VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 307 



soon after it appeared in England, with the tourmalin belonging to Dr. Sharp, 

 being the only one known of here at that time : and though but a small one, 

 compared with /^pinus's, yet it was large enough to satisfy Mr. W. that his 

 opinion was well founded. 



The largest tourmalin Mr. W. had from Dr. Heberden, and with which he 

 made the following experiments, weighed above 120 grs. It was of an oval 

 form, and polished : the greatest diameter an inch and a quarter, and the least 

 one inch. One side plain, the other convex, but cut into several small planes or 

 facettes, something like a rose diamond ; the thickest part of which is near one 

 third of an inch. To make experiments with the tourmalin requires the greatest 

 attention, as the appearances sometimes are scarcely sensible, so that he has been 

 obliged to employ the tenderest kind of apparatus, and even interpose a sort of 

 screen to prevent his breath, or other like motions of the air, from disturbing 

 the experiment. 



Mr. W.'s apparatus for making many of these experiments, consisted of 2 very 

 small balls made of the pith of elder, and suspended by two linen threads of the 

 finest kind : the ends of these threads were fastened to a slip of wood about 3 

 inches long, and half an inch broad: then on a stick of sealing wax, Q inches 

 long, fixed upright on a table, or any other convenient place, was fastened the 

 slip of wood, from one end of which the threads &c. hang down 5 inches, so 

 that the pith balls are about 4 inches from the table. These balls are always 

 supposed to be electrified plus, except where the contrary is mentioned : but 

 with no greater power than to make them recede from each other about 1 inch 

 in every experiment. 



Mr. W. prefers the wax stand to a glass one, as the latter when cold acquires 

 moisture very soon, and therefore becomes a conductor: whereas wax, when it 

 is once in good order, will continue a non-conductor for a long time. Before 

 setting down my experiments, he mentions 3 truths that are commonly known, 

 and which for the present he calls general laws, viz. ] . Two bodies equally elec- 

 trified plus recede from each other, or are repelled. 2. Two bodies electrified 

 minus recede also from each other, or are repelled. 3. One body electrified 

 plus and another body electrified minus, to an equal degree, accede, or move 

 towards each other, or as it is generally expressed, are attracted. 



Experiments on the Tourmalin. 



Exp. 1. One edge of the tourmalin being properly fastened to a long stick of 

 sealing wax, Mr. W. dipped the stone into boiling water, and continued it there 

 near one minute. On taking it out, and presenting the convex side near the 

 pith balls, they immediately receded from it, but not very strongly. On turn- 

 ing the plain side towards the balls, it caused them to accede to it, but rather at 

 a greater distance in this case, than they receded from it in the last. When the 



