436 Dr. W. Huggins and Mrs. Huggins. Relative Behaviour 



4. Two sets of similar experiments were made ; in one case with 

 electrodes of platinum, and in the other with electrodes of iron. In 

 the latter case the chief lines of iron were present with those of 

 calcium. 



Method adopted for reducing the Density of the Calcium Vapour. 



(a) The spark was taken between electrodes of metallic calcium. 

 Tt was assumed, as was confirmed by the appearance of the spark, 

 that with metallic calcium for electrodes, the largest amount of 

 calcium vapour would be present. 



(6) The tips of the electrodes, iron or platinum, were slightly 

 moistened with a strong solution of calcic chloride. 



(c) The tips were slightly washed with pure water. 



(d) The tips were again washed with pure water. 



(e) The tips were then slightly moistened with a very weak solution, 

 made by adding a drop of the strong solution to 2 ounces of water. 



Our expectations were completely confirmed. Under the condi- 

 tions (a) of greatest density of the calcium vapour, when metallic 

 calcium was employed, the blue line was as strong and possessed the 

 same diffuse character as H and K. 



As the density of calcium was reduced, the lines were not found to 

 be equally enfeebled, but, on the contrary, the blue line and the greater 

 number of the lines were increasingly reduced in intensity relatively 

 to H and K, until at last with the twice washed electrodes (d) the 

 spectrum was simplified to the condition usually existing in the pro- 

 minences, in which H and K only are present. 



We now proceed to a more precise statement of the changes of 

 relative intensity as they are presented in the photographs which 

 accompany this paper. 



Description of the Photographs on the Plate. (Plate 4.) 



A. Photograph of the spark when both electrodes consist of 

 metallic calcium. Here we have present doubtless the largest 

 amount and greatest density of calcium vapour. The winged 

 character of EL and K, of the blue line, and of the pair more refran- 

 gible than H and K, is well seen, showing that this appearance 

 comes out when the gas is dense. If the greater extension of the 

 wings of H is allowed for, and the line H carefully distinguished 

 from the fine lines close to it, it will be seen to possess very nearly 

 the same strength, both as regards width and length, as the blue line 

 at 4226*9. The strength of this blue line under this condition of 

 density is about the same as that of the line at 3737, and rather 

 greater than the line beyond at 3706. 



