September 8, 1923] 



NA TURE 



Z^Z 



directly with the percentage of organic matter and 

 inversely with the percentage of tricalcium phosphate. 

 It is thereforemore rational to assume that this lime 

 is combined with the organic matter than to state 

 (as has been done) that it is present as a compound of 

 the type ;i;(Ca3P208).3;(CaO).2'H20. 



Dahllite, 4Ca3P2O8.2CaCO3.H2O, is not mentioned 

 in my paper. The idea of its occurrence is not 

 rejected by me, however, as its presence is not in 

 conflict with the chemical analyses, no excess lime 

 (as defined above) being required for it. 



That the calcium fluoride shown in the analyses 

 (about 3 per cent.) is present combined as apatite to 

 any considerable extent is unlikely, from the behaviour 

 of the phosphate to reagents ; the purer varieties of 

 Ocean Island phosphate being, for example, almost 

 completely soluble in cold, dilute hydrochloric acid. 



Launcelot Owen. 

 Monte ria, 

 Republic of Colombia, 



South America, July 21. 



I HOPE that no one who consults Mr. Owen's inter- 

 esting paper will have been much troubled by misinter- 

 pretations on my part. The words " normal 88 per 

 cent." are based on the analysis on p. 13 of the paper, 

 which is said to be " representative " and gives 87-5 

 per cent. I hope, again, that no reader of my note 

 would suppose that a level exists in the rock at which 

 the phosphate-content drops suddenly to 79 per cent. 

 I should have written " has sunk " for " sinks." 



In suggesting on p. 13 of the paper, and in his letter, 

 that lime is associated with the organic matter in the 

 phosphate, Mr. Owen raises a question of wide 

 importance. The special adsorptive influence of 

 organic colloidal gels in soils is now well known, and 

 Mr. Owen doubtless sets a good example in not 

 presuming the presence of dahllite or any other 

 t^mineral unless it can be recognised by specific 

 [characters in the mass. The rather delicate fibrous 

 [crystallisation of dahllite may be looked for. We 

 imust remember that A. Lacroix and other mineralo- 

 j^ists recognise definite species of mineral " calcium 

 ^carbo-phosphates." E. Blackwelder, on the other 

 |hand {Amev. Journ. Sci., ser. 4, vol. 42, p. 294, 

 ^1916), regards the less definite coUophane as the 

 icommon product of the reaction between phosphoric 

 [acid and lime salts, especially calcium carbonate, in the 

 jresence of ammonia. Collophane, as Rogers shows, 

 [can associate fluorine with its colloidal substance, and 

 Rmay thus suggest the presence of apatite. In his 

 fresearches on the chemistry of phosphatised reefs, 

 [Mr. Owen is opening up a very interesting petrological 

 meld. The Writer of the Note. 



The Metric Campaign. 



In reviewing Drury's " World Metric Standardisa- 

 tion" (Nature, August 18, p. 234), the statement is 

 made that " far less opposition has been raised to the 

 adoption of the litre and gram than to the metre, 

 which is very much more closely related to industrial 

 processes than the units of mass and measure." 



Perhaps the following will serve to indicate to 

 fnetric campaigners why those who are directly in- 

 terested in industrial processes are in such an im- 

 penetrable fog over the question. 



A few days ago, in a retail tool shop in a provincial 

 town, I was shown a narrow steel measure, in four 

 folding sections, the total length being one foot, which 

 was divided into 305 minor, and 30J major and 

 numbered divisions, the first two engraved thus : — 



NO. 2S1O, VOL. 112] 



1 1 meter 1 2, so that to the purchasers of such an 

 instrument 30J metres are represented as equivalent 

 to 12 inches, instead of 100 feet ! The stock included 

 the carpenter's ordinary foot-rule, divided along one 

 edge into inches and sixteenths of an inch, and the 

 other into millimetres and 30^ (centi) metres. All 

 the shopkeeper could say was that the scales were as 

 supplied by the best makers, and must therefore be 

 accepted as correct — the word meter had no other 

 meaning than that the makers used it instead of 

 saying the scale was French ! 



Four years ago, in a western London suburb, I had 

 exactly the same experience, but if my memory serves 

 me the makers were different. The shopkeeper in- 

 formed me that in his two shops (one nearer the West 

 End) he had already sold many hundreds of these 

 scales. 



From time to time the London and provincial Press 

 report meetings at which there have been discussions 

 on the great advantages of the metric system, but 

 there the matter ends — apparently it is nobody's 

 business, not even of the Board of Trade or the Board 

 of Education, to take action which would ensure 

 the circulation of correctly marked scales. All the 

 wrongly engraved ones ought to be recalled, to have 

 centi- engraved above meter. Hy. Harries. 



August 20. 



Direction of /3-rays Produced by Polarised X-rays. 



In an abstract (Nature, July 7, p. 26) of a paper 

 read recently before the Royal Society, Mr. C. T. R. 

 Wilson discusses some results on /3-ray ionisation 

 tracks which he has obtained by his cloud method. 

 Among other things he notes (i) " Partial polarisa- 

 tion of the primary beams is indicated by the direction 

 of ejection of a number of the /3-particles being in 

 one plane — that containing the direction of the 

 cathode rays in the X-ray tube," and (2) " Of the 

 ordinary long-range tracks, the majority have a 

 large forward component comparable with the lateral 

 component." 



During the past year the present writer, using a 

 beam of scattered X-rays about 90 per cent, polarised 

 (Wilson's primary beam was probably about 10 per 

 cent, polarised), has obtained stereoscopic photographs 

 of /3-ray ionisation tracks by the cloud method. These 

 photographs show that most of the /3-particles are 

 ejected in a direction nearly parallel to that of the 

 electric force of the polarised beam of X-rays. There 

 is, however, a variation on either side of this direction. 



The photographs also support Wilson's conclusion 

 that a large majority of the i3-particles have a velocity 

 component in the direction of propagation of the 

 X-rays. F. W. Bubb. 



Washington University, Saint Louis, 

 July 30. 



Proposed International Surveyjof the Sky. 



I AM informed by the director of the Office National 

 M^t^orologique de France that, with the approval of 

 Sir Napier Shaw, president of the International 

 Commission for the Study of Clouds, the dates for 

 taking the photographs of clouds have been postponed 

 by one week. Photographs will be taken at the three 

 specified hours from September 24 to October i 

 inclusive. Volunteers are much needed to help in the 

 work, and I shall be glad to send full instructions to 

 those who will send me their names. 



C. J. P. Cave. 



Stoner Hill, Petersfiield, Hants, 

 August 27. 



