

 pigment, while the entries in the diaries are either 

 in ink or, less frequently, in pencil. Referring to the 

 latter entries, the Rev. Andrew Clark remarked (" Life 

 and Times of Anthony Wood," I. 3 Oxf. Hist. 

 Soc, 1891) : " Wood's pencil, I assume, was not 

 graphite, but actual lead. It has left a faint mark, 

 almost illegible, except for the indentation of the 

 paper." Microscopical examination of these entries 

 down to 1685 showed that this assumption was 

 correct, none of the writing having the characteristics 

 of graphite. 



At a later date, however. Wood appears to have 

 had a graphite pencil. " A Collection of Poems on 

 Affairs of State," London, i68g (Wood, 382) has a 

 note on its flyleaf in the writing of Wood : " Bought 

 at Oxon. 26 Feb. 1688." (The discrepancy between 

 the dates is explained by the use of the old style for 

 one of them.) This writing has the appearance of 

 ordinary graphite, the masses of black pigment being 

 uniformly distributed, and none of the particles 

 showing the lustre or striation of lead or its alloys. 



With the very doubtful exception of the markings 

 in the MS. of Johannis Dastyn {supra), this is the 

 earliest writing in graphite pencil noted in the 

 Bodleian Library. 



It will be recalled that the earliest graphite writing 

 found in the British Museum was in two Notebooks 

 of Sir Thomas Cotton, one of about 1 630-1 640 and 

 the other 1640-1644. 



C. AiNswoRTH Mitchell. 



195 Victoria Street, S.W.i. 



Haloes and Earth-History : A New Radioactive 

 Element. 



In the Archaean black mica of Yitterby, very 

 small, colourless, spherical, halo-like forms occur. 

 The mica is, as it were, bleached within these halo- 

 spheres. Many show a central opaque particle or 

 nucleus. In the case of others it is difficult to be 

 sure of the presence of a nucleus, or it may take the 

 form of a limpid refracting particle. The optical 

 appearances suggest that in these spheres the re- 

 fractive index of the mica has been raised. 



They are very small. A cluster of these minute 

 haloes, amounting to many hundreds, presents with 

 a low power the appearance of a starry sky. The 

 average diameter appears to be o-oi mm., no allow- 

 ance being made for the nucleus. The greater 

 number are remarkably uniform in diameter. Thus, 

 readings of 23 of these haloes, taken at random, 

 range from 50 to 59 scale divisions of the micrometer. 

 A few larger ones are present, but, as in most cases 

 their greater size is directly referable to an aggrega- 

 tion of nuclear particles occupying a large central 



NO. 2738, VOL. 109] 



volume, it seems safe to conclude that these limpid 

 spheres are in reality of one size only. 



The mica flake in which this cluster occurs is 

 o-oi8 mm. thick, or perhaps a little less. The 

 minute spheres are located at various depths in the 

 mica — some below the surface, some more or less 

 truncated. Many are sharp and easily measured ; 

 others, as would be expected, are diffuse upon the 



I have been for some years aware of the existence 

 of these haloes — since a period before the war — 

 but, during the occupation of my laboratory by 

 troops in 1916, among other things lost was the 

 specimen of Yitterby mica from which my slides 

 were obtained. A few other samples of this mica 

 did not appear to contain them. Nor could I form 

 any probable theory to account for them. Their 

 uniform size convinced me that they were radio- 

 active. The hypothesis that they might be due to 

 slow /3-radiation had really nothing to support it : 

 no similar effect having been found in any other 

 mica. Later I concluded that they must be reversed 

 haloes such as I found in the Devonian mica of Co. 

 Carlow. There were other appearances to support 

 this view. Thus in the very similar Archaean mica 

 from Arendal, bleached uranium haloes in various 

 stages were found. Again, in both the Yitterby 

 and Arendal mica, semi-bleached bands occur having 

 a centrally placed line of what are probably radio- 

 active particles disposed like a central moraine on a 

 'glacier. But with these appearances of reversal were 

 vigorous haloes in all stages of development. 



Recently, Dr. Prior was so good as to send me 

 some flakes of Yitterby mica. In them I found all 

 the appearances described above, including a few 

 of the minute bleached halo-spheres. 



The idea that this bleaching of the Archaean micas 

 might be due to former high temperature conditions 

 led to a test of the behaviour of the Carlow mica at 

 different temperatures. It was found that a halo- 

 rich specimen of this mica after an hour's exposure 

 to a temperature of about 730° C. had acquired many 

 of the characters of the Archaean micas. Most of 

 the haloes had disappeared, and some had left a 

 bleached area giving quite characteristic readings. 

 Bleached bands, also, had taken the place of linear 

 radioactive staining; and the originating radioactive 

 particles were exposed to view forming a central line. 



There is, I think, no doubt that these (and other) 

 appearances show that the Yitterby and Arendal 

 Archaean micas were at some remote period subjected 

 to a temperature probably not much exceeding 700° C. 

 during which a prior-existing crop of haloes were 

 obliterated or reversed, and that, therefore, the 

 existing haloes are a second crop which originated 

 from the same nuclei when the thermal conditions 

 permitted their development. This history has a 

 good deal that is fortuitous connected with it. Some 

 three hundred additional degrees of temperature 

 would have reduced the mica to a slag. 



But what is to be inferred as to the nature of the 

 minute halo-spheres ? A chance find seems to bear 

 upon their origin. 



In the mica of Arendal uranium haloes are fairly 

 abundant ; they occur in every stage of development. 

 Within some of the earliest rings having radial 

 dimensions reading from 0-0150 to o-oi6o (correspond- 

 ing to rings which in Devonian mica read 0-0145) a 

 very minute ring is sometimes seen. It is a difi&cult 

 object and needs good lighting and good sight to 

 detect. This ring surrounds the point-like nucleus 

 with perfect centricity, an intervening band of 

 unstained mica being apparent. The radius of this 

 ring has been read from 0-0049 to 0-0057. The 

 effect of the nuclear dimensions on a ring so small 



