Mineralogy and Geology. 415 



1 



vary as — 7 — , . - As the distance between the spheres increases, 

 ' a (a-\-2r) r ' 



the points q q' recede from the surface, and would finally coincide with 

 the centres of the spheres; which would limit the distance at which a 

 charge could possibly occur. The author exhibited to the Section the 

 striking agreement of the calculation with experiment by means of this 

 electrical balance : consisting of a delicate scale-beam, with a duly 

 poised suspended sphere. The weights requisite to balance the force 

 between this sphere and a second sphere placed beneath it, and charged 

 with a given quantity of electricity, were predicted with great precision. 

 In the course of this paper some magnetic experiments were exhibited, 

 illustrative of the action of the forces between similarly placed and 

 opposite points. The author observed that every kind of case in ordi- 

 nary electricity could be easily and simply investigated upon these 

 principles ; and that without the air between conductors could become 

 charged, no attractive force would be apparent, and this would of course 

 limit the distance at which such force could exist. It has been shown 

 by Faraday, that with the same constraining power, induction takes place 

 more readily, or with more difficulty, as the extent of the intervening 

 dialectric particles is diminished or increased. We might have electri- 

 cal currents in a space devoid of resistance; but without an interposed 

 dialectric medium it is doubtful whether those phenomena indicative of 

 attraction between the bodies would exist. 



II. Mineralogy and Geology. 



1. On an Amorphous Boracite ; by Dr. M. Karsten, (Rep. Brit. 

 Assoc, for 1847, Athen., No. 1028.)— When boring for rock salt at 

 Neusalzwerk, in the neighborhood of Minden, in Prussia, at the depth 

 of about 1,400 feet, a bed of amorphous boracite was found, of which 

 specimens were brought out by the boring apparatus. The chemical 

 analysis, which proves that the composition of the amorphous mineral 

 is exactly the same as that of the well-known crystallized body, was 

 wade by Dr. Karsten in Berlin, particulars of which may be seen in the 

 monthly reports of the Berlin Academy. It seemed to him interesting 

 to examine if that uncrystallized species would show the pyroelectric 

 quality which in so high degree is to be seen in boracite crystals. Sir 

 D. Brewster has poinTed out a way by which the pyroelectric quality 

 °f pulverized tourmaline may be shown. By heating that substance the 

 fine particles cohere together, and show that a polarization has taken 

 Place in them. The same phenomenon is to be seen in the particles 

 °f the amorphous boracite by pulverizing and heating it on a metallic 

 Plate. These boracite particles show by their pyroelectric properties 

 that they must be crystallized, although by microscopic examination the 

 crystallization cannot be discovered. The conclusion must be, that the 

 difference between the crystallized and the amorphous states cannot be 

 exactly determined, since the microscope shows in this case no crystal- 

 lization where the pyroelectricity is a proof that we must suppose a 



cr ystalline structure. 



2. On Sulphato-Chlorid of Copper, a new Mineral; by Prof. Con- 



N£ ll, (Proc. Brit. Assoc, 1847, Athen., No. 1027.)— Amongst some 



