between the Lines of various Spectra. 361 



with wave-lengths 4266-3, 3919*5, 3881'9, 3875*7, 3870*7, 

 3590, 3585, 3583*5, 2746*6, and 2640 are absent without any 

 apparent reason. The first five of these lines are much 

 lengthened and strengthened when carbon dioxide is the 

 atmosphere surrounding the electrodes and are unquestionablv 

 carbon lines (Phil. Trans, vol. clxxv. p. 49, 1884). The 

 spectra of magnesium, zinc, and cadmium have been photo- 

 graphed repeatedly, and that of cadmium has been observed 

 by me many thousands of times during the last twelve years, 

 yet not only have the spectra been always characteristic, but 

 they have never varied in any single particular. 



I should infer from this that molecules of the same metal 

 always act alike under the influence of a properly regulated 

 spark-discharge, in spite of the fact that the motion of the 

 spark is oscillatory. Mr. Ames further remarks : — " It is to be 

 regretted, too, that in general so little attention has been paid 

 to the physical properties of the lines, when homologous 

 groups or series were selected. It is only, I think, by means 

 of the physical similarities that correct inferences can be 

 made. For these two reasons a great many of the relations 

 given by Professor Hartley are inaccurate." 



Nothing but the physical similarities of the lines, and their 

 evident arrangement in pairs and triplets in the three spectra 

 above mentioned, led to my calculations and the discovery of 

 those relations in spark-spectra which have been recently 

 observed by Mr. Ames in the arc-spectra of zinc and cad- 

 mium. It was therefore at least seven years prior to the 

 publication of his paper that the importance of taking into 

 account the physical similarities of lines and groups of lines 

 was fully recognized. 



Mr. Ames at another point in his communication makes 

 the following statement : — " The resemblance of the spectrum 

 of magnesium to those of zinc and cadmium is much less 

 marked than the one just noted. In fact, I think nearly all 

 the points of similarity given by Professor Hartley are wrong. 

 Magnesium has, it is true, the groups of triplets ; but their 

 physical properties bear no apparent relation to those of the 

 zinc and cadmium triplets. The beautiful series of five lines 

 at wave-length 2779*9 in the magnesium spectrum surelv ha> 

 no companions in the zinc and cadmium spectra. There is, 

 however, one striking feature common to all three. This is 

 the presence of one broad reversed line. In magnesium it is 

 at wave-length 2S5'2"2 ; in zinc, at wave-length 2138*3 : in 

 cadmium, at wave-length 2288*1 ." I do not agree with Mr. 

 Ames, and think he is not justified in stating the opinion 

 which is the second sentence in the foregoing quotation. The 



