[ 3°8 ] 
It may farther be obferved, that the whole lur- 
faces of thefe two ftones, except the parts of them 
where they lay in contact with each other, or where 
they continually preffed upon the lower furface of the 
urethra, or bed, which they had formed for themfelves 
in this chanel, are rough, and have feveral inequa- 
lities, or eminences ariling from them. 
Fig. 2. reprefents the inferior fmooth parts of the 
two Rones, as they appeared when feparated from 
each other, as well as the fmooth or polilhed furfaces 
of the ends of thefe hones, which lav in contadt with 
* «/ 
each other, and upon motion rubbed againft each 
other. 
XXXV. Experiments on the Tourmalin : hy 
Mr. Benjamin Wilfon, F. R. S. In a letter 
to Dr, William Heberden, F. R . S . 
SIR, 
Read Dec. 6. TT Have the pleafure to communicate to 
you fome experiments made upon the 
* Tourmalin , or AJhJlone, which you were fo kind as to 
procure me, together with fome others, relpedting the 
vitreous and refinous electricities, as they are called, 
and the obfervations I have made thereupon. 
The more I am acquainted with eledtricity, the 
more I admire a wonderful limplicity which feems 
to prevail in nature, at lead: in this part which abounds 
with phenomena of a very curious kind ; whereof 
many that have paffed under my examination of late 
are fo extremely nice, that I avoid venturing to relate 
them, becaufe I would not willingly fubjedt myfelf 
to the cenfure of incautious obfervers. 
4 
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