I have here a number of aluminium discs, the size of sixpences, stamped out of thin metal, and these are arranged in a sort of semi-cylindrical trough between two terminal screws, so that the discs are very lightly pressed together. Under these circumstances the pile of metal discs is not a conductor, and it will not pass the electric current from a battery which is joined up in series with an electric bell and the pile of discs (see Fig. 70). Supposing, however, that I make an oscillatory spark in proximity to this pile of metal discs, as I can do by taking the discharge from a large Leyden jar near it; the pile of discs at once becomes a conductor; the electric current from the battery can then pass through it, and the bell rings. Such an arrangement has been named by Sir Oliver Lodge a coherer, because, under the action of the oscillatory spark, the discs cohere or stick together. We can separate the discs by giving them a sharp rap, and then the operation can be again repeated.

A much more sensitive arrangement can be made by taking a small box of wood through the bottom of which pass two nickel wires which are parallel to one another, but not in contact. In this box is placed a small quantity of very finely powdered metallic nickel or nickel filings, and if the quantity of these filings is properly adjusted, it is possible to make an arrangement which possesses the property that there is no conductivity between the two nickel wires under ordinary circumstances, but that they become conductively connected to one another the moment an oscillatory electric spark is made in the neighbourhood. We shall speak of this contrivance as an electric wave indicator, and we shall employ it in subsequent experiments to enable us to detect the presence of an electric wave.

We must then return for a moment to the consideration of the production of electrical oscillations in circuits of various kinds. I trust it has been made plain to you that if two metallic surfaces, separated by a non-conductor such as air or glass, are acted upon by an electromotive force, the non-conductor becomes electrically strained. Another way of stating this is to say that a positive charge of electricity exists on one metal surface, and a negative charge on the other. The only objection which can be raised to expressing the facts in this manner is that it fastens attention rather upon the conductors than upon the insulator, which is the real storehouse of the energy. If these two metal surfaces are then connected together by a conductor of low resistance, the charges disappear by a series of oscillations, and the result is an electric current in the conducting circuit connecting the plates, which rushes backwards and forwards in the circuit, but gradually diminishes in strength until it completely dies away. You may picture to yourselves the electrical effect as analogous to the following experiment with two air-vessels: Supposing we have two strong steel bottles, into one of which we compress a quantity of air, and in the other we make a vacuum by pumping out nearly all the air. These vessels would correspond with two conductors, one charged with positive electricity and the other with negative. Imagine these vessels connected by a wide pipe in which is placed a tap or valve, which can be opened suddenly so as to permit the air to rush over from the full vessel to the empty one. If this is done, it is a matter of experience that the equality of pressure between the two vessels is not at once established, but in virtue of the inertia quality of the air, it only takes place after a series of oscillations of air in the pipe. In rushing over from the full vessel to the empty one the air, so to speak, overshoots the mark, and the state of the vessels as regards air-pressure is exactly interchanged. The air then rushes back again, and it is only after a series of to-and-fro movements of the air in the pipe that an exact equality of pressure in the two vessels is attained.

The electrical actions which take place in connection with an electric discharge between two conductors, one of which is charged positively and the other negatively, are exactly analogous to the above-described experiment with two air-vessels, one of which has air in it under compression, and the other has had the air removed from it. You will notice, however, that the oscillations of the air in the pipe in the air-vessel experiment depend essentially upon the fact that air is a substance which has inertia, or mass, and you will naturally ask what is it which has inertia, or its equivalent, in the electrical experiment? The answer to this question is as follows: Every electric circuit has a quality which is called inductance, in virtue of which an electric current cannot be started in it instantly, even under any electromotive force, and conversely when the current is started it cannot be immediately brought to rest. From the similarity of this quality of the circuit to the inertia of ordinary material substances, it has been sometimes called the electric inertia of the circuit. The word “inertia” really means inactivity, or laziness, but the term as used in mechanics implies something more than mere inactivity. It involves the notion of a persistence in motion when once the body is set moving.

When a material substance is in motion it possesses energy, and has the power of overcoming up to a certain point resistance to its motion. This energy-holding power, or capacity for storing up energy of motion, which is characteristic of all material substances, is a consequence of their inertia. The fact is otherwise expressed by stating that the mass of a material substance is one element in the production of energy of motion.

An electric current in one sense resembles a moving substance, for it is an exhibition of energy in association with matter. The current-energy is measured by the product of two factors: one is half the square of the current-strength, and the other is the inductance of the circuit. The analogy between the two cases may be more exactly brought out by pointing out that the energy of motion of a moving body is measured by the product of its mass and half the square of its velocity. Hence it follows that the power of overcoming resistance, or, in other words, of doing useful work or mischief, which is possessed by a heavy body in motion is proportional, not simply to its speed, but to the square of its speed. If a bullet, moving with a certain speed, can just pass through one plank 1 inch thick, then, when moving with twice the speed, it will pass through four such planks, and if moving with three times the speed, through nine planks of equal thickness. The energy of an electric current is similarly measured by the product of the inductance of the circuit and half the square of the current-strength. In the same or equal circuits two currents, the strengths of which are in the ratio of 1 to 2, have energies in the ratio of 1 to 4. The greater, therefore, the inductance of an electric circuit, the greater is the tendency of an electric current set flowing in it to run on after the electromotive force is withdrawn. The inductance of a circuit is increased by coiling it into a coil of many turns, and decreased by stretching it out in a straight line.

The important idea to grasp in connection with this part of the subject is that, just as there are two forms of mechanical energy, viz. energy of mechanical strain and energy of motion, so also there are two forms of electrical energy, viz. energy of electro-static strain and electric-current energy.

If, for instance, we bend a bow or extend a spring, this action involves the expenditure of mechanical energy, or work, and the energy so spent is stored up as energy of strain, or, as it is called, distorsional energy in the distorted bow or spring. When, however, the bow communicates its energy to the arrow or the spring to a ball, and so sets these in motion, we have in the flying arrow or ball a store of energy of motion. If a slip of steel spring is fixed at one end, and then set in vibration, we have a continual transformation of energy from the motional to the distorsional form. At one moment the spring is moving violently, and at the next it is bent to its utmost extent; and these states succeed each other. The store of energy in the vibrating spring is, however, gradually frittered away, partly because the continual bending of the steel heats it, and this heat dissipates some of the energy; but also because the spring, if vibrating quickly enough, imparts its energy to the surrounding air, and creates air waves, which travel away, and rapidly rob the vibrating spring of its stock of energy.

In a precisely similar manner all electrical oscillation effects depend upon the fact that electric energy can exhibit itself in two forms. In one form it is electro-static energy, or energy of electric strain. In this form we have it when we charge a Leyden jar. The glass is then, as explained, in a state of electrical strain, and its condition is analogous to that of a stretched spring. The same holds good when we have two conductors insulated from each other in air. We have then an electrical strain in the air. It is important, however, to notice that, since a perfect vacuum can support electric strain, it follows that, in the cases where air or glass constitute this non-conductor, or dielectric, of a condenser, the whole of the energy cannot be stored in the material substance, the glass or the air. The real storehouse of the energy is the æther, as modified by the presence of the ordinary matter in the same place.

When we discharge the Leyden jar or condenser, the electro-static energy in the dielectric disappears, and we obtain in its place an electric current in the connecting conductor; and this, as described, is an exhibition of energy in another form. If the resistance of the connecting conductor is small, then we have electrical oscillations established which consist in an alternate transformation of the energy from an electro-static form to the electric-current form.

At each oscillation some energy is frittered away into heat in the conductor, and if the conductor and condenser have a special form, energy may be rapidly removed from the system by the electric waves which are formed in the surrounding æther or dielectric. These waves consist in the propagation through the medium of lines of electric strain, just as an air wave consists in the propagation through the air of regions of air-compression, or a water wave consists in the propagation of an elevation on the surface.

Returning again to the discussion of the production of electrical oscillations, it is necessary to consider a little more in detail the manner in which we can create an electrical oscillation in what we have called an open electric circuit. Let me begin with an experiment, and it will then be easier for you to understand the particular points to be explained.

Before me are two long brass rods, each of them about 5 feet in length, and the ends of these rods are provided with polished brass balls (see Fig. 71). The rods are placed in one line and supported on pieces of ebonite, and are so fixed that the two balls are separated from one another by a space of about ¹⁄₄ inch. The two rods constitute, therefore, two insulated conductors. These rods are connected by coils of wire with the terminals of an instrument called an induction coil, which I shall not stop to describe, but which you may regard as a kind of electrical machine for producing electromotive force. If we set the induction coil in action, it creates between its terminals an intermittent but very powerful electromotive force, which gradually increases up to a certain value, at which it breaks down the conductivity of the air-gap between the two balls. Let us think carefully what happens as the electromotive force of the induction coil is increasing. One of the rods is in effect being electrified with positive, and the other with negative, electricity, and these charges are increasing in magnitude. The two rods constitute, as it were, the two coated surfaces of a kind of Leyden jar, or condenser, of which the surrounding air is the non-conductor. Accordingly, by all that has been previously explained, you will easily understand that there is an electric strain in the air which exists along certain lines, called lines of electro-static strain, and this state in the air is exactly similar to the condition in which the glass of a Leyden jar finds itself when the jar is charged. If we were to delineate the direction of this electric strain by lines drawn through the space around the rods, we should have to draw them somewhat in the fashion represented by the dotted lines in Fig. 71. As the electrical state of the rods gradually increases in intensity, a point is reached at which the air between the balls can no longer maintain this strain, and it breaks down and passes into a conductive condition. The state of affairs round the rods is then similar to that of a Leyden jar being discharged. An electric current is produced across the air-gap, moving from one rod to the other, and the intensely heated air in between the balls is visible to us as an electric spark. This spark, if photographed, would be found to be an oscillatory spark. The electric current in the rods cannot continue indefinitely: it gradually falls off in strength, but as it flows it creates in the space around the rods an electric strain which is in the opposite direction to that which produced it, although taking place along the same lines.

After a very short time, therefore, the electrical conditions which existed at the moment before the air broke down are exactly reproduced, only the direction of the strain is reversed. In other words, the rod which was positively electrified is now negatively, and vice versâ. Then this state of strain again begins to disappear, producing in the rod an electric current, again in the reverse direction; and so the energy, which was originally communicated to the space round the rods in the form of an electric strain, continually changes its form, existing at one moment as energy of the electric current passing across the spark gap, and the next moment as energy of electric strain. We may ask why this state of things does not continue indefinitely, and the answer to that question is twofold. First because the rods possess a property called electrical resistance, and this acts towards the electric current just as friction acts towards the motion of material substances; in other words, it fritters away the energy into heat. So at each reversal of the electric current in the rod a certain quantity of the original store of energy has disappeared, due to the resistance.

There is, however, a further and more important source of dissipation of energy, and this is due to the fact that an electrical oscillation of this kind taking place in a finite straight circuit, or, as it is called, an open electric circuit, creates in the space around an electric wave. The rapid reversal of the electric strain in the air results in the production of an electric wave, just as in the case of an explosion made in air, the rapid compression of the air results in the production of an air wave. It is not easy for those who come to the subject for the first time to fully grasp the notion of what is implied by the term “an electric wave.”

In the first lecture, you will perhaps remember, I pointed out that the production of a wave in a medium of any kind can take place if the medium possesses two properties. In the first place, it must elastically resist some change or distortion, and, in the second place, when that distortion is made it must tend to disappear if the medium is left to itself, and in so doing the displacement of the medium must overshoot the mark and be reproduced in the opposite direction, owing to some inertia-like quality or power of persistence in the medium.

It would lead us into matters beyond the scope of elementary lectures if we were to attempt to summarize all the evidence which exists tending to show that the phenomena of electricity and magnetism must depend upon actions taking place in some medium called the electro-magnetic medium. All the great investigators at the beginning of the last century, when electrical and magnetic phenomena were beginning to be explored, came to this conclusion, and in the writings of Joseph Henry, of Ampère, and of Faraday we find references again and again to their conviction that the phenomena of electricity imply the existence of a medium exactly in the same way as do the phenomena of optics. It is only, however, in recent years that we have had evidence before us, some of which will be reviewed in the next lecture, which affords convincing proof that the luminiferous æther and the electro-magnetic medium must be the same. The consideration of the simplest electrical effects is sufficient to show that, if this medium exists, it possesses at least two properties, one of which is that it offers an elastic resistance to the production of electric strain in it by means of electromotive force. A question which is sure to arise in the minds of those who consider this subject carefully is, What is the nature of an electric strain? And the only answer which we can give at the present moment is that we must be content to leave the question unanswered. We do not know enough yet about the mechanical structure of the electro-magnetic medium, or æther, to be able to pronounce in detail on the nature of the change we call an electric strain. It may be a motion of some kind, it may be a compression or a twist, or it may be something totally different and at present unthinkable by us, but, whatever it is, it is some kind of change which is produced under the action of electromotive force, and which disappears when the electromotive force is removed.

Clerk-Maxwell, to whom we owe some of our most suggestive conceptions of modern electricity, coined the phrase electric displacement to describe the change which we are here calling an electric strain. One essential element in Maxwell’s theory of electricity is that an electric strain or displacement, whilst it is being made or whilst it is disappearing, is in effect an electric current, and it is for that reason sometimes spoken of as a displacement current. We have seen that every electric circuit possesses a quality analogous to inertia, that is to say, when a current is produced in it it tends to persist, and it cannot be created at its full value instantly by any electromotive force.

Just as we cannot, at the present moment, pronounce in detail on the real nature of electric strain, so we cannot say whether that quality which we call inductance of a circuit is dependent upon a true inertia of the electro-magnetic medium or on some entirely different quality more fundamental.

It may be remarked, in passing, that there is a strong tendency in the human mind to seek for and be satisfied with what we called mechanical explanations. This probably arises from the fact that the only things which we can picture to ourselves in our minds very clearly are movements or changes in relative positions. If we can in imagination reduce any physical operation to some kind of movement or displacement taking place in some kind of material, we seem to arrive at a kind of terminus of thought which is more or less satisfactory. We invariably aim at being able to visualize an operation concerning which we are thinking, and it requires some mental self-control to be able to content ourselves with a general expression which does not lend itself readily to visualization. There are plenty of indications, however, that this mental method of procedure, and this endeavour to reduce all physical operations to simple mechanics and to movements of some kind, may in the end be found to be unjustifiable; and the time may arrive when we may be more satisfied to explain mechanical operations in terms of electrical phraseology rather than aim at dissecting electrical effects into mechanical operations. Thus, for instance, instead of speaking of electric inertia, it may be really more justifiable to speak of the inductance of ordinary matter. The final terms in which we endeavour to offer ourselves an explanation of physical events are in all probability very much a matter of convenience and custom. We may, however, for present purposes rest content by thinking of the electro-magnetic medium as in some sense like a heavy elastic substance which is capable of undergoing some kind of strain or distortion, the said strain relieving itself as soon as the distorting force is withdrawn; but, in addition, we must think of the medium as possessing a quality analogous to inertia, so that as distortion vanishes it overshoots the mark, and the medium only regains its state of equilibrium at the particular point considered, by a series of oscillations or alternate distortions, gradually decreasing in amount. Any medium which possesses these two qualities has, in virtue of explanations already given, the property of having waves created in it, and what we mean by an electric wave is a state of electric strain which is propagated through the æther with a velocity equal to that of light, just as an air wave consists of a state of compression which is propagated through the air with a velocity of 1100 feet a second.

To sum up, we may then say that whenever rapid electrical oscillations are created in open circuit, such as the two rods above described, the arrangement constitutes a device for creating an impulse or effect in the surrounding space called an electric wave in the æther or electro-magnetic medium; just as an organ-pipe or piano-string or other musical instrument constitutes a device for creating waves in the air by means of mechanical oscillations. The existence of these electric waves, and their transference to distant places, can be rendered evident by their action as already described upon finely powdered metals. An apparatus which shows this effect very well is now arranged before you. At one end of the table I have a pair of rods connected to an induction coil, constituting a Hertz radiator, the action of which has just been described. At the other end of the table are two similar long rods, but their inner ends are connected to two small plates of silver, which form the sides of a very narrow box, and between these plates is placed a very small quantity of metallic powder. The construction of this little box is as follows: A thin slip of ivory has a little gap cut out of it (see Fig. 72), and on the two sides of this slip of ivory are bound two silver plates bent in the shape of the letter L, forming, therefore, a very narrow box with silver sides. The two silver plates are connected to the two long rods. As already explained, the metallic filings or finely powdered metal are not in their ordinary condition an electric conductor. Accordingly, if we connect to one of the silver plates one terminal of a battery joined in series with an electric bell, the other end of the bell being connected to the second silver plate, this battery cannot send a current through the bell, because the circuit is interrupted by the non-conductive metallic powder in the little box. Supposing, then, that we cause a spark to pass between the balls of the radiator, and start an electric wave. When this electric wave reaches the long rods connected to the receiving arrangement, it sets up in these rods a sudden electromotive force, and this electromotive force, as already explained, if of sufficient magnitude, causes the loose mass of metallic filings to pass from a non-conductive to a conductive condition. At that moment, therefore, the battery is able to send an electric current through the bell, and to cause it to ring. We can, however, stop the ringing by giving the little box containing the metallic filings a tap, which separates them from one another and interrupts the electric conductivity. The function of the two rods connected with the receiver is not quite the same as the function of the two rods connected to the radiator. In order to create a vigorous electric wave, we must have a radiator which possesses what is called considerable electric capacity, and also considerable inductance, and we can only do this in general by using long rods. On the other hand, at the receiving end the efficacy of the rods is due to the fact that they, so to speak, add together the electric strain taking place over a considerable distance; in other words, the electromotive force which is set up in the receiving circuit is dependent on the length of the rods. The longer, therefore, these rods, the greater is the distance at which we can obtain the effect which is shown to you with a given spark-length.

One point it is important to notice, and that is, that the rods of the receiver must be parallel to the rods of the radiator if we are to obtain any effect at a distance. If we turn the rods of the radiator round so that they are at right angles to those of the receiver, you see that no sparks produced at the radiator balls cause the bell in connection with the receiver to ring. The reason for this is because the electric strain, which is propagated out into the space, exists in a direction parallel to the radiator rods all along a line drawn perpendicular to the rods through the spark-gap. The receiver rods will not have electromotive force produced in them by this travelling line of electric strain unless they are parallel to its direction.

It is to be hoped that the above explanations have afforded indications of what is meant by an electric wave. On the other hand, there may be many who find it exceedingly difficult to derive clear ideas when the subject is presented to them clothed in such general terms as we have been obliged to use.

It may assist matters, therefore, if, before concluding this chapter, a word or two is said on the subject of recent investigation into the inner mechanism of an electric current and an electric strain. It is impossible to do this, however, without making mention, in the briefest possible way, of modern researches into the constitution of matter. If you can imagine yourselves furnished with a little crystal of ordinary table salt, chemically called chloride of sodium, and the means of cutting it up under an immensely powerful microscope, you might go on dividing it up into smaller and smaller pieces. If this process could be continued sufficiently far, we should ultimately obtain a very small fragment of salt, which, if still further divided, would yield two portions of matter not alike and not salt. This smallest possible portion of salt is called a molecule of sodium chloride. Chemical facts teach us that this molecule is made up of two still smaller portions of matter, which are called respectively atoms of chlorine and sodium.

We have good reason to believe that all solids, liquids, and gases are composed of molecules, and these are built up of atoms, few or many.

In the case of some substances, such as salt, the molecule is very simple and composed of two atoms. In other substances, such as albumen or white of egg, the molecules are very complicated and composed of hundreds of atoms. The word atom means something which “cannot be cut,” and until comparatively recent time the opinion was held that atoms of matter were the smallest indivisible portions of matter which could exist.

More than twenty-five years ago, Sir William Crookes showed, by numerous beautiful experiments, that in a vacuum tube, such as you have seen used to-day, a torrent of small particles is projected from the negative terminal when an electric current is passed through the tube. This stream of particles is called the cathode stream, or the cathode radiator. Within recent times, Sir Joseph Thomson has furnished a proof that this cathode stream consists of particles very much smaller than chemical atoms, each particle being charged with negative electricity. These particles are now called corpuscles, or electrons.

It has been shown that these electrons are constituents of chemical atoms, and when we remove an electron from an atom we leave the remainder positively electrified. An atom can, therefore, by various means be divided into two portions of unequal size. First, a very small part which is charged with negative electricity, and, secondly, a remaining larger portion charged with positive electricity. These two parts taken together are called ions, i.e. wanderers. The negative ions, or electrons, or corpuscles, taken together constitute what we call negative electricity, and up to the present no one has been able to show that the corpuscle can be unelectrified. Hence the view has been expressed that what we call electricity is a kind of matter, atomic in structure, and that these negative ions or corpuscles collectively are, in fact, the atoms of the electric fluid. These corpuscles can move freely in the interior of some solids, moving between the molecules of the solid just as little dogs can run about in and amongst a crowd of people in a street. In these cases the substance is called a conductor of electricity. In other substances the movement of the corpuscles is more restricted, and these constitute the various kinds of so-called non-conductors.

The corpuscle, being a small charge of negative electricity, creates in all surrounding space a state called electric force. It is impossible to expound this action more in detail without the use of mathematical reasoning of a difficult character. Suffice it to say that this electric force must be a particular condition of strain or motion in the æther. If the corpuscle is in rapid motion, it creates in addition another kind of strain or motion called magnetic force. The electric force and the magnetic force are related to each other in free space in such a manner that if we know the difference between the values of the electric force at two very near points in space, we are able to tell the rate at which the magnetic force is changing with time in a direction at right angles to the line joining these near points in space. We cannot specify in greater detail the exact nature of these states or conditions which constitute magnetic force and electric force, until we know much more than we do at present about the real nature of the æther. The two fundamental qualities of the æther are, however, its capacity to sustain these states we call the magnetic force and the electric force.

The electrons of which we have spoken not only give rise to electric and magnetic force when in movement, but they are themselves set in motion by these forces. Thus electric force at any point moves electrons placed at that spot, and an electron in motion is affected and has its direction of motion changed when magnetic force acts on it.

Leaving further remarks on the relations of atoms, electricity, and æther until the end of the last lecture, we may conclude the present one by explaining the manner in which the observed facts connected with a Hertz oscillator are interpreted in terms of this electron hypothesis of electricity.

Take the simple case of two long insulated metal rods separated by a spark-gap. The process of charging one rod positively and the other negatively consists in forcing more corpuscles, or negative ions or electrons, into one conductor and removing some from the other. Any source of electromotive force, such as a dynamo or induction coil, is, on this hypothesis, a sort of electron-pump, which pumps electrons from one conductor and puts them into another. One conductor, therefore, gains in electron-pressure, and the other loses.

The excess of electrons in one conductor endeavour to escape, and a strain is produced on the electrons or atoms in the surrounding dielectric or air, which may be looked upon as the effort of the electrons, more or less tethered to the atoms, to escape. The air in the spark-gap is subjected to the most intense strain, and when this reaches a certain intensity some of the electrons are torn away from their atoms, and the air in the gap then becomes a conductor. The excess of electrons in one conductor rush through the channel thus prepared, and this constitutes an electric current. The first rush carries over too many electrons to equilibrate the electron-pressure, and hence the first torrent of migrating electrons in one direction is followed by a back-rush in the opposite one, this again in turn by another in the original direction, and so the equality in the number of electrons in each conductor is only established after a gradually diminishing series of to-and-fro rushes of electrons across the air-gap. This action constitutes a train of electrical oscillations. At the same time that these operations are going on in and between the conductors, the electrons attached to the atoms of the air or other dielectric all around are being violently oscillated. These oscillations may not proceed to such an extent as to detach electrons from their atoms, but they are sufficient to create rapidly reversed electric and magnetic forces. It appears that the very rapid movement to and fro of an electron causes a wave in the æther, just as the rapid movement of the hand through water causes a wave in water, or the vibration of the prong of a tuning-fork creates a wave in the air.

The electron has some grip on the æther, such that the sudden starting or stopping of the electron makes a disturbance which we may popularly describe as a splash in the æther. Hence, if a large number of electrons are suddenly started into motion in the same direction, the effect on the æther is something like casting a multitude of stones on the surface of still water, or the simultaneous action of a number of small explosions in the air. Anything, therefore, which, so to speak, lets the electrons go gradually, or softens the first rush, is inimical to the production of a vigorous electric wave. On the other hand, anything which causes the first rush of electrons from one conductor to another across the air-gap to be very sudden is advantageous, and results in a powerful wave. Experience shows that the nature of the metal surfaces, whether polished or rough, has a great influence on the wave-making power of the radiator. If the spark-balls or surfaces are rough and not polished, it seems to tone down the violence of the first electron rush, and the wave-making power of the oscillator is not so great as if the balls are polished.

At this point, however, it will be best to withhold further discussion on points of theory until we have considered the facts to be brought before you in the next lecture, showing that the electric radiation manufactured by means of electric oscillations is only one variety of a vast range of æther waves, some forms of which are recognizable by us as light and radiant heat.