396 * PHILOSOPHICAL TRANSACTIONS. [aNNO 175Q. 



tied together; and by proper assistants he was held in the same position as is 

 done in the operation of cutting for the stone in the urinary bladder. He then 

 proceeded to divide the urethra longitudinally by incision. The extent of the 

 incision was from one end of the swelling to the other : the length and size of 

 the wound enabled him to take away the stones without any violence or diffi- 

 culty. After the stones were removed, he brought the lips of the wound to- 

 gether, and with the twisted suture he retained them in that situation. By this 

 method, and by occasionally passing a bougie of a proper size into the urethra, 

 beyond the farther extent of the incision, the patient went happily on, till the 

 cure of the wound was completed, which was effected in about 3 weeks ; and 

 there afterwards remained no inconvenience at all to the patient in voiding or 

 retaining his urine. 



JVeight of the extracted Stones. 



In air. In water. Spec, gravity. 



Large stone 308. 5 92.85 1 .43 1 



Small stone 44.35 10. 4 1 .326 



Both stones 350. 8 102. 9 1.415 



XXXV. Experiments on the Tourmalin.* By Mr. Benj. Wilson, F.R.S. p. 308. 



The first account met with of the tourmalin, and the remarkable properties 

 belonging to it, was from a memoir in the Berlin acts, printed in the year 1758; 

 where it appears that F. V. T. jfEpinus, professor of natural philosophy, made 

 several very curious and judicious experiments on it; the most material of which 

 prove a plus electricity on one side of it, and at the same time a minus electri- 

 city on the other side; provided the tourmalin is moderately warmed, and even 

 by hot water. These appearances are the more extraordinary, as the like means 

 employed in the same manner upon diamonds, glass, and all other electric bodies 

 hitherto tried, produce no such appearances. 



The Duke de Noya, who visited this kingdom in 1758, wrote a small treatise 

 on the subject, and published it at Paris on his way to Italy : in this work he 

 mentions ^Epinus's experiments, but does not admit of a plus and minus elec- 

 tricity belonging to the tourmalin when heated. On the contrary, he says that 

 the two sides are electrified plus, but one of them is more so than the other ; 

 and that it is the difference between those degrees which has led -^pinus into the 

 mistake. 

 . Mr. W. repeated most of the experiments mentioned in the Berlin memoir, 



* The tourmalin belongs to the genus of siliceous stones, and is a species of shorl (schorlus elec- 

 tricus) found at the Brasils, Ceylon, in the Tyrol, &c. and generally in a crystallized form. The 

 crystals are truncated prisms with 3, 6, or 9 faces or sides, and are either of a green or blue colour. 

 See the 3d vol. of Hauy's Traite de Mineralogie. 



