July 19, 1906] 



NA TURE 



271 



The Action of "u" Radiation on Diamonds. 

 Irir, action of the " o rays " on diamonds is of con- 

 -idorable interest, for while the fluorescence caused by the 

 tJ ;ind 7 radiation from radium is probably similar to that 

 caused by the X-rays, the appearance of a diamond made 

 luminous by the impact of a stream of " o " particles 

 sujigpsts some considerations as to the possible action of 

 " a " radiation on fluorescent crystals in general. The 

 fluorescence of a fairly large stone (cut and polished) when 

 viewed with a suitable lens shows practically nothing of 

 the spinthariscopic action, although the stone may be 

 lirightly luminous. Instead of the familiar scintillations, 

 the whole crystal, or at least tha whole surface exposed to 

 the rays, appears to give out a steady bluish-while light. 



The thought at once occurs to one that this seemingly 

 continuous flow may be the collective etTect of the very 

 numerous scintillations produced by a too intense stream 

 of "a" rays; such an action is well shown on a zinc 

 sulphide screen when there is an e.xcessive quantity of 

 radium used. That this action is really the collective 

 fluorescence of scintillations is at once evident by removing 

 the fragment of radium to a greater distance, reducing 

 the quantity used, or increasing the magnifying power 

 employed to view the screen. In the case of a diamond, 

 however, this does not appear to be so. The use of a 

 higher power to view the fluorescence still shows a seem- 

 inglv steady glow, and the increase of the distance between 

 the radium and the stone merely causes the light to be- 

 come gradually fainter, while still preserving its .steady 

 character. Yet it is certain that the diamond responds 

 readily to the " a " particles, and also that, as the quantity 

 of radium is so very small, the action of the /3 and •y 

 r.idiation is quite negligible. It is, of course, well known 

 that when an " a " particle strikes a fluorescent screen, the 

 point of impact becomes the centre of a luminous area, 

 which is simply enormous in extent when compared with 

 the size of the atomic projectile which causes it. 



The following may perhaps be suggested as a possible 

 explanation of this action : — When an " a " particle strikes 

 a homogeneous fluorescent crystal (say a diamond), the 

 energy which excites the fluorescence finds equal conduction 

 in .all directions. The fluorescence caused thus tends to 

 fill the whole volume of the crystal. If there are many 

 such atomic projectiles incident on the same crystal, they 

 are all tending to do the same thing, and consequently 

 their spheres of influence mingle with one another. .As 

 such spheres of fluorescence find equal conduction of all 

 sides, they extend indefinitely within the limits of the 

 crystal in , question. .As fluorescence is apparently a 

 molecular property, and probably electrical in its nature, 

 it is not diflicult to imagine that such may be the case. 

 A still pond, into which a handful of gravel is scattered, 

 may present an approximate analogy. The ring-waves 

 (neglecting the time they take to travel) would mingle with 

 one another, and yet each one might be said separately to 

 occupy the whole area of the pond. In the case of a zinc 

 sulphide screen, or one coated with minute fragments of 

 diamond crystals, the energy received by one crystal or 

 fragment of a crystal is confined exclusively to the volume 

 of that fragment. Moreover, it is impossible for an " a " 

 particle to strike more than one crystalline fragment at a 

 time, for it is a body of atomic dimensions compared with 

 which the most minute fragment of the fluorescent com- 

 pound would be enormous. 



The whole of the available energy is thus confined to 

 the limits of the fragment struck, and is not, apparently, 

 extended to the neighbouring crystals, which are only in 

 loose and indifferent contact with it. When such a crystal- 

 line fragment is of a size which is comfortably visible 

 with the aid of a lens magnifying about 20 diameters to 

 30 diameters, the resulting fluorescence will be visible as 

 a scintillation. To diminish the size of the crystals beyond 

 a certain point in order to increase the brightness of the 

 scintillations is apparently not advantageous, as it requires 

 the higher powers of a compound microscope to render the 

 areas properly visible, and there would be a corresponding 

 loss of light.' 



On the analogy of the pond given above, the spinthari- 

 scopic effect may be compared to throwing a handful of 



NO. 191 6. VOL. 74] 



gravel into a collection of small puddles. The disturbance 

 caused in each would be strictly confined to its own area, 

 and would be correspondingly intense within that area. 

 With a given stream of " a " radiation, a small stone 

 appears to give a very slight scintillating effect which is 

 not seen in a larger stone, except at the edges and angles 

 of the facets, where the area of fluorescence is abruptly 

 terminated, and even here it is very faint. The above 

 remarks would, of course, only apply to perfect crystals. 

 If a crystal is full of flaws and imperfections, the areas 

 or spheres of fluorescence would not find easy conduction 

 across the faults, and would therefore become localised in 

 their action. It may be noted that a lump of willemite 

 (natural), which is of a semi-crystalline character, does 

 show scintillations, though very imperfectly, while the 

 powdered mineral answers much better. This may be ex- 

 plained on the assumption that the areas of conduction are 

 restricted to the size of the particles. (". W. R. 



June 20. 



The Day of \A/eek for any Date. 



The following method for finding the day of the week 

 for anv given date (new or Gregorian style) may interest 

 your readers. We assign a number for each month in 

 accordance with the oU style, beginning with March, so 

 that the last four months are numbered according to their 

 Latin names, as follows : — 



January, o; February or March, i ; April, 2; May, 3; 

 June, 4; July, 5; August, 6; September, 7; October, 8 ; 

 November, 9; December, 10; next January, 11; next 

 February, 12. 



For a Leap Year, January and February must count as 

 II and 12 respectively in the preceding year. 



It is only in dealing with the month-number that any- 

 thing not straightforward and obvious is involved. 



The rule then runs as follows : — 



A. For the century : divide by 4, and calculate 5 times 

 the remainder. 



B. For the year : add to the number the quotient obtained 

 from divisor 4. 



C. For the month : multiply by 4, and negate the units 

 digit {i.e. subtract instead of adding it). 



D. For the day : retain the number unchanged. 



Then add together the results A, B, C, D (casting out 

 sevens, of course, as you proceed), and the result gives 

 the required day of the week. 



The rule holds without modification, not excepting such 

 years as 1600, 2000, &c., as well as 1700, 1800, iqoo, &c. 



Examples — 1906, September 19. 



A. For century : 3 :< 5 = 1 5 ^ ' 



B. Yoxyear: 06+1=7 ^^ 



C. Ya\- month : 4 >: 7 gives 20-8=12 ^5 



D. Vox day: 19 ^S 



A 4- B -I- C ^ D = 11—4, i.e. Wednesday. 

 1815, June 18 (Battle of Waterloo). 



A. Yo\ century : 2x5=10 ==3 



B. For.iwr: 15-1-3 = 18 =4 



C. ¥ox month : 4 x 4 gives 10- 6= 4 ... ... ^4- 



D. Vox day: iS =4 



A-l-B + C + D = I5=I,!.f. Sunday. 

 1784,* January 12 (Pitt's appearance as Prime Minister). 



A. Yot etnlury : 1x5 = 5 s^5 



B. Vox year: 83" -f 20 =103 =5 



C. Fox month: 1 1' x 4 gives 40- 4 = 36 ^I 



D. Vox day: 12 ... =5 



* Leap year. 



A-)-B4-C + D=i6=2, i.e. Monday. 



W. E. Johnson. 

 King's College, Cambridge, July 11. 



