516 Prof. E. F. Herroun on the Electromotive 



spectrum of sodium, and that the same relation prevails be- 

 tween the two series of triple lines in the spectra of Zn, Cd, 

 and Hg. This is suggested by the circumstance that the line 

 which we now know to be the first term of Series S stands in 

 a position in all these spectra which appears to be related in 

 the same way to the positions in them of the lines of Series D. 

 But what the connexion is we do not yet know. 



Description of the Diagrams. 



A sketch on a very small scale of the primary curves of the 

 three sodium series is given in Plate VII. fig. 4, and a sketch 

 of their derived curves is given in fig. 5. In both these figures, 

 as in the diagrams of the corresponding hydrogen curves in 

 Plate VI., the horizontal lines represent the oscillation- 

 frequencies of the successive lines of each series, when 

 measured from the vertical line to the curve belonging to 

 that series. A small circle is placed round those dots that 

 correspond to lines that have not yet been observed, and the 

 cross on the upper line between —500 and —600 indicates 

 the distance to which Professor Hartley has succeeded in 

 photographing in the ultra-violet. 



To judge what the approximation is that has been obtained, 

 imagine each of the diagrams enlarged, until the vertical line 

 becomes ten metres long. Each of the diagrams would then 

 occupy the side of a large house. Even on this immense 

 scale the greatest deviation of the observed ends of the 

 horizontal lines from the curve would be less than six 

 millimetres in the case of Series S, would be under three 

 millimetres in Series P, and would be only a fraction of a 

 millimetre in Series D. Although these deviations are very 

 small, modern spectroscopic work is carried out with such 

 accuracy that they may not be attributed to errors of observa- 

 tion, and, accordingly, we are justified in drawing the first 

 and second of the inferences on p. 514. 



LIX. A Note on the Electromotive Forces of Gold and of 

 Platinum Cells. By E. F. Herroun, Professor of Natural 

 Philosophy in Queen's College, London*. 



IN nearly all modern text-books of Physics the metal pla- 

 tinum is placed after gold in Volta's Electropositive 

 Series. This no doubt is partly owing to the well-known 

 fact that gold is attacked by chlorine or nitrohydrochloric 

 acid more readily than platinum, and it might therefore be 



* Communicated by the Physical Society : read March 25, 1892. 



