left a long space of varnished glass above the top of the coatings. If it is desired to accumulate a very great charge of electricity, a number of jars must be employed, all their inner coatings being connected together, and all their outer coatings being united. This arrangement is called a battery of Leyden jars, or Leyden battery (Fig. 45). As it has a large capacity, it will require a large quantity of electricity to charge it fully. When charged it produces very powerful effects; its spark will pierce glass readily, and every care must be taken to avoid a shock from it passing through the person, as it might be fatal. The "Universal Discharger" as employed with the Leyden battery is shown at the right of the figure. 63. Seat of the Charge. Benjamin Franklin discovered that the charges of the Leyden jar really reside on the surface of the glass, not on the metallic coatings. This he proved by means of a jar whose coatings could be removed (Fig. 46). The jar was charged and placed upon an insulating stand. The inner coating was then lifted out, and the glass jar was then taken out of the outer coating. Neither coating was found to be electrified to any extent, but on again putting the jar together it Fig. 46. was found to be highly charged. The charges had all the time remained upon the inner and outer surfaces of the glass dielectric. 64. Dielectric Strain. — Farady proved that the medium across which influence takes place really plays an 66 important part in the phenomena. It is now known that all dielectrics across which inductive actions are at work are thereby strained.* Inasmuch as a good vacuum is a good dielectric, it is clear that it is not necessarily the material particles of the dielectric substance that are thus affected; hence it is believed that electrical phenomena are due to stresses and strains in the so-called 'ether," the thin medium pervading all matter and all space, whose highly elastic constitution enables it to convey to us the vibrations of light though it is millions of times less dense than air. As the particles of bodies are intimately surrounded by ether, the strains of the ether are also communicated to the particles of bodies, and they too suffer a strain. The glass between the two coatings of tinfoil in the Leyden jar is actually strained or squeezed, there being a tension along the lines of electric force. When an insulated charged ball is hung up in a room an equal amount of the opposite kind of charge is attracted to the inside of the walls, and the air between the ball and the walls is strained (electrically) like the glass of the Leyden jar. If a Leyden jar is made of thin glass it may give way under the stress; and when a Leyden jar is discharged the layer of air between the knob of the jar and the knob of the discharging tongs is more and more strained as they are approached towards one another, till at last the stress becomes too great, and the layer of air gives way, and is "perforated" by the spark that discharges itself across. The existence of such stresses enables us to understand the residual charge of Leyden jars in which the glass does not recover itself all at once, by reason of its viscosity, from the strain to which it has been subjected. It must never be forgotten that electric force acts across space in consequence of the transmission of stresses and strains in the * In the exact sciences a strain means an alteration of form or volume due to the application of a stress. A stress is the force, pressure, or other agency which produces a strain. medium with which space is filled. In every case we store not electricity but energy. Work is done in pushing electricity from one place to another against the forces which tend to oppose the movement. The charging of a Leyden jar may be likened to the operation of bending a spring, or to pumping up water from a low level to a high one. In charging a jar we pump exactly as much electricity out of the negative side as we pump into the positive side, and we spend energy in so doing. It is this stored energy which afterwards reappears in the discharge. LESSON VII. - Other Sources of Electrification 65. It was remarked at the close of Lesson I. (p. 13) that friction was by no means the only source of electricity. Some of the other sources will now be named. 66. Percussion. — A violent blow struck by one substance upon another produces opposite electrical states on the two surfaces. It is possible indeed to draw up a list resembling that of Art. 6, in such an order that each substance will take a + charge on being struck with one lower on the list. 67. Vibration. Volpicelli showed that vibrations set up within a rod of metal coated with sulphur or other insulating substance, produced a separation of electricities at the surface separating the metal from the non-conductor. 68. Disruption and Cleavage. If a card be torn asunder in the dark, sparks are seen, and the separated portions, when tested with an electroscope, will be found to be electrical. The linen faced with paper used in making strong envelopes and for paper collars, shows this very well. Lumps of sugar, crunched in the dark between the teeth, exhibit pale flashes of light. The sudden cleavage of a sheet of mica also produces sparks, and both laminæ are found to be electrified. 69. Crystallization and Solidification. Many substances, after passing from the liquid to the solid state, exhibit electrical conditions. Sulphur fused in a glass dish and allowed to cool is violently electrified, as may be seen by lifting out the crystalline mass with a glass rod. Chocolate also becomes electrical during solidification. When arsenic acid crystallizes out from its solution in hydrochloric acid, the formation of each crystal is accompanied 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 solidifies without becoming electrical, but on cooling a little further the crystalline mass begins to fly to powder with an instant evolution of electricity. 70. Combustion. - Volta showed that combustion generated electricity. A piece of burning charcoal, or a burning pastille, such as is used for fumigation, placed in connexion with the knob of a gold-leaf electroscope, will cause the leaves to diverge. 71. Evaporation. The evaporation of liquids is often accompanied by electrification, the liquid and the vapour assuming opposite states, though apparently only when the surface is in agitation. A few drops of a solution of sulphate of copper thrown into a hot platinum crucible produce violent electrification as they evaporate. 72. Atmospheric Electricity. The atmosphere is found to be always electrified relatively to the earth: this is due, in part possibly, to evaporation going on over the oceans. The subject of atmospheric electricity is treated of separately in Lesson XXV. 73. Pressure. A large number of substances when compressed exhibit electrification on their surface. Thus cork becomes + when pressed against amber, guttapercha, and metals; while it takes a charge when pressed against spars and animal substances. Péclet found the degree of electrification produced by rubbing two substances together to be independent of the pressure and of the size of the surfaces of contact, but depended upon the materials and on the velocity with which they moved over one another. Rolling contact and sliding friction produced equal effects. 74. Pyro-electricity. There are certain crystals which, while being heated or cooled, exhibit electrical charges at certain regions or poles. Crystals thus electrified by heating or cooling are said to be pyro-electric. Chief of these is the Tourmaline, whose power of attracting light bodies to its ends after being heated has been known for some centuries. It is alluded to by Theophrastus and Pliny under the name of Lapis Lyncurius. Tourmaline is a hard mineral, semi-transparent when cut into thin slices, and of a dark green or brown colour, but looking perfectly black and opaque in its natural condition, and possessing the power of polarizing light. It is usually found in slightly irregular three-sided prisms which, when perfect, are pointed at both ends. It belongs to the "hexagonal" system of crystals, but is only hemihedral, that is to say, has the alternate faces only developed. Its form is given in Fig. 47, where a general view is first shown, the two ends A and B being depicted in separate plans. These two ends differ slightly in shape. Each is made up of three sloping faces terminating in a point. But at A the edges between these faces run down to the corners of the prism, while in B the edges between the terminal faces run down to the middle points of the long faces of the prism. The end A is known as the analogous pole, and B as the antilogous pole. While the crystal is rising in temperature A exhibits + electrification, B ; but if, after having been heated, it is allowed to cool, the polarity is reversed; for during the time that the temperature is falling B is + aud A is -. If the temperature is |