288 History of Luminescence 



dons of the Royal Society for 1752, a contribution full of observa- 

 tions on electroluminescence. The experiments were on a grand 

 scale. Watson first used a glass cylinder three inches in diameter 

 and three feet long, containing a movable and a fixed brass plate, 

 the two connected with an electrical machine. The cylinder could 

 be evacuated and the brass plates brought near or far from each 

 other for different experiments. Watson (1752) wrote: 



It was a most delightful spectacle when the room was darkened to see 

 the electricity in its passage; to be able to observe, not, as in the open 

 air, its brushes or pencils of rays an inch or two in length, but here the 

 coruscations were of the whole length of the tube between the plates . . . 

 and of a bright silver hue. These did not immediately diverge, as in the 

 open air, but frequently from a base apparently flat, divided themselves 

 into less and less ramifications, and resembled very much the most lively 

 coruscations of the aurora borealis. 



Watson also discharged a Leyden phial through the evacuated 

 tube and " at the instant of the explosion yoti saw a mass of very 

 bright embodied fire, jumping from one of the brass plates in the 

 tube to the other." 



To produce the vacuum, Watson had used the air pump of Mr. 

 John Smeaton (1724-1792) , " by which we are empowered to make 

 Boyle's vacuum much more perfect than heretofore " but this was 

 not good enough, and Watson proposed to use the best vacuum 

 known, that of the barometer. He wrote: 



The difficulty however of applying the Torricellian vacuum to these 

 experiments has been happily got over by the right honorable Lord 

 Charles Cavendish, ^^ our worthy vice-president. This noble lord, who 

 to a very complete knowledge of the sciences joins that of the arts, and 

 whose zeal for the promotion of true philosophy is exceeded by none, 

 has applied it in the following manner, and his lordship has had the 

 goodness to put his apparatus into my hands. 



The Cavendish device was a double barometer, consisting of a 

 long glass tube bent in the middle, filled with mercury, and then 

 inverted, with the two ends dipping into two cups of mercury. The 

 Torricellian vacuum in the bend and the two mercury cups which 

 could be connected with an electrical machine, made a perfect dis- 

 charge tube. Watson wrote " while the [electrical] machine was in 

 motion the electricity pervaded the vacuum, in a continued arch of 

 lambent flame, and as far as the eye could follow it, without the least 

 divergency." Many experiments were performed with this tube and 



^* Father of Henry Cavendish (1731-1810), whose notable researches on gases and 

 on electricity did not include a study of luminescence. 



