452 Mr. H. W. L. Absalom on the Ultra-violet 



like it in this respect also. The colour is, however, super- 

 ficial instead o£ being diffused through the mass. 



6. This pale violet variety of Chili saltpetre, occurring 

 naturally, can be decolorized by heat, like the blue rock- 

 salt. The colour may then be restored by cathode-ray 

 bombardment. The same colour can be in some measure 

 imparted to a piece of the ordinary white Chili saltpetre by 

 bombardment, but in this case the coloration is not uniform 

 but banded, suggesting that its development depends on some 

 impurity. 



7. The specimen was originally greenish in colour, but the 

 violet colour imparted by bombardment was very much deeper. 



II. Solutions of Metals in Liquid Ammonia. 



It has been known for some years that the metals sodium, 

 potassium, lithium, rubidium, caesium, barium, strontium 

 (and magnesium to a less extent), react with liquid ammonia, 

 producing fine blue colorations. The blue colour is not 

 permanent ; its duration varies with the metal and with the 

 purity and dryness of the metal and the ammonia. It is 

 believed that the metal dissolves in the ammonia forming the 

 blue colloidal solution, and that then a reaction occurs 

 resulting in the formation of an amide with the liberation 

 of hydrogen — the disappearance of the colour marking the 

 final conversion of the metal in solution into amide. The 

 stability of solutions increases in the order potassium, 

 calcium, lithium, sodium. Considerable attention has been 

 devoted to the reaction which takes place and to the physico- 

 chemical properties of the solutions. (Moissan, Comptes 

 Eendus, 1898 ; Krauss, J. Am. Chem. Soc. 1907, 1908, 1914.) 



Recently Argo and Gibson (Phys. Rev. Feb. 1916) have 

 studied the absorption of the blue solutions of sodium, 

 potassium, and magnesium by means of the spectro-photo- 

 meter. There appears to be no record of observations 

 carried out in the ultra-violet region. 



In the present instance attempts were made initially to 

 obtain the blue solutions of sodium in liquid ammonia. Some 

 *880 solution of liquid ammonia was heated and the gaseous 

 ammonia led through a long tube packed with soda-lime, 

 and thence by a very thin drawn-out tube into a silica tube 

 surrounded by a freezing-mixture of solid carbon dioxide 

 and methylated spirits. When sufficient liquid ammonia had 

 collected, a small piece of sodium, as free as possible from 

 oxide and moisture, was introduced and the silica tube then 

 sealed off. As soon as the tube was removed from the 

 freezing-mixture the metal seemed to swell somewhat, and 

 the contents of the tube soon assumed a deep blue clotted 

 appearance. Similar results were obtained with potassium 



