DR. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 403 



The former gives with = 4, d = 0, W = 200'3'3, q = 361*423 1*61, and the 



latter 3G2'09 with a large uncertainty owing to 127 "2 



D,,(2) gives q = 361*501*28. The large possible error is due to the uncertainty 

 in the atomic weight of Hg. Adopted value, 36T50r33. 



Al. In Al also there is a large possible variation due to the uncertain atomic 

 weight. W = 27'10-05. From v , q = 361*88 1*5. There is an order difference 

 117 A between m = 2 and 3 for the D series (see Table II). This gives 

 q = 361*871 + 1-30. The denominator of D (2) gives q = 3617771'92, or if there 

 is a satellite D 12 (2) gives 3617171'92. Adopted value, 361'8711'33. 



In. From , with = 1, dv = '25, W = 114*8*5, q = 361*9473*31. 



From D,,(2), if 22A-16<S, q = 361-871, but the theory is uncertain. Adopted 

 value, 36 1'947 3'33. 



TL From v , q = 362'00'20. From D,,(2), q = 361 '913 '6, but with somewhat 

 doubtful theory. Value adopted, 362'00 '20. 



These values for the 17 elements weighted according to the possible errors now 

 give q = 361*890. This is the same as our first approximate value, but its probable 

 error is much less. If the determination of the value depended only on questions of 

 errors of calculation and of observation in spectral and atomic weight data, the above 

 number would probably be extremely close to the actual one. .It must lie remem- 

 bered, however, that our theory of the constitution is not yet complete. For 

 instance, in [II.] it was seen that the supposition that N was not constant for the 

 ^-sequence, but that the value for the first line was slightly larger explained the 

 introduction of a term in the denominator. A similar explanation might explain the 

 fact that the value of q appears to deviate from the mean by alwnt the same amount 

 in each group of elements, and if it were justified, the value of q calculated as above 

 would receive a slight modification. I believe it will be found ultimately that the 

 true value will lie within the limits given by 361'890'05 or 90*4725*0125. 



If the existence of the oun as a definite proportion of the (atomic weight) 3 be 

 considered as established, the best and most direct method of determining the value 

 of the factor q would be from the discussion of an element in which the spectroscopic 

 data are good and in which the atomic weight has been determined with great 

 accuracy. For this purpose we naturally turn to silver. Regarded as the ultimate 

 standard of atomic weight determinations, no error in the atomic weight enters the 

 value of q is determined in terms of W = = 107*88. Moreover its separation is large, 

 so that any error of measurement is a small fraction of its total value, and in addition 

 the actual error is extremely small. It is therefore tantalising to find that the lines, 

 D (2) excepted, are not susceptible of such exact measures as in many others, that 

 the typical series are not well developed, and that in fact there may be a doubt 

 whether the lines generally accepted as the P, S, D series follow laws altogether 

 analogous to those in other groups. In KAYSER and RUNGE'S measures four lines are 

 assigned to D a (2-5) and three to D, (2-4), the possible errors for D M being much less 



3 F 2 



