348 Lightning Conductors in Ships. 
ies are broken; whilst, at the same time, there is such an evolution 
of heat, that substances directly in the line of action are sometimes 
inflamed, fused, and ignited. 
3. This easy and elementary view of electrical action may not be 
altogether useless; for to investigate any branch of physical science 
with success, it is always advantageous to arrange our ideas in some 
determinate order, by means of which the details assume a clear and 
connected form; for although it must be admitted, that every theory 
is merely a way of picturing to ourselves the course of nature, it may 
be always sufficient, and admissible, .so long as it is consistent with 
the observed phenomena, and not contradicted by any known fact. 
4. In the progress of electrical inquiries, it has been found, that 
some substances oppose but comparatively little resistance to the pas- 
sage of the electrical agency, whilst, on the contrary, other substan- 
ces seem to arrest its course altogether; a fact which induced elec- 
iricians to consider bodies as possessed of these peculiar properties, 
and to classify them in relation to this conducting or non-condueting 
wer. Substances which 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 conducting 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 elec- 
trical machine, whilst a current of sparks is passing from It, the 
sparks will immediately cease ; in consequence of the electric mat- 
ter being transmitted by them to the earth:—an easy and striking 
experiment. Non-conductors of electricity, or insulators, are all vit- 
reous and resinous substances ;—dry, permanently elastic fluids, such 
as air; baked wood, silk, pure carbon, and most precious stones, oils, 
dry vegetable substances, as also, dry marble, chalk, and lime, wool, 
hair, feathers, dry paper, parchment, and leather. If, whilst a cut 
rent 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 will 
continue. 
" 5. Although for general purposes, the various bodies in nature may 
be considered as belonging to one or the other of these classes, @ gra- 
dation of effect is observable from one class to the other ; so that the 
