362 PHILOSOPHICAL TKANSACTIONS. [aNNO 1774. 



nations, have generally been furnished with weather-fanes, which commonly end 

 in sharp points; for had they been terminated with large round balls of metal, 

 perhaps many more of them might long since have been demolished. Here 

 therefore I cannot but express my earnest wishes, that on all future occasions, 

 where lofty public edifices are to be erected, a good pointed conductor for the 

 lightning, may be considered by every architect, or surveyor, as an essential part 

 of the edifice itself. 



Exper. 6. I attemped to ascertain the conducting power of different metals, 

 in the following manner. I took a thick piece of paste-board, across which I 

 ruled lines, exactly an inch asunder. On these lines crosswise I placed the wires, 

 which were confined by heavy weights: the edges of which weights just touched 

 the ruled lines; leaving exactly an inch of wire between them (see fig. 10). The 

 kinds tried were, pure gold, silver, brass, copper silvered, and iron. They were 

 all drawn through the same hole, except the iron, which was somewhat larger 

 than the others. They were proved by 2 jars, containing 1 1 square feet of 

 coated surface; and the charges were adjusted by an electrometer graduated in 

 divisions of a 10th of an inch each, the diameter of the scale being 2 inches. 

 The result was as follows: 



Pure gold "\ f * 'l 



Brass / \ 6 I 



Copper silvered > was melted at •< 8 > divisions. 



Pure silver I I '"^ V 



Iron J (. 10 J 



When I gave either of the wires a division less than the number above speci^r 

 fied, it was not melted ; when I gave either of them a division more, it was ex- 

 ploded; the greater part vanishing in smoke; whereas these charges just burst 

 them into balls. 



Having lately been presented, by Dr. Lewis, with 6 specimens of his platina, 

 in as many different states, I selected the largest grains, from one of the parcels 

 which he informed me had been repeatedly exposed to long-continued vehement 

 fires ; the most intense he had been able to excite, or any vessels he could pro- 

 cure would support: and after a few small globules, consisting doubtless in great 

 part of heterogeneous metal, had melted out, repetitions of the operation pro- 

 duced no further change. It was afterwards boiled successively in oil of vitriol, 

 aquafortis, and spirit of salt, in order to its further purification ; and which in- 

 deed reduced it to a state the most pure of any that excellent chemist had been 

 able to produce. Having ruled a line with a blunt ended wire, over the surface 

 of a plate of white wax; 



Exper. 7- I pressed in the grains of platina lightly, and in contact with each 

 other, so as to form a regular line, half an inch long. At each end of the line 

 of platina, and in contact with it, I placed a thick wire, with its ends nicely 



