called frictional electricity. At first sight nothing scems more remote from the phenomena produced by rubbing a cat's fur, or the phenomenon of lightning, than the action of the battery which rings bells, decomposes liquids, and produces the mysterious magnetic effect in the neighbourhood of wires which we have noted. We have now good reasons for believing that when we stroke the fur of a cat the mechanical action breaks up the arrangement of the molecules of the hair and of the surface of the hand, just as the electro-motive force of a battery breaks up the arrangement of molecules in a conducting fluid and produces an electric charge on the two metals of the battery. When a lady produces a spark by walking across a carpeted floor, the molecules of her silk skirts and dry garments are rudely disarranged and an electric charge results. This charge does not result from any peculiar electricity of the body. It is not what is termed animal electricity. It results merely from the rubbing of the clothes or of the dry slippers upon the carpet, and it can be produced by men as well as women, being merely a question of the proper clothing necessary for its production. In our furnace-heated houses in winter the phenomena of electrical charges produced by friction is of common occurrence. A pair of silk undergarments suddenly withdrawn from a pair of trousers diverge under their electrical charge. A sheet of paper briskly rubbed adheres to objects presented to it. The sheet of paper, together with the object which is brought near it, really constitute a battery-the sheet being one pole and the neighbouring object being the other pole, while the air between takes the place of the fluid in the ordinary battery. The Franklin electrical machine can be considered a battery, and can be made to produce all the effects produced by an ordinary voltaic cell. We shall see, when we consider the dynamo, that the latter also can produce all the effects due to batteries, and it can be made to give sparks five feet long which are identical with discharges of lightning. In general, a change of molecular aggregation produces a manifestation of electricity, and when we reflect upon this fact we see the immense importance to the chemist of the study of electricity. If we should suddenly bend a ring of metal wire we can produce a current of electricity in the wire. Moreover, if we hold it over a lamp and heat it at one point we can also produce a current in it. In these cases also the current is produced by the change in molecular arrangements in the wire ring. We are apt to hastily conclude, from observations upon the electricity produced by differences in molecular activity, that electricity is a motion or vibration of molecules. As we continue, however, our study of what electricity is, we perceive that there are other and more significant ways of producing electricity than by chemical action, or by any operation which breaks up and changes the arrangement of molecules. A voltaic cell is not our only means of obtaining a Copper Iron FIG. 7. B Copper current of electricity without the use of a dynamo. One of the most valuable means for an experimenter consists in the employment of heat. If we join any two metals—for instance, iron and copper-and heat one junction-for instance, A, Fig. 7--and cool the other, B, an electric current results which flows from the hot junction through the copper wire to the cold junction. The new alloy termed constantan, with copper, gives a very sensitive combination. These thermoelectric junctions can be used as very delicate thermometers. One can easily determine the one hundredth of a degree Centigrade by means of them. To detect the current, it is merely necessary to have a delicate galvanometer. Prof. Tyndall, in his remarkable treatise on Heat as a Mode of Motion, devotes much space to the description of his thermopile and the galvanometer he employed, for this apparatus served to illustrate throughout his treatise the various manifestations of heat due to motion. He used it instead of a thermometer. Since the date of the publication of Prof. Tyndall's treatise, which gave to the world in a popular form the great generalization of the conservation of energy, this form of electrical thermometer has been much simplified and made more sensitive. Instead of the needle galvanometer employed by Prof. Tyndall, we now have the mirror galvanometer, and we are able to detect electrical currents which his instrument would not respond to. If Thermal currents always arise when there is a difference of temperature between any two points in an electric circuit. It is not necessary to employ the junction of the different metals to produce these currents. If a piece of copper, for instance, is heated at one place and cooled at another, a thermo-electric current results. a knot is tied in it and it is heated on one side of the knot, a current results. In other words, any change in the molecular aggregation of the metal at the points will produce a current of electricity. An attempt has been made to apply this principle to practical use; for instance, a furnace has been constructed with thermal junctions set in its walls, with the other set of junctions outside. It is possible, by having a large number of junctions in the pot of a furnace, to produce an electric light. The heat of an ordinary house furnace is sufficient to afford a supply of electricity for the house bells and for running a sewing-machine, and, indeed, for a certain amount of electric lighting; but no practical way has been devised of preserving the junctions from injury due to expansions and contractions. The energy in the coal we use in ordinary furnaces is amply sufficient both to heat and light our houses if it could be economically transformed. It has been proposed by various investigators to employ the electro-motive force developed between iron and carbon which are immersed in a hot alkali. The carbon wastes away or is consumed under the action of a current of air which is forced through the melted alkali. If we could break up the carbon into its constituents, or ions, as we can water, we could produce electricity directly from coal. The use of the thermal junction seems at present the only way worthy of consideration by means of which we can produce electricity direct from coal without the use of a steam engine. The electro-motive force produced by heating the junction of two metals, however, is very small even with the best combination-bismuth and antimony-far less than with the employment of copper and zinc in the ordinary voltaic cell. Both with the employment of two different metals and in the case of an electrolyte, outside the evidence of the increased molecular activity at the surface of the metals and an increase of heat throughout the liquid of the cell, there is no evidence of any flow in one direction or the other, or of any commotion in the liquid. At a sufficient distance from the plate of the battery the liquid is in repose. A beam of light sent through it is not absorbed by the liquid more or less when the current flows. A photograph taken by means of light which has passed through a layer of liquid traversed by an electric current differs in no respect from one taken through an equal layer of the liquid not traversed by a current. No effect of strain can be observed in the liquid. Faraday examined this question carefully by means of the polarization of light. There are certain substances which confine the vibrations of light in one plane. We can think of such substances as similar in internal arrangement to a Venetian blind with its horizontal slats. If we should attempt to thrust a hand moving to and fro through the blind, it would be possible to do so only when the hand is moving to and fro horizontally. Now the vibrations of a ray of light are in all directions, so that a section of the ray would be a star-shaped figure. Only those vibrations which are in the horizontal plane would therefore pass through the blind. When this light emerges from such an arrangement of matter, therefore, it is said to be polarized, and it can be tested by another substance exactly similar to that which polarizes the ray; for if the analyzer is turned so that its molecular arrangement, in regard, for instance, to a horizontal plane, is the same as the polarizer-that is, if two Venetian blinds are hung vertically one behind the other the hand moving horizontally can be made to pass through both. If one blind, however, is not vertical, and the slats are at right angles with the other, the movements of the hand will be intercepted. Two Nicol prisms form such an arrangement. Although they are perfectly limpid, like two pieces of glass, yet when we look through both and turn one we shall find that all the light is cut off in a certain position of the prism next the eye, which is called the analyzer. When a piece of glass submitted |