24 



Prof. R. Threlfall on the Electrical 



Experimental Sparh-Tuhe. — The first successful tube was 

 furnished with external electrodes and sparked between Feb. 24 

 and March 2, 1891. Detail are (pressure 8*8 millim.) : — 



Feb. 24. Sparked 1 



30 



ong and thin, laro-e 



Feb. 25. 

 Feb. 28. 



Mar. 2. 



4 



Sparks being 



resistance in circuit. 



Ditto. 



Ditto. Then 3 hours 15 minutes 

 with less resistance. 



The least perceptible discharge. 



No effect was noticed at any time. The tube was left for 

 three days in order that the temperature might reach the 

 exact value it had when the gauge was sealed, and it happened 

 that this did not occur for three days. There was still no 

 effect. This tube was sealed from the pump in series with 

 the spectrum-tube and is still in my possession. 



Tube 2. — After experimenting with many tubes mostly of 

 the form shown in the diagram (fig. 1), with the armed ends 

 in mercury cups, a satisfactory tube was got on April 21st. 



Fig. 1. 



The difficulty arose from my determination to have the elec- 

 trodes white hot for several hours by a preliminary sparking at 

 about '1 millim. before the tube was finally filled with gas and 

 sealed off. The excessive heating of the electrodes generally 

 broke the glass. During this time I noticed a thing which 

 was new to me, but which I daresay is well known, viz., the 

 ease with which the colour of the fluted spectrum-discharge 

 may be made to change. Thus on the 20th I found that at a 

 prassure of less than 1 millim., and with a very intense dis- 

 charge from a coil giving 12-in. sparks without condenser, I 

 could cause the golden-coloured discharge to become pinkish 

 by heating the tube with a bare flame to about 200° C, a 

 necessary step in the preparation. The spectrum, of course, 

 merely showed a shift in relative luminosity from the red 

 towards the blue end. The action was reversed on cooling 

 the tube. This could be repeated once or twice. However, 

 with the next filling of gas the experiment did not succeed at 

 all, no heating of the tube caused any change in the colour of 



