J'rom a Natural Magnet. 



51 



inches long, and containing four layers in thickness, which 

 were carefully separatetl by insulating partitions of cloth and 

 sealing-wax. The one termination rfe of the wire, passed 



into the bottom of a glass tube h, half filled with mercury, in 

 which the wire terminated, and the purity of the mercurial 

 surface is of great consequence to the experiment. The other 

 extremity^ of the helical wire communicated by means of the 

 cup of mercury i, with the iron-wire g, the fine point of which 

 may be brought by the hand into contact with the surface of 

 the mercury in h, and separated from it at the instant when 

 the contact of the connecter a b with the poles of the magnet 

 is effected. The spark is produced in the tube h. 



The success of the experiment clearly depends on the syn- 

 chronism of the production of the momentary current by con- 

 necting the magnetic poles, and the interruption of the gal- 

 vanic circuit at the surface of the mercury. This might be 

 pretty nearly ensured by a variety of simple mechanical con- 

 trivances which suggest themselves, — but as these would re- 

 quire very considerable nicety in their execution, I have been 

 satisfied with the precision which may be insured by a good 

 ear and an accurate assistant, — as I have thus, with a little 

 practice, been able to produce, for many times in succession, 

 at least two sparks from every three successive contacts. 



These sparks have generally a fine green colour ; that I 

 obtained on the 30th of March was in every respect similar to 

 those I afterwards procured. The intensity of light varies 

 considerably, as it depends on the degree of accuracy with 

 which the circuit is broken at the moment of contact. Some- 

 times it is highly vivid, and has been seen some yards off in a 

 dark place. 



As soon as I had the circumstances under my command, 

 I hastened to show the experiment to my brother, who was 



f resent, and to Dr. Gregory, acting secretary of this Society, 

 afterwards had the satisfaction of showing it to Dr. Hope, 

 to Sir John Leslie, and several other gentlemen. 

 H2 



