If one end of an elastic string be made to oscillate to and fro a series of waves travels along the string. If a large number of these strings are attached to an oscillating point and stretch out in all directions, the waves will travel along each string, and if the strings are all exactly alike will travel at the same speed along all of them. Any particular crest of a wave will thus at any instant lie on the surface of a sphere whose centre is the oscillating point. If now we imagine that the strings are so numerous that they fill the whole of the space we have a conception of the transmission of waves by an elastic solid. +Electromagnetic Waves.+--Since Maxwell published {18} his electromagnetic theory in 1873 it has been universally held that heat and light consist of electro-magnetic waves. These are by no means so easy to imagine as the elastic waves, as there is no actual movement of the medium; an alternating condition of the medium is carried onward, not an oscillation of position. When a stick of sealing-wax or ebonite is rubbed with flannel it becomes possessed of certain properties which it did not have before. It will attract light pieces of paper or pith that are brought near to it, it will repel a similar rubbed piece of sealing-wax or ebonite and will attract a rod of quartz which has been rubbed with silk. The quartz rod which has been rubbed with silk has the same property of attracting light bodies which the ebonite and sealing-wax rod has, but it repels another rubbed quartz rod and attracts a rubbed ebonite or sealing-wax rod. +Positive and Negative Electrification.+--The ebonite is said to be negatively electrified and the quartz positively electrified. When the two rods, one positively and the other negatively electrified, are placed near to one another, we may imagine the attraction to be due to their being joined by stretched strings filling up all the space around them. If a very small positively electrified body be placed between the two it will tend to move from the quartz to the ebonite, _i.e._ in the direction of the arrows. [Illustration: FIG. 8.] +The Electric Field. Lines of Force.+--The space {19} surrounding the electrified sticks in which the forces due to them are appreciable is called the electric field, and the direction in which a small positively electrified particle tends to move is called the direction of the field. The lines along which the small positive charge would move are called lines of force. The conception of the electric field as made up of stretched elastic strings is, of course, a very crude one, but there is evidently some change in the medium in the electric field which is somewhat analogous to it. [Illustration: FIG. 9.]