314 Conductors and Non-conduftors. [Book I\ r . 



Electric bodies, or thofe fubftances which emit this 

 fiuid, are the following : 



Amber, jet, fulphur, 



Glafs, and all precious 'ftones, 



All refmous compounds, 



All dry animal fubftances, as filk, hair, wool, 



paper. &c. 



M. Achard, of Berlin, has proved by experiment, 

 that certain circumftances will caufe a body to conduct 

 electricity, which before was a non- conductor. The 

 principal of thefe circumftances are the degrees of 

 heat to which the body is fubjected. This gentleman 

 agrees in opinion with M. Euler, that the principal dif- 

 ference between conductors and non-conductors con- 

 fifts in the fize of the pores of the conftituent parts of 

 the body. 



It muft be obferved, that electrics and non-electrics 

 are not fa ftrongly marked by nature as to be defined 

 with precifion j for the fame fubflance has been diffe- 

 rently claffed by different writers ; befides, the electric 

 properties of the fame fubftance vary according to 

 changes of circumftances j thus a piece of green wood 

 is a conductor, and the fame piece, after it has been 

 baked, becomes a non-conductor j when it is formed 

 into charcoal it again conducts the electric matter ; but 

 when reduced to afhes is impervious to it. Indeed, it 

 might perhaps be generally laid, that every fubftance 

 is in a certain degree a conductor of this fiuid, though 

 fome conduct it \vith much more facility than 

 others. 



The inftruments ufed in electricity are of five kinds ; 

 firfl, tubes of glafs, or cylinders of fealing wax j the 

 fecond confift of a fingle winch or of a multiplying 

 wheel, by means of which, globes, cylinders, and plates 

 of glafs, of fulphur, or of fealing wax, are made to 



turn 



