210 



Mr. W. Crookes. 



[May 19, 



a green light, and gives a spectrum which chiefly consists of two 

 bright green lines in the same place as the dark lines seen by reflected 

 light. 



A curious phenomenon is presented by erbia when the spark passes 

 over it at a high exhaustion. The particles of earth which have acci- 

 dentally covered the poles are shot off with great velocity, forming 

 brightly luminous lines, and, striking on the sides of the tube, rebound, 

 remaining red-hot for an appreciable time after they have lost their 

 velocity, They form a very good visible illustration of radiant 

 matter. 



Titanic acid phosphoresces dark brown, with gold spots in places. 

 Stannic acid gives no phosphorescence. 



Chromic, ferric, and eerie oxides do not appreciably phosphoresce. ^ 

 Magnesia phosphoresces with a pink opalescent colour, and shows no 

 spectrum lines. 



Baryta (anhydrous) scarcely phosphoresces at all. Hydrated 

 baryta, on the contrary, shines with a bright orange-yellow light, but 

 shows no discontinuity of spectrum ; only a concentration in the 

 yellow-orange. 



Strontia (hydrated) phosphoresces with a beautiful deep blue 

 colour, and when examined in the spectroscope the emitted light 

 shows a greatly increased intensity at the blue and violet end, with- 

 out any lines or bands. 



Lime phosphoresces of a bright orange-yellow colour, changing to 

 opal-blue in patches where the molecular discharge raises the tem- 

 perature. In the focus of a concave pole the lime becomes red and 

 white-hot, giving out much light. This earth commences to phos- 

 phoresce more than 5 millims. below the vacuum, and continues to 

 grow brighter as long as the electricity is able to pass through the 

 tube. On stopping the discharge there is a decided residual glow. 

 No lines are seen in the spectrum of the light. 



Calcium carbonate (calcite) shows a strong phosphorescence, which 

 begins to appear at a comparatively low exhaustion (5 millims.). 

 The interior of the crystal shines of a bright straw colour, and the 

 ordinary and extraordinary rays are luminous with oppositely polarised 

 light. Calcite shows the residual glow longer than any substance I 

 have as yet experimented with. After the current has been turned 

 off the crystals shine in the dark with a yellow light for more than a 

 minute. 



Calcium phosphate generally gives an orange-yellow phospho- 

 rescence and a continuous spectrum. Sometimes, however, a yellow- 

 green band is seen superposed on the spectrum. 



Potash phosphoresces faintly of a blue colour. The spectrum shows 

 a concentration at the blue end, but the light is too faint to enable 

 lines, if any, to be detected. 



