[mclennan-edwards] absorption SPECTRA OF MERCURY 



171 



in the arc spectrum of cadmium given by n = 1-5, S — m, P and 

 n = 2, p2 — m, S, i.e., they are analogous to the lines in the mercury 

 arc spectrum at A = 1849-6 A°.U. and A = 2536-72 A°.U. 



Some photographs were also taken of the spectrum of the cadmium 

 arc. In taking these the form of arc used is that shown in Fig. 7. 



Figure 7 



The apparatus consisted of a tube of fused quartz possessing 

 three arms, R, S and MN together with a receptacle L. The metal 

 to be used in the arc was placed in the receptacle, L, and two rods of 

 the same metal FE and DC were attached to two wires and these 

 latter were in turn fastened to two brass plugs A and B which were 

 sealed into the tubes R and S with mastic wax. A small sheet of 

 platinum was attached to two wires which constituted a heating 

 circuit and these were sealed with platinum wire into a glass tube 

 PQ at H and K. The open end of the glass tube was ground so as 

 to fit exactly into the end of the quartz tube MN as shown in the 

 diagram. The arms MN, R and S were each about 40 cms. long and 

 it was found with this length that when the receptacle L was strongly 

 heated with a Bunsen burner the wax joints at A and B and the ground 

 one at the end of the tube MN remained quite cool. 



In the experiments the plate G was coated with a thin layer of 

 either calcium oxide or barium oxide. When the tube was in oper- 

 ation the terminals of an auxiliary heating circuit were attached at 

 H and K, B and K were joined by a wire and the arcing voltage was 

 applied between B and A, the latter being the positive terminal. With 

 this arrangement G and D constituted a double cathode. The tube 

 was highly exhausted with a Gaede mercury pump through a glass 

 tube sealed into an opening in the brass end piece at A. 



In taking the photographs the plate G was brought to incandes- 

 cence by means of the auxiliary heating circuit; the metal in it was 

 strongly heated with the flame of a Bunsen burner so as to keep the 

 plate G surrounded with the vapour of the metal, and the collimator 



