Transparency of Certain Coloured Media. 



453 



i 



and calcium. In all cases the colour persisted but for a 

 few days, and sometimes disappeared very rapidly if only a 

 small piece of the metal had been used. A white preci- 

 pitate remained. The almost opaque nature of the contents 

 of the tube rendered the photography of absorption spectra 

 difficult. By supporting the tube horizontally and rotating 

 steadily it was hoped to obtain a sufficiently transparent and 

 uniform film, but the liquid did not 

 adhere sufficiently well to the walls of 

 the tube. Ultimately attempts to obtain 

 photographs were made as the blue 

 colour became less dense. The tube was 

 fitted tightly into a deep cork. A hole 

 at right angles to the former allowed 

 light to traverse the liquid. The results 

 with a calcium solution were the most 

 satisfactory, but even these were not 

 altogether consistent. 



More recently solutions of magne- 

 sium have been obtained by Cottrell 

 (J. Phys. Chem. 1914.) He found that 

 with freshly scraped metal and dry 

 ammonia blue solutions resulted which 

 were of considerable permanence, per- 

 sisting in one case for several years. 



The apparatus in the present case is 

 shown. 



CE was of silica, the remainder of 

 glass. A few small pieces of freshly 

 scraped magnesium ribbon were intro- 

 duced into E, which had been pre- 

 viously cleaned and dried. A con- 

 striction was made at D and the silica 

 tube then joined to the glass by means 

 of a sealing-wax joint at C. F was 

 connected to a Gaede pump, and when 

 the pressure was very low E was heated 

 to disengage any gases. (Cottrell, ibid.) The purnp was then 

 disconnected, B immersed in liquid air and ammonia collected 

 as a solid in the way described above. A few pieces of 

 sodium were dropped into B to account for any water which 

 may have passed over with the ammonia. The pump was again 

 connected and the tube sealed off at A. B was removed from 

 the liquid air, and when the ammonia had liquefied E was 

 immersed in the liquid air and the ammonia distilled over. 

 The liquid contents ofE were afterwards washed back into B. 

 and then distilled over to E again. The silica was sealed off 

 Phil. Mag. S. 6. Vol. 33. No. 197. May 1917. 2 I 



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