1871.] 



through Rarefied Media, fyc. 



239 



by cell, the changes took place abruptly and suddenly. For example, 

 when the power was so low that the positive pole only was visible, the 

 current was feeble, and kept augmenting in power as cell after cell was 

 added on. Suddenly the luminous red flame (phot. 3) made its appearance, 

 and the galvanometer showed that the current had suddenly augmented 

 three or four times in power. As the power was again further increased 

 cell by cell, the current again steadily augmented in proportion, until the 

 luminous tongue suddenly disappeared, and the appearance in 6 was 

 shown, the galvanometer showing a still further sudden increase in the 

 current. 



The phenomena shown in 4 and 5 can only be obtained by inserting a 

 large resistance. 



Nature of the luminous cloud.— Pliicker has shown that when such an 

 exhausted tube, with a current through it, is placed between the poles of 

 an electro-magnet, a luminous arch is produced, which arch follows the 

 course of the magnetic rays. (See photograph 8, in which the negative pole 

 was a small ring. Photograph 9 shows the arch when the large ring was 

 negative.) 



As the electro-magnet is magnetized, the tube, which before was full of 

 a luminous cloud, is seen gradually to change ; the magnet gathers up this 

 diffused cloud, and builds up the arch shown in 8 and 9. 



Inasmuch as the electricity was passing in a continuous current from the 

 battery, from wire to wire, it is evident the light is projected right and left 

 into those parts of the tube where there is no electric current flowing. 



To endeavour to ascertain the nature of this arch, a tube (PI. III. 

 fig. 2) was constructed. A piece of talc, bent into the form U, had a fibre 

 of silk stretched across it ; on this fibre of silk was cemented a thin strip 

 of tale, 1 inch in length, j 1 ^ inch broad, weighing about of a grain. The 

 tube was sealed up and exhausted ; carbonic acid and potash were used to 

 get a high vacuum. When the magnet was not magnetized, the passage of 

 the current from wire to wire did not affect the piece of talc. When the 

 magnet was charged, and the luminous arch was made to play upon the 

 lower portion of the talc, it repelled it, no matter which way the electric 

 current was passing. 



When the tube was shifted over the poles of the magnet so as to project 

 the luminous arch against the upper part of the talc, the upper end of the 

 talc was repelled in all instances; the arch, when projected against the 

 lower part of the talc, being near the magnet, was more concentrated, and 

 the angle of deviation of the talc was as much as 20°. Wlien the upper 

 part of the arch, which was much more diffused, was thrown upon the 

 upper part of the talc, it was repelled about 5°. 



This experiment, in the author's opinion, indicates that this arcji is com- 

 posed of attenuated particles of matter projected from the negative pole 

 by electricity in all directions, but that the magnet controls their course ; and 



