ELECTRICAL PROPERTIES OF SILK. 159 
attached by very fine brass and iron wire, but, to 
their utter astonishment, found the effect exactly 
the same as when they used the hempen cord ; the 
electrical virtue utterly passed away ; while, on the 
other hand, when the packthread was supported by 
a silken cord, they were able to convey the electric 
virtue seven hundred and sixty-five feet. 
It was evident, therefore, that these effects de- 
pended upon some peculiar quality in the silk, 
which disabled it from conducting away the elec- 
trical power, as the hempen cord and the wire had 
done. 
The accidental discovery of Mr Gray led to 
the knowledge of the non-conducting powers of 
various other substances; and since the nature 
of electricity has been more deeply investigated, 
the true electric properties of most substances have 
become known, and are now divided into elec- 
tries and non-eleectrics. The following substances 
are among the principal conductors of the electric 
fluid; namely, stony substances in general, more 
especially those of a calcareous nature, such as lime, 
marble, &e., sulphuric acid, black pyrites, black 
lead, alum, charcoal, all the metallic ores, the ani- 
mal fluids, and all other fluids excepting air and 
oils. 
The electric bodies are those substances which, 
when excited, collect or omit this fluid, such as 
amber, sulphur, jet, glass, and all precious crystal- 
