430 



NATURE 



[October 6, 1910 



from 3.30 p.m. until 4.55 p.m. (French time), and on the 

 north and south at a considerable distance through the 

 mist there appeared two arcs of a circular halo, showing 

 on the north colours from red to green, and on the south 

 colours from red to j'ellow, only the other spectrum colours 

 were absent. 



About 4.30 p.m. a dark grey cloud hid the whole pheno- 

 menon, but about 4.45 p.m. until the end of the display 

 the grey cloud passed, and the sight was both beautiful 

 and brilliant. 



The colour-bands were very wide ; the green was the 

 widest and palest, and I only observed it on the northern 

 arc. R. Asiii.sGTON Bullen. 



Bordeaux, September 28. 



RADIUM STANDARDS AND NOMENCLATURE. 



'PHE International Congress of Radiology and 

 -*• Electricity, held at Brussels, September 12th to 

 15th, afforded an excellent opportunity of discussing 

 several important questions of general interest to 

 workers in radio-activity. Tfie need of a definite 

 radium standard, in which all results should be ex- 

 pressed, has been growing more acute with the 

 increase of accuracy of radio-active measurements. 

 At the present time, scientific results are expressed in 

 many cases in terms of arbitrary radium standards kept 

 in each laboratory, and it has been diflicult to be certain 

 of the accuracy or relative value of such standards. 

 Mr. C. E. S. Phillips several ^'ears ago pointed out 

 to the Rontgen Society the desirabilitv of adopting 

 a fixed radium standard, and arranged for the pre- 

 paration of several small radium standards which 

 were compared with the working standard adopted 

 by Rutherford and Boltwood. Dupli|Cates of the 

 latter standard have been used for several vears bv 

 a number of English, .\nierican, and Continental 

 workers. 



At the opening meeting, Prof. Rutherford read a 

 report on the desirability of establishing an inter- 

 national radium standard. He pointed out that he 

 had compared by the y ray method the radium 

 standards used by several important European labora- 

 tories, and had found that there was a considerable 

 difference amongst them, amounting in some cases to 

 20 per cent. It is now possible to measure with con- 

 siderable precision a number of magnitudes connected 

 with radium ; for example, the volume of the emana- 

 tion, the heating effect, the rate of production of 

 helium, and the rate of emission of a and /3 particles. 

 The values of each of these quantities is dependent 

 on the accuracy of the radium standard in which the 

 results are expressed. For the comparison of results 

 obtained by workers in different laboratories, it is 

 necessary that thev should all be expressed in terms 

 of the same standard. For example, at the present 

 time it is not possible to compare the results obtained 

 on the heat emission of radium by various observers 

 until the radium standards employed have been 

 accurately compared. When once a standard has 

 been adopted, it is relatively a simple matter to deter- 

 mine the radium contents of substandards bv the y 

 ray method or modification of it, without opening 

 the tube containing the radium. 



A special international committee was appointed to 

 report to the congress on the best means to be 

 adopted to fix an international radium standard. 

 This committee comprises the following workers in 

 radio-activity representative of a number of countries : 

 Mme. Curie, Debierne, Rutherford, Soddv, Hahn, 

 Geitel, Meyer, Schweidler, Eve, and Boltwood. No 

 doubt representatives of other countries who are pre- 

 pared to assist in the work will be added later. This 



NO. 2136, VOL. 84] 



committee reported to the congress at its final meet- 

 ing and their suggestions were adopted by the con- 

 gress. As a member of the committee, Mme. Curie 

 agreed to prepare a radium standard containing about 

 20 milligrammes of radium enclosed in a suitable 

 sealed tube. This standard is somewhat large, but 

 the amount was considered necessary on account of 

 the difficulty of weighing small quantities of radium 

 salt with tfie requisite accuracy. The thanks of all 

 workers in this subject are due to Mme. Curie in 

 undertaking the full responsibility of preparation of 

 a standard, and for the large expenditure of time and 

 labour its preparation will involve. The committee 

 agreed to reimburse Mme. Curie for the cost of the 

 radium and its preparation, after which the standard 

 becomes the property, and is under the control, of 

 the international committee. It was suggested that 

 the standard should be suitably preserved in Paris. 

 The initial cost of preparation of this standard will 

 be somewhat heavy (about 500!.), but it is 

 hoped that scientific societies and Governments 

 of various countries will assist in defraying the 

 expenses. 



.\s soon as the primary standard has been prepared, 

 it is proposed to approach through the committee 

 the various national laboratories to ask them to 

 acquire a radium standard accuratelv determined in 

 terms of the primary standard. In this way it was 

 thought that any Government interested in the ques- 

 tion could acquire an accurate radium standard to be 

 used as a basis for standardisation of quantities of 

 radium in use in scientific laboratories, or to be sold 

 commercially. As the primary standard is somewhat 

 large for use in ordinary laboratories, the committee 

 propose to investigate the question of the best method 

 of comparing^ accuratelv in terms of the primary 

 standard smaller substandards containing one or two 

 milligrams of radium. 



The committee also has under consideration the 

 question of the preparation of very small substandards 

 to be used for the determination of minute quantities 

 of radium and of radium emanation. It is proposed 

 that special investigations be made by the committee 

 to determine the most suitable method of preparation 

 and preservation of such standards. There is at pre- 

 sent some uncertainty of how far radium solutions 

 are affected by time in consequence of the tendency 

 of radium to be precipitated out of the solution. No 

 doubt before long it should be possible to secure 

 accurate standard solutions to distribute amongst 

 scientific workers. 



In the course of the congress it was suggested that 

 the name Curie, in honour of the late Prof. Curie, 

 should, if possible, be employed for a quantity of 

 radium or of the emanation. This matter was left 

 for the consideration of the standards coinmittee; the 

 latter suggested that the name Curie should be used 

 as a new unit to express the quantity or mass of 

 radium emanation in equilibrium with one gram of 

 radium (element). For example, the amount of 

 emanation in equilibrium with one milligram of 

 radium would be called i/iooo Curie or one millicurie. 

 The adoption of this unit will avoid much circumlocu- 

 tion, and will prove useful since the radium emana- 

 tion is now sn widely used in all kinds of experi- 

 ments. 



The committee has under consideration the ques- 

 tion whether special names should be given to a very 

 small quantity of radium, and also to the emanation 

 in equilibrium with it. For example, the quantity 

 Ki-'- gram radium seems a natural unit for expres- 

 sion of the radium content of rocks and soils. At 

 the same time, the Large amount of investigation on 



