586 



NA TURE 



[May 6, 1922 



and a thin strip of paraffin wax or other hydrogen 

 material is placed over the source. The number of 

 H-scintillations observed for a given absorption depends 

 only on the amount of hydrogen^ and is quite in- 

 dependent of chemical combination. This is to be 

 expected, for the forces required to set the H-nucleus 

 in rapid motion are enormous compared with the 

 weak forces involved in chemical combination. We 

 thus conclude that, for a-particles of range 7 cm., 

 no H-atoms from hydrogen in the free state or in chemi- 

 cal combination can be detected for an absorption 

 greater than 30 cm. of air. 



The oxygen which gives no scintillations is now 

 replaced by dry air. At once we observe for an 

 absorption of 30 cm. more than 100 scintillations per 

 minute when for hydrogen we did not observe one. 

 By adding mica screens we find that the scintillations 

 cease for an absorption of 40 cm. It is clear that these 

 particles, which come from nitrogen, have a greater 

 range than free H-atoms bombarded by a-rays, so 

 that the effect observed beyond 30 cm. cannot be 

 ascribed to any hydrogen impurity in the nitrogen. 



The air is now replaced by neutral oxygen, and thin 

 foils of say copper, iron, silver, gold of stopping power 

 corresponding to about 3 cm. of air are placed suc- 

 cessively over the source. Not a single H-atom can be 

 ©bserved for an absorption of 30 cm. A piece of 

 aluminium foil is substituted and at once the number 

 of scintillations jumps to more than 100 per minute. 

 Some of the scintillations are very bright, and we 

 find that some of the particles are so swift that the 

 absorption must be increased to 90 cm. before the 

 scintillations vanish. It is clear that aluminium must 

 give rise to a number of very long-range particles. 



Thus if we examine the number of scintillations 

 beyond the range of ordinary H-atoms, we are quite 

 independent of any possible contamination of hydrogen 

 in the material under examination. This is a great 

 advantage, for we need not concern ourselves about 

 the purity of the material as regards hydrogen. In 

 this way. Dr. Chadwick and I have examined a large 

 number of elements to test whether they emit particles 

 of range more than 32 cm. When the element was 

 not available, a compound of the element with an 

 " inactive " element like oxygen was used. The 

 material in the form of a fine powder is dusted on a 

 thin gold foil, an adhesive film being used so that the 

 average absorption of the material corresponded with' 



3-4 cm. of air, and was then exposed to the source 

 of rays. With the exception of helium, neon and 

 argon, all the elements up to atomic weight 40 have 

 been tested. No element of atomic weight greater 

 than phosphorus, 31, was found to give any effect, 

 although it should be said that only a few of the 

 elements of higher atomic weight have so far been 

 examined. 



A list of the elements examined in this way, from 

 lithium to sulphur inclusive, is given in the following 

 table. The second column gives the number of 

 scintillations per minute per milligram activity of the 

 source, namely, radium-C, for an absorption of 32 cm. 

 of air. These numbers afford only a rough comparison 

 of the effects given by different elements, for the con- 

 ditions of the experiment, for example, the thickness 

 and distribution of the film of material, varied from 

 element to element. The fourth column gives the 

 approximate range of the particles. 



Zr-^J^^tl Maximum range 

 Element. Material. ^l^^.^^^J^^' of particles i« 



Lithium 



Glucinum 



Boron . 



Carbon 



Nitrogen 



Oxygen . 



Fluorine 



Sodium . 



Magnesium 



Aluminium 



Silicon . 



Li^O 



GIO 



B 



CO2 



Air 



O, 



CaFj 



Na.,0 



MgO 



Al.AlaOg 



Si 



for the micro 

 scope used. 



0-4 



0-2 



0-7 



ca. 45 



over 40 

 ca. 42 



90 

 ca. 65 



Phosphorus . . P (red) 



Sulphur. . . S.SOj — — 



In addition to these, the following elements of higher 

 atomic weight were examined : chlorine as MgClg ; 

 potassium as KCl ; calcium as CaO ; titanium as 

 TigOg ; manganese as MnO, ; iron, copper, tin, silver, 

 and gold in the form of metal foils. In no case were 

 any particles observed of range greater than 32 cm. 

 of air. The question whether any of these elements 

 give particles of range less than 32 cm. has not been 

 examined. 



It will be seen that the elements which give scintilla- 

 tions for an absorption of 32 cm. are boron, nitrogen, 

 fluorine, sodium, aluminium, and phosphorus. The 

 numbers for boron and sodium were distinctly less than 

 for the other elements. 



(To he continued^ 



The Royal Academy. 



THE representative of Nature looking for points 

 of scientific interest amongst the fourteen hun- 

 dred or so annual exhibits at the Royal Academy may be 

 excused if he sometimes feels depressed and is reminded 

 of the proverbial searcher after a needle in a haystack, 

 in so few of the pictures do objects having any direct 

 connection with science appear. It has in past years 

 been remarked that purely scientific work does not yet 

 appeal to the Academy artist, and it is necessary to 

 turn for points of interest to nature scenes such as 

 may be found in pictures of sea, sky, snow, and country 

 life.' 



NO. 2740, VOL. 109] 



In snow scenes J. Farquharson frequently has suc- 

 cessful effects and " The Edge of the Forest " (239) 

 this year is quite up to his standard. Another good 

 snow effect, in this case associated with water, is con- 

 tained in " A Yorkshire Bridge in Winter," by F. E. 

 Home (884). A successful landscape somewhat of the 

 Leader type, the central feature of which is a group of 

 pine trees, is shown by Frank Walton in 591. It is a 

 pity that there is only this one example of his work 

 in the exhibition. The title which A. J. Munnings has 

 chosen for No. iii does not lead one to expect a 

 landscape, but the setting of the portraits which give 



