402 PHILOSOPHICAL TRANSACTIONS. [anNO 1759. 



softest and weakest always minus. It was from this theory, that he was de- 

 sirous of trying to electrify the tourmalin minus by rubbing, not having at that 

 time been able to do it. He fixed on a brilliant diamond for this purpose, as 

 being the hardest body, and strongest electric, that he was acquainted with ; 

 and on rubbing the tourmalin with it, his expectations were answered : for both 

 sides of the tourmalin were electrified minus, and the diamond plus. 



Exper. 32. These experiments succeeding, he rubbed glass against glass, 

 and found that they electrified each other : but one of them was plus, and the 

 other minus. 



Exper. 33. Two pieces of amber, treated in the same manner, were also 

 electrified plus and minus. 



Exper. 34. When he rubbed glass against amber, the former was plus, and 

 the latter minus. 



Exper. 35. Two tourmalins being rubbed against each other, one became 

 plus, and the other minus. 



As electrics rubbed against electrics, occasioned electrical appearances, he was 

 encouraged to try what would be the effect of air, if he rubbed, or rather forced 

 it against electrics ; for he supposed the particles of air to be surrounded with a 

 medium of the same kind as grosser bodies, which is the cause of their being 

 so elastic. 



Exper. 36. To do this, he only made use of a common pair of bellows, and 

 having brought the tounnalin near to the end of the pipe, he found after it had 

 receivexl about 20 blasts, it was electrified plus on both sides. Air therefore 

 seems to be less electric than the tourmalin. 



Exper. 37. Into the place of the tourmalin he brought a pane of glass, and 

 blew against it the same number of times as in the former experiment. When 

 he examined both sides, they were electrified plus also, but less than the 

 tourmalin. 



Exper. 38. Amber treated in the same manner, was electrified less than the 

 glass. 



Exper. 39. He had recourse next to a smitli's bellows. The difference these 

 occasioned was only a much stronger electricity in the tourmalin. Amber was 

 still weaker than the glass, and the glass weaker than the tourmalin. 



Still having in view the inedium on the surfaces of the particles of air, he 

 considered that heat would rarify it : by which means, air having its resistance 

 lessened, would more readily part with the electric fluid, and of consequence 

 electrify more powerfully. Exper. 40. The pipe of the bellows being made 

 red-hot, he blew against the tourmalin 12 times only. In this experiment like- 

 wise the tourmalin was electrified plus on both sides, but to a considerable de- 

 gree more than was done in the 36th and 39th experiments. The hot air had 



