Mr Harris on Lightning Conductors in Ships. 155 



ducting or non-conducting power. Substances whicli oppose 

 but comparatively little resistance to an electrical explosion, 

 have therefore been termed conductors, whilst those which offer 

 resistance to its progress, have been termed non-conductors, or, 

 occasionally, from the same cause, insulators. In the conduct- 

 ing class, we find, all the metals, concentrated acids, water, well 

 burnt charcoal, wood, diluted acids, and saline fluids, most 

 earths and stones, flame, smoke and steam. If any of these 

 substances resting on the ground, be put into contact with an 

 electrical machine, whilst a current of sparks is passing from 

 it, the sparks will immediately cease ; in consequence of the 



electric matter being transmitted by them to the earth : an 



easy and striking experiment. Non-conductors of electricity, or 

 insulators, are all vitreous and resinous substances ; — dry, per- 

 manently elastic fluids, such as air ; baked wood, silk, pure car- 

 bon, and most precious stones, oils, dry vegetable substances, as 

 also, dry marble, chalk, and lime, wool, hair, feathers, dry pa- 

 per, parchment, and leather. If, whilst a current of sparks is 

 passing from the electrical machine, any of these bodies be put 

 into contact with it, and rest as in the former instance on the 

 earth, little or no difference will be perceived, the sparks wil 

 continue. 



5. Although for general purposes, the various bodies in nature 

 may be considered as belonging to one or the other of these 

 classes, a gradation of effect is observable from one class to the 

 other ; so that the conducting or insulating power of some sub- 

 stances, compared with that of others, may be considered as im- 

 perfect : hence has arisen a third class, which consists of the re- 

 mote extremes of the other two, and which may be considered 

 in the power of arresting or transmitting certain electrical actions 

 as appertaining to either. Thus wood, hemp, stone, and the 

 like, may become insulators to a state of low electrical action, 

 and conductors to a high one. 



6. The manner in which accumulations of atmospheric elec- 

 tricity proceed, may be referred to the following principle: 

 When two substances of the conducting class are directly opposed 

 to each other, and are separated by a substance of the non-con- 

 ducting or insulating class, leaving the one free and the other 

 insulated, the proportionate state of electrical distribution may 



