These

this thread remain covered with gold after the passage of the electricity parts are at equal distances, or distances which are multiples of one another, and these distances vary with the intensity of the discharge. The gold which is seen still adhering to the silk thread must have been less agitated than that which has undergone volatilization; here, therefore, were the points of minimum agitation.

By substituting for the threads of gilt silk strips of tin, maintained in place between two glasses, analogous effects are obtained. Under the influence of the discharge the particles undergo fusion, impressing their traces on the glasses. We then discover an innumerable quantity of small and very fine filaments, which group themselves around the strip like the filings of iron around the poles of a magnet.

Sometimes, from the point of the explosion shoot in all directions rectilinear trails of a copper color and semi-transparent, forming a sort of divergent rays constituted by metallic particles reduced to a state of extreme tenuity by the discharge. Examined with the microscope these trails are of a structure truly exquisite, and are disposed with admirable symmetry on each side of the central band, along the length of which are to be observed points of maximum and of minimum intensity. A series of figures thus obtained are annexed: different systems of linear filaments, of circular figures, of elliptical figures, continuous and discontinuous, affording a beautiful exemplification of the coexistence in the same body of several kinds of movement, preserving their individual character in all its integrity.

It is quite surprising to see that certain of the electric rays arriving on the edge of the glasses, between which the strips of tin are confined, rebound on the surface of separation of the two mediums, and are reflected in making an angle of reflection equal to the angle of incidence.

Is it possible, at sight of these figures, not to admit that the metallic vapor thus distributed has been subjected to a vibratory agitation, to a species of undulatory movement? Nothing can be more beautiful than these fine and granulated curves which cross each other in a thousand ways on the interior faces of the glass, and along which are disposed very minute metallic particles, visible, however, to the naked eye when looked at on being transferred to the diffused light of heaven. We are forced by this to conclude that the vibratory movement is transmitted by waves which increase as they are propagated, which are reflected to the surface of separation of the two mediums, and which reveal to the eye the route traversed by the reflected wave.

The discoveries of Faraday on the relation of magnetism to light, on the illumination of magnetic lines, and on diamagnetism, the sounds which, according to the obserations of Wertheim, of De la Rive, as well as my own, accompany temporary magnetism and interrupted currents, are phenomena of the same order, tending to prove that the forces of matter are in a reciprocal dependence so intimate that they are capable of producing one another by equivalent quantities.

But the intervention of determinate conditions for the production of such or such a kind of movement is necessarily subordinate to the pre-existence, in every species of matter, of a molecular agitation which, under the influence of the different circumstances we have passed in review, may take any character whatever, pass from one species of movement to another, or be added to other movements which may exist at the same time in the same body, and thus give rise to luminous, calorific, electro-magnetic or chemical phenomena, or to all these movements at once.

Thus we are led to admit, in the atoms of ponderable bodies, and with still stronger reason in the particles of imponderable matter, an interior movement, a primordial property-that is to say, a general property which has always existed.

The intensity of this interior agitation varies with the nature and dimensions of the body, with the volume and density of the atoms, with their individual

separation within the limits of their appropriate agitation. The nature of this movement must vary with the nature of the bodies, each of them having a particular movement which constitutes its normal state and impresses on it a special character. We can comprehend, therefore, that when two bodies are placed in contact, there must be a communication, a transference of their oscillatory and rotary movements, with loss on the one side and gain on the other. Let us take an example: suppose the initial molecular movement of copper to be more intense than that of zinc; if the copper comes into collision with the zinc it will lose, according to the laws of mechanics, a part of its inherent force equal to that which the zinc acquires. We conclude hence that the copper, considered as the colliding body, is negative, while the assailed zinc is positive.

Would it be rational to suppose that mechanical laws are true when perceptible masses are in question, and yet control neither atoms nor the particles of subtile matter? If it be true that there is nothing absolutely large nor absolutely little in creation, dimensions can never constitute a difference in relation to the forces which produce and the laws which govern phenomena. We consider then the action of light on the object which it renders visible, and that of heat which elevates its temperature, as something of an analogue to the sympathetic vibration of a chord at the moment when the sound of another chord traverses the air. After an analogous manner, all actions at a distance and electro-magnetic inductions present themselves as natural consequences of one same mechanical principle, conformably to the general economy of the system of the universe.

One of the most important consequences of this study is that sound, light, heat, electricity, are not real entities, but simply modes of action and movements of matter communicated to our brain through the medium of the nerves. The human organism may therefore be considered as an elastic system, of which the different parts receive the shocks of elastic mediums, and vibrate in unison with a certain number of undulations each of which produces its complete effect independently of others. Here science stops! The mysterious influence of matter on mind we must be content to regard as a secret which will yet be long hidden from us.

Be it remarked that in the order of animated beings there are those which might be clustered in myriads on the point of a needle, and which live but a few seconds; and yet to them their life seems long and complete, and during those few seconds they may have a perception of millions of shocks such as those which constitute heat. There may very probably exist other beings which can never have a perception of a complete undulation of the ether, and which scarcely distinguish a feeble portion of one; in fine, even these elementary portions will always appear too complex to certain other beings which perceive nothing but the individual movements and displacements of atoms. These considerations enable us to appreciate how very limited are our senses, for, as has just been seen, the human race occupies but a few degrees of the indefinite scale of sensibility. Herein, perhaps, is one of the principal motives why man is often diverted from truth, even while seeking it; the instinctive repugnance which he feels to meditate upon simple and common phenomena, and to extend the laws which govern the domain that he is able to explore beyond the limits of his own sensibility, has led him to imagine complicated systems and to have recourse to the hypothesis of mysterious principles.

These reflections lead us to another important consequence, namely, that if the philosophers who have considered our world as an atom in creation are right, under a certain point of view, we ought yet to recognize, with other philosophers, that each atom is a world. Each atom possesses a proper activity, and is the seat of all the natural forces; these never manifest themselves outside of matter,

[* The author is not warranted by the present state of science to include electricity in the same class with sound, light, and heat. In statical electrical repulsion, which manifests itself at a great distance, we have a phenomenon entirely unlike any effect exhibited by sound, light, or heat.-J. H.]

and matter without these forces is a pure abstraction. If there were but a single atom in the universe, it would always remain identical with itself; but the tendency of each atom to maintain its original activity is continually countervailed by the action of other atoms and the undulations of the other. We are henco forced to admit that inertia and activity are two facts inseparable from matter. We all know that in bodies which revolve upon an axis, inertia is manifested by a tendency of the particles to withdraw from that axis, a tendency which varies in intensity with the velocity of the rotation. We may equally admit that in the molecular groups which constitute bodies, the force antagonistic to molecular gravitation is nothing else than the centrifugal force due to a rotary movement of each of the molecules around the centre of gravity of the group, and variablo with the velocity of the molecules themselves, a velocity which is proportional to their temperature.

Since heat generates electricity, either directly or indirectly, and electricity heat; since all other forces are transformed among themselves and are resolved into different forms of movement, we must thence necessarily conclude that any phenomena whatsoever can only proceed from the varied evolutions of the primordial force which the Creator has given to matter, of which it is the active and inseparable principle; as to the nature of that force it will, perhaps, always be hidden to the humani ntellect.

RADIATION.

BY JOHN TYNDALL.*

I. VISIBLE AND INVISIBLE RADIATION.

Between the mind of man and the outer world are interposed the nerves of the human body, which translate, or enable the mind to translate, the impressions of that world into facts of consciousness and thought,

Different nerves are suited to the perception of different impressions. We do not see with the ear, nor hear with the eye, nor are we rendered sensible of sound by the nerves of the tongue. Out of the general assemblage of physical actions, each nerve, or group of nerves, selects and responds to those for the perception of which it is specially organized.

The optic nerve passes from the brain to the back of the eye-ball and there spreads out, to form the retina, a web of nerve filaments, on which the images of external objects are projected by the optical portion of the eve. This nerve is limited to the apprehension of the phenomena of radiation, and, notwithstanding its marvellous sensibility to certain impressions of this class, it is singularly obtuse to other impressions.

Nor does the optic nerve embrace the entire range even of radiation. Some rays, when they reach it, are incompetent to evoke its power, while others never reach it at all, being absorbed by the humors of the eye. To all rays which, whether they reach the retina or not, fail to excite vision, we give the name of invisible or obscure rays. All non-luminous bodies emit such rays. There is no body in nature absolutely cold, and every body not absolutely cold emits rays of heat. But to render radiant heat fit to affect the optic nerve a certain temperature is necessary. A cool poker thrust into a fire remains dark for a time, but when its temperature has become equal to that of the surrounding coals it glows like them. In like manner, if a current of electricity of gradually increasing strength be sent through a wire of the refractory metal platinum, the wire first becomes sensibly warm to the touch; for a time its heat augments, still, however, remaining obscure; at length we can no longer touch the metal with impunity; and at a certain definite temperature it emits a feeble red light. As the current augments in power the light augments in brilliancy, until finally the wire appears of a dazzling white. The light which it now emits is similar to that of the sun.

By means of a prism Sir Isaac Newton unravelled the texture of solar light, and by the same simple instrument we can investigate the luminous changes of our platinum wire. In passing through the prism all its rays (and they are infinite in variety) are bent or refracted from their straight course; and as different rays are differently refracted by the prism, we are by it enabled to separate one class of rays from another. By such prismatic analysis Dr. Draper has shown that when the platinum wire first begins to glow the light emitted is a pure red. As the glow augments the red becomes more brilliant, but at the same time orange rays are added to the emission. Augmenting the temperature still further, yellow rays appear beside the orange; after the yellow, green rays are emitted; and after the green come, in succession, blue, indigo, and violet rays.

*The Rede Lecture, delivered in the senate house, before the university of Cambridge, England, May 16, 1865.

To display all these colors at the same time the platinum wire must be whitehot; the impression of whiteness being in fact produced by the simultaneous. action of all these colors on the optic nerve.

In the experiment just described we began with a platinum wire at an ordinary temperature, and gradually raised it to a white heat. At the beginning, and before the electric current had acted at all upon the wire, it emitted invisible rays. For some time after the action of the current had commenced, and even for a time after the wire had become intolerable to the touch, its radiation was still invisible. The question now arises, What becomes of these invisible rays when the visible ones make their appearance? It will be proved in the sequel that they maintain themselves in the radiation; that a ray once emitted continues to be emitted when the temperature is increased, and hence the emission from our platinum wire, even when it has attained its maximum brilliancy, consists of a mixture of visible and invisible rays. If, instead of the platinum wire, the earth itself were raised to incandescence, the obscure radiation which it now emits would continue to be emitted. To reach incandescence the planet would have to pass through all the stages of non-luminous radiation, and the final emission would embrace the rays of all these stages. There can hardly be a doubt that from the sun itself rays proceed similar in kind to those which the dark earth pours nightly into space. In fact, the various kinds of obscure rays emitted by all the planets of our system are included in the present radiation of the sun.

The great pioneer in this domain of science was Sir William Herschel. Causing a beam of solar light to pass through a prism, he resolved it into its colored constituents; he formed what is technically called the solar spectrum. Exposing thermometers to the successive colors he determined their heating power, and found it to augment from the violet or most refracted end to the red or least refracted end of the spectrum. But he did not stop here. Pushing his thermometers into the dark space beyond the red, he found that, though the light had disappeared, the radiant heat falling on the instruments was more intense than that at any visible part of the spectrum. In fact, Sir William Herschel showed, and his results have been verified by various philosophers since his time, that besides its luminous rays, the sun pours forth a multitude of other rays more powerfully calorific than the luminous ones, but entirely unsuited to the purposes of vision.

At the less refrangible end of the solar spectrum, then, the range of the sun's radiation is not limited by that of the eye. The same statement applies to the more refrangible end. Ritter discovered the extension of the spectrum into the invisible region beyond the violet; and, in recent times, this ultra-violet emission has had peculiar interest conferred upon it by the admirable researches of Professor Stokes. The complete spectrum of the sun consists, therefore, of three distinct parts: 1st, of ultra-red rays of high heating power, but unsuited to the purposes of vision; 2d, of luminous rays which display the following succession of colors: red, orange, yellow, green, blue, indigo, violet; 3d, of ultra-violet rays which, like the ultra-red ones, are incompetent to excite vision, but, unlike them, possess a very feeble heating power. In consequence, however, of their chemical energy, these ultra-violet rays are of the utmost importance to the organic world.

II. ORIGIN AND CHARACTER OF RADIATION.

THE ETHER.

When we see a platinum wire raised gradually to a white heat and emitting in succession all the colors of the spectrum, we are simply conscious of a series of changes in the condition of our eyes. We do not see the actions in which these successive colors originate, but the mind irresistibly infers that the appearance of the colors corresponds to certain contemporaneous changes in the wire.