Page:EB1922 - Volume 30.djvu/669

Rh T. W. Richards and others carried out similar observations with leads separated from uranium minerals; and from these obtained the low value of 206-05. I n another case, Sir J. J. Thomson was led by an entirely special method to conclude that neon was a mixture of two gases; Mr. Aston then succeeded in separating the gas, by diffusion, into two portions differing slightly in den- sity (see GASES, ELECTRICAL PROPERTIES OF).

Sir Joseph Thomson's method involved the projection at a photographic plate of molecules carrying a positive charge of electricity, moving with great velocity. Molecules carrying the same charge but differing in mass produced tracks on the plate more or less apart. Mr. Aston developed the method: subjecting the charged particles to the controlling influence of an electric as well as of a magnetic field, he was able so to focus the rays upon the sensitive plate that they produce sharply defined spectra; from the position of the lines it is possible to deduce the masses of the exciting particles. His results are more than remarkable. In the case of chlorine, the chief line is in a position exactly cor- responding to that of a molecule of mass 35, a line of less intensi- ty appearing at 37 ; the hitherto received weight of chlorine being practically 35-5, we have to suppose that the gas is a mixture of two kinds of molecule in the proportion of about 3:1. Bromine (79-92), strange to say, appears to be a mixture in about equal proportions of molecules relatively of mass 79 and 81. Fluorine and iodine, however, behave as simple species.

Atomic Numbers. Before- discussing the bearing of these as- tounding developments, it is necessary to consider another ad- vance, that made by Moseley whose death in the World War was one of the most irreparable of British losses in the discovery of a method of determining the order of succession of what must now be spoken of as atomic species.

2. He 4 3. Li 6.94

(7-6) 10. Ne 20-2 11. Na 23

(20, 22) o

18. A 39-88 19. K&-I (40, 36) (39, 40

The X-rays are regarded as vibrations set up by the impact of the electrons upon material surfaces, the character of the rays being determined by the nature of the material which is bom- barded. Moseley's method involved the study of the X-ray spectra of the elements; these he found were characterized by an orderly progression from element to element, so that it was possible from the spectra to arrange them in true order and even to foresee gaps. The spectra are simple and the relationship be- tween successive terms is unmistakable. The numbers indic- ative of the place of an element in the successional series are spoken of as atomic numbers. The unfilled gaps seem to be few. We have to recognize 92 species of elements; of these only five are missing numbers 43, 61, 75, 85, 87.

These results are a complete vindication of the policy long followed by chemists of classifying the elements in accordance with the periodic law of Mendeleeff. Tellurium, it had always been insisted, must be placed in the oxygen-sulphur series, in ad- vance of iodine. The " number " assigned to tellurium is 52, which places it in advance of iodine (53), although the accepted atomic weights are i275and 126-92. Now that iodine is regarded as whole, it may safely be predicted that tellurium is a mixture of homologues; an infra-tellurium has yet to be discovered. In like manner, cobalt has always been ranked before nickel, al- though the atomic weights were against this order; the atomic numbers they have received (17 and 18) are in accordance with this view. Recently Mr. Aston has obtained evidence that nickel has two constituents, one of mass 68, the other of mass 70; the intensity of the spectral lines are approximately as 2:1, in accord- ance with the atomic mass (68-68) hitherto assigned to nickel.

Assuming Moseley's generalization to be correct and that our knowledge of elementary species is nearly complete, it is possible

4.

Be 9-1

5.

Genetic Table of Elements 1. Hydrogen 1.008 o B 10-9 6. C 12

(II, 10)

o

12.

Mg 24-32

13.

Al

27-1

14.

Si 28-3

(24, 25, 26)

(28, 29)

20.

Ca 40-07

21.

Sc

44-1

22.

Ti 48-1

7. N 14-01

o 15. P 31-04

o 23. V 51-06

8. Oi6

o 16. 832-06

o 24. Cr 52

9.

29. Cu 63-57 30. Qu 65-37 31. Ga 69-9 32. Ge 72-5

36. Kr 82-92 37. Rb 85-45

(78, 80, 82, 83, (85, 87) 84, 86)

33. As 74-96 34. Se 79-2

o 38. Sr 87-83 39. Yt 88-7 40. Zr 90-6 41. Nb 93-5 42. Mo 96

Fl 9

17. Cl 35-46

(35, 37)

25. Mn 54-93

26. Fe 55-85

27. 0058-97

28. Ni 58-68

(58, 60)

35. Br 79-92

(79. 81) 43.

47. Ag 107-88 48. Cd 1 12-4 49. In 114.8 50. Sn 118-7 51. Sb 120-2 52. Te 127-5

54. Xe 130-2 55. Cs 132-81 (129, 132, 131, o

134, 136)

56. Ba 137-37

57.

La

139

58.

Ce

140-25

59.

Pr

140-6

60.

Nd

144-3

62i

Sa

150-4

63.

Eu

152

64.

Gd

157-3

65.

Tb

159-2

66.

Dy

162-5

67.

Ho

163-6

68.

Er

167-7

69.

Tm 1 68- 5

70.

Yb

173-5

71.

Lu

175

44. Ru 101-7

45. Rh 102-9

46. Pd 106-7

53. I 126-92 o

72. Kt 73. Ta 181-5 74. W 184

79. Au 197-2

80. Hg 200-6 81. Tl 204 82.

(6) 197 to 204

83. Bi 208

75.

76. Os 190-9

77. Ir 193-1

78. Pt 195-2

84. Polonium 85.

86. Nt 222 87.

(Th Em 220 Ac Em 2I8 1 )

88. Ra 226

(Th X 224

Ac X 222)

Pb 207-2

206, 208 (various radio (various radio (various radio (various radio

elements) elements) elements) elements)

89. Ac 226 90. Th 232-15 91. 92. U 238-2

(Ms Th II 228) (various radio UX2 (U II 234)

elements)