862 Mr. G. P. Thomson on the Application of 



In other cases the intensities seem much too nearly equal. 

 There seems to be a tendency for the contrast in intensity to 

 be greater when the potential difference in the discharge is 

 small than when it is larger. 



On account o£ the change in contrast due to reproduction, 

 no attempt has been made to reproduce the evidence for the 

 variation in relative intensity of the lines mentioned above. 

 The two photographs of lithium shown on the plate were 

 chosen simply for their suitability for reproduction. 



Beryllium. 



As this element shows no well-marked spectrum in the 

 visible, it was necessary to mix it with a salt of another 

 metal in order to make the ra} T s visible. Owing to the low 

 boiling-point of beryllium iodide, it was found that, when an 

 iodide was present in the mixture, gas was given off to such 

 an extent that the pressure could not be kept down suffi- 

 ciently, even after many hours running. The chloride is 

 unstable and the stable oxychloride gave only an extremely 

 faint line. A mixture of sodium chloride and beryllium 

 fluoride gave better results and showed a well-defined line 

 at 9 taking sodium as 23. The atomic weight of beryllium 

 is generally given as 9*1, and so the possibility of an isotope 

 at 10 or 11 must be considered, either of which would 

 coincide in atomic weight with boron. To investigate the 

 possibility of this, it is desirable to get the line of 9 as strong 

 as possible in order to be able to see a line of a small fraction 

 of its strength. As the photographs so far taken were not 

 black enough, several further experiments were made. The 

 best results were obtained with an anode of sodium bromide 

 and beryllium fluoride, which gave an appreciably stronger 

 line at 9 but no sign of a line at 10 or 11. This seems to 

 indicate that a very faint trace seen before on one photo- 

 graph in the neighbourhood to 10 was either a fault in the 

 plate or due to some other cause: e. <?., HF with two charges 

 (see PI. XX VI.). 



Calcium. 



For this metal I tried a mixture of equal parts calcium 

 fluoride, lithium iodide, and graphite. This gave a remark- 

 ably strong beam of red light, due to the lithium. When 

 a photograph was taken, the lithium appeared very strongly, 

 and there were also lines at 28 due to CO, at 23 to an 

 impurity of sodium, and at 40 to calcium. The calcium line 

 gradually increased in strength during the subsequent photo- 

 graphs, but no line at 20, corresponding to calcium with a 



