CHAP. I.] ELECTRICITY AND MAGNETISM. 63 



hydrochloric acid, the formation of each crystal is accom- 

 panied by a flash of light, doubtless due to an electrical 

 discharge. A curious case occurs when the sulphate of 

 copper and potassium is fused in a crucible. It solidi- 

 fies without becoming electrical, but on cooling a little 

 further the crystalline mass begins to fly to powder with 

 an instant evolution of electricity. 



62. Combustion. Volta showed that combustion 

 generated electricity. A piece of burning charcoal, or a 

 burning pastille, such as is used for fumigation, placed in 

 connection with the knob of a gold-leaf electroscope, will 

 cause the leaves to diverge. 



63. Evaporation. The evaporation of liquids 

 causes the production of electricity, the liquid and the 

 vapour assuming opposite states. A few drops of a 

 solution of sulphate of copper thrown into a hot plati- 

 num crucible produce violent electrification as they 

 evaporate. 



64. Atmospheric Electricity. Closely connected 

 with the electricity of evaporation is the atmospheric 

 electricity always present in the air, and due, in part 

 at least, to evaporation going on over the oceans. The 

 subject of atmospheric electricity is treated of sepa- 

 rately in Lesson XXIV. 



65. Pressure. A large number of substances when 

 compressed exhibit electrification on their surface. Thus 

 cork becomes + when pressed against amber, gutta- 

 percha, and metals ; while it takes a charge when 

 pressed against spars and animal substances. Abb 

 Haiiy found that a crystal of calcspar pressed between 

 the dry fingers, so as to compress it along the blunt 

 edges of the crystal, became electrical, and that it re- 

 tained its electricity for some days. He even proposed 

 to employ a squeezed suspended crystal as an electro- 

 scope. A similar property is alleged of mica, topaz, 

 and fluorspar. Pressure also produces opposite kinds of 

 electrification at opposite ends of a crystal of tourmaline, 



