206 



Mr. W. Crookes. 



[May 19, 



determination of the intensities of the spot and flame lines, and the 

 heights in the solar atmosphere at which they are visible at the 

 moment of disappearance and reappearance of the sun. The spectrum 

 of the lower parts of the corona itself should also be observed, with a 

 proper amount of dispersion with instruments affording a great 

 quantity of light. 



I stated some time ago to the Royal Society that the iron lines 

 visible in the arc but not in the spark, in all probability owe their 

 existence among the Fraunhofer lines to an absorption going on in 

 the cooler levels, above the flame and spot regions. A look at the map 

 will show how important it will be to adjust a spectroscope on the 

 lines shown in the figure, and contrast their behaviour with the other 

 twelve adjacent arc lines which are prominent neither in spark nor 

 spot, nor flame. It is to be feared that any observer who attempts 

 to do more than such a restricted piece of work as this will doubt his 

 own results. The time has arrived when minute observations must 

 take the place of mere general ones. 



May 19, 1881. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The Right Hon. William Ewart Gladstone was admitted into the 

 Society. 



The following Papers were read : — 



J. " On Discontinuous Phosphorescent Spectra in High Vacua." 

 By William Crookes, F.R.S. Received March 31, 1881. 



In a paper which I had the honour of presenting to the Royal 

 Society in March, 1879,* I drew attention to the fact that many sub- 

 stances, when in high vacua and submitted to the molecular discharge 

 by means of an induction coil, emitted phosphorescent light ; and I 

 especially mentioned the phosphorescent sulphides, the diamond, the 

 ruby, and various other forms of alumina, crystalline and amorphous. 



Pure alumina chemically prepared has very strong phosphorescence. 

 Sulphate of alumina is dissolved in water, and to it is added an excess 



* " Phil. Trans.," vol. 170. 



