ELECTRICAL CHARACTERS 67 



of minerals assumed their true importance, while their electrical characters became a minor 

 consideration, as may be gathered from their brief mention in this place. 



For the purpose of demonstrating the existence of a charge of electricity upon the 

 surface of a stone an instrument known as an electroscope may be used ; for very feeble 

 charges an electrometer of complicated construction will be necessary. Hatty employed 

 for this purpose an " electrical needle " ; it consisted simply of a brass rod, terminated at 

 either end by a small brass ball, and balanced on a vertical fine steel point, on which it could 

 turn freely like a magnetic needle. An electrically charged body, when presented to either 

 of the balls, would attract it. By giving an electric charge to the balls, they would be 

 attracted or repelled on the approach of a body according as its charge was unlike or like 

 that of the balls. The electric pendulum, consisting of a pith ball suspended by a silk 

 thread, served the same purpose. With the help of such instruments it is easy to 

 demonstrate that minerals, including precious stones, become, under various conditions, 

 charged with electricity ; the fact of itself is, however, of little note. 



After rubbing on cloth all precious stones, like glass, become positively electrified. 

 Topaz and tourmaline become strongly electrified after such treatment, diamond less 

 strongly, and the majority of precious stones only feebly. Smooth faces are more 

 susceptible of electrification than are rough ones, and hence cut stones furnish the most 

 favourable material for this purpose. In perfectly dry air, some precious stones retain an 

 electrical charge for a comparatively long period ; this is especially so in the case of topaz, 

 the electrification of which can be detected after an interval of thirty-two hours ; sapphire will 

 retain its charge for from five to six hours, and diamond for half an hour. Colourless topaz, 

 colourless sapphire, and diamond may be distinguished by this difference in their behaviour ; 

 after imparting a charge by rubbing with a cloth, the stones should be laid on a metal plate 

 and their electrical state tested from time to time. The majority of precious stones lose 

 their charges with great rapidity, some, indeed, after only a few moments. 



Amber, like other resinous substances, becomes negatively electrified on rubbing, and so 

 strongly that it attracts to itself any light bodies, such as pieces of paper. These, after 

 contact with the amber, themselves become charged and are then repelled by it. This 

 particular character of amber is of value as a means whereby it may be distinguished from 

 its imitations, which will be mentioned later under the special descriptions. 



The electricity developed on some precious stones when under the influence of changes 

 of temperature is known as pyroelectricity. The charge produced in this way on the 

 surface of a stone varies in sign at different areas of the surface, the charge at one point 

 being positive while that at another is negative. Those parts of the surface which become 

 positively electrified on heating become negatively electrified on cooling, and vice versd. 

 Tourmaline and topaz are remarkable for the strength of the pyroelectrical charge they 

 acquire ; and this distinguishes them from other precious stones, which when exposed to the 

 same influences acquire but feeble charges, or none at all. Thus, with the help of one of 

 the electrical instruments mentioned above, a red tourmaline can be distinguished from a 

 ruby, and a greenish-blue topaz from an aquamarine of the same colour ; for the former 

 in each case will show a strong pyroelectrical charge, and the latter none. During the 

 gradual cooling of tourmaline after being heated, it assumes the power of attracting light 

 bodies to itself, as does amber after being rubbed. 



3. Magnetism. 



Some minerals, such as magnetite, are magnetic and respond to the influence of a 

 magnet, being attracted by it. Magnetite has a black metallic lustre, and a certain 



