208 



Mr. W. Crookes. 



[May 19, 



strong ammoniacal solution filtered from the precipitated alumina 

 was now boiled. The alumina which the excess of ammonia had dis- 

 solved was thereby precipitated. This was filtered off, ignited, and 

 tested in the molecular discharge. It gave no red light whatever, 

 but phosphoresced of a pale green, and on examination with a prism 

 the light showed no lines, but only a concentration of light in the 

 green. 



Two earthen crucibles were tightly packed, the one with sul- 

 phate of alumina, the other with acetate of alumina. They were 

 then exposed, side by side, to the most intense heat of a wind furnace 

 — a heat little short of the melting-point of platinum.* The resulting 

 aluminas were then tested in the molecular stream. 



The alumina from the sulphate gave the crimson glow and the 

 spectrum line. 



The alumina from the acetate gave no red glow or line, but a pale 

 green phosphorescence. 



In my examination of rubies, many pounds of which have passed 

 through my apparatus, I have been fortunate enough to meet with one 

 solitary crystal, not to the eye different from others, which emits a 

 green light when tested in the molecular stream. All others act as I 

 may call normally. The spectrum of this green-glowing crystal shows, 

 however, a trace of the red line, and on keeping the discharge acting 

 on it for a few minutes the green phosphorescence grows fainter and a 

 red tino-e is developed, the spectrum line in the red becoming more 

 distinct. 



Besides the ruby, other native forms of crystallised alumina phos- 

 phoresce. Thus corundum glows with a pink colour. The sapphire 

 appears to be made up of the red-glow and green-glow alumina. Some 

 fine crystals of sapphire shine with alternate bands of red and green, 

 arranged in layers perpendicular to the axis. Unfortunately it is im- 

 possible to prepare a tube for exhibition containing this variety of 

 sapphire, as it is constantly evolving gas from the numerous fissures 

 and cavities which abound in this mineral. 



The red glow of alumina is chiefly characteristic of this earth in a 

 free state. Few of its compounds, except spinel (aluminate of mag- 

 nesium), either natural or artificial, show it in any marked degree. 

 All the artificially crystallised aluminas give a strong red glow and 

 spectrum line. An artificially crystallised aluminium and barium 

 fluoride phosphoresces with a blue colour, but shows the red alumina 

 line in the spectrum. Spinel glows red, and gives the red line almost 

 as strong as the ruby. 



The mineral spodumene (an aluminium and lithium silicate) phos- 

 phoresces very brilliantly with a rich golden-yellow colour, but shows 

 no spectrum line, only a strong concentration of light in the orange 



* This operation was kindly performed for me by Messrs. Johnson and Matthev. 



