SQO I'HILOSOPHICAL TRANSACTIONS. [anNO I76I. 



' That we may judge the better of this, Mr. D. mentions the circumstances of 

 one of those experiments particularly. When a common tobacco-pipe, or any 

 other slender body of the like kind, is heated red-hot, it conducts the electric 

 fluid as perfectly as when cold: on cooling, it gradually arrives at its most per- 

 fect electric state in 2 minutes; and in less than 2 minutes more it entirely loses 

 its electric property again, though at that time it is not cold ; it cannot therefore 

 in that interval have imbibed a moisture sufficient to have destroyed its electricity. 

 Nor are any of the substances, employed in the experiment, of that kind of 

 bodies, which are apt suddenly to draw moisture from the air. 



In confinTiation of particular bodies requiring particular degrees of heat, to 

 render them electric or n on- electric, independent of moisture, Mr. D. mentions 

 a substance, which is affected by heat in an opposite manner to the former in- 

 stances; for the degree of heat necessary to render the other substances electric, 

 makes tliis non-electric. This substance is island crystal, well known for its sin- 

 gular property of a double refraction, on a piece of which he made the following 

 observations. 1 st. After this piece of crystal has been rubbed, when the heat of 

 the air is moderate, it shows signs of electricity, though not very strong ones. 

 2d Obs. If the heat is increased, so as to be a little greater than that of the 

 hand, it destroys its electric power entirely. 3d Obs. By cooling the ston^ again 

 the electric power is restored. He immersed this piece of crystal into a vessel 

 filled with quicksilver, and surrounded by ice, where it remained near 2 hours, 

 when the weather was very cold; on taking it out with a pair of tongs, that it 

 might not be altered by the heat of his hands, and rubbing it again, it was more 

 strongly electric than he had at any other time experienced; but on placing it for 

 a few minutes on the hearth, at some distance from the fire, its electric property 

 was again destroyed, for rubbing would not occasion any signs of it. Thus we 

 see two different kinds of fixed bodies, the one of which acquires an electric 

 property, with the same heat with which another loses it; while a 3d set of sub- 

 stances, as glass, &c. retain their electricity through both the degrees of heat 

 necessary to the other two. 



Some pieces of island crystal, which he had procured from different places, had 

 not the property of losing their electricity by a moderate heat. He had in par- 

 ticular a piece of that crystal, one part of which, when gently heated, becomes 

 non-electric, while the other part, with the same heat, or even with a much 

 greater one, remains perfectly electric. There are several other earthy substances 

 he found, whose electricity is destroyed by very different degrees of heat. 



Lf^J. Of an Encr'muSf or Star-Jish, with, a jointed Stem, taken on the Coast of 

 Barhadoesy which explains to what kind of minimal those Fossils belong, called 



