The Marquess of Salisbury. 321 



" If instead of the jar a sheet of copper, insulated between two 

 India rubber pads, be used, the same effect may be pr6duced in a 

 slighter degree. Also, if instead of an india rubber pad, a long coil of 

 insulated wire, in a flat case, be laid on the copper, after contact broken, 

 if the tube be passed sharply across the coil lengthways not touching, 

 but very close light will be produced at each stroke for a little time. 

 The difference in effect probably results from the fact that a jar is a 

 form capable of more insulation than any other. 



" If the poles of a strong coil be one connected to earth through a 

 jar or with considerable Interval the other not at all the whole 

 surrounding space near the jar and the coil, or anything in connection 

 with either, will be in a state of excitement. The tube, if held any 

 way in the air will show it, but most if held normally. A thermometer 

 will show it, giving a mercury light. The thermometer will show it, 

 even if enclosed in an upright tall narrow jar (dry). If a coil so 

 arranged is in action, and an india rubber disc be laid on the table not 

 far from the jar, and a tube be laid upon it, the action can be so 

 regulated that the tube will be just short of lightening. Put the 

 finger near it in any direction, and the part of the tube nearest the 

 finger will lighten. If a thermometer, standing as above described, be 

 grasped by the finger and thumb, the part grasped will have a thicker 

 and stronger light than the rest of the tube. 



" Hang the tube over a plate of copper in connection with one pole ; 

 the other pole to earth; tube to earth. Approach the tube (using 

 negative to plate) till there is a brush discharge from tube, with 

 occasional sparks. Let tube be a hydrogen tube. Examine the light 

 in its strongest part by spectroscope. As long as the brush discharge 

 is going on it is a mercury light (?) ; but when the spark comes, C and 

 F shoot across the spectrum." 



In further experiments a bell-jar was placed on the plate of an air 

 pump and exhausted ; over it was inverted a Ley den jar from which 

 the interior connecting rod had been removed, and the outside of the 

 jar and the air pump plate were connected to the coil ; a brilliant dis- 

 charge was found to be produced in the bell-jar when the coil was in 

 action. When the coil was stopped, and the wire attached to the 

 Leyden jar was removed, occasional glimmers were seen in the bell-jar, 

 which lasted for some time and were renewed on touching the jar with 

 the finger. This experiment was modified by charging a Leyden jar 

 by an electric machine, removing the interior connecting rod and 

 inverting the jar over a small bell jar on the air pump plate, the edge 

 of the jar resting on the plate. A flickering discharge, which was 

 called " summer lightning," was seen, and lasted for some time, and 

 was renewed on touching the outside of the jar, from which a small 

 spark was drawn. An exhausted and sealed tube, without electrodes, 



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