neath the cloudless heaven. By the heat-intensely luminous green bands. Here it ing of the gas and vapour a more vigorous is as if two forks only, of slightly different motion - -a greater amount of vis viva, as pitch, were vibrating. The length of the we term it is imparted to the ultimate waves which produce this first band is such particles of both. Will the oxygen impart that 47,460 of them, placed end to end, its motion as freely to the ether as the aque- would fill an inch. The waves which proous vapour? No. The difference between duce the second band are a little shorter; them in this respect is enormous. When it would take of these 47,920 to fill an inch. their temperatures are alike, the amount of In the case of the first band, the number heat radiated, or, in other words, of mole- of impulses imparted in one 'second to cular motion lost by the vapour in a given every eye which now sees it, is 577 millions time, is at least nine thousand times the of millions; while the number of impulses amount lost in the same time by the oxy- imparted in the same time by the second gen. So great is this power on the part of band is 600 millions of millions. I now the vapour, that I profoundly doubt the cast upon the screen before you the beauticorrectness of the simple formula assigned ful stream of green light from which these to it by chemists. The molecule of water bands were derived. This luminous stream represents a sound-board of vast dimensions, is the incandescent vapour of silver. You otherwise it never could generate waves of cannot by any possibility render that vathe extraordinary magnitude that experi- pour white hot. The rates of vibration of ence has proved it competent to produce.* its atoms are as rigidly fixed as those of two The pitch of a musical note depends upon tuning-forks; and to whatever height the the rapidity of its vibrations, or, in other temperature of the vapour may be raised, words, on the length of its waves. Now, the rapidity of its vibrations, and consethe pitch of a note answers to the colour of quently its colour, which wholly depends light. Taking a slice of white light from upon that rapidity, remain unchanged. the beam of an electric lamp, I cause that light to pass through an arrangement of prisms. It is decomposed, and we have the effect obtained by Newton, who first unrolled the solar beam into the splendours of the solar spectrum. At one end of this spectrum we have red light, at the other violet, and between those extremes lie the other prismatic colours. As we advance along the spectrum from the red to the violet, the pitch of the light-if I may use the expression-heightens, the sensation of violet being produced by a more rapid succession of impulses than that which produces the impression of red. The vibrations of the violet are not quite twice as rapid as those of the red; in other words, the range of the visible spectrum is not quite equal to an octave.

There is no solution of continuity in this spectrum; one colour changes into another by insensible gradations. It is as if an infinite number of tuning-forks, of gradually augmenting pitch, were vibrating at the same time. But turning to another spectrum that, namely, obtained from the incandescent vapour of silver-you observe that it consists of two narrow and

* Bulk for bulk, that wonderful substance ozone probably transcends ordinary oxygen in radiant power a hundred thousand times. This shows that the atoms of an element can be so grouped as to be have towards the ether as a highly complex compound. May not the molecule of water, from which its vast radiant power is derived, be a mole cule of molecules, the chemical formula stamping only a single member of the group?

The vapour of water, as well as the vapour of silver, has its definite periods of vibration, and these are such as to disqualify the vapour from being raised to a white heat. The oxyhydrogen flame, for example, consists of hot aqueous vapour. It is scarcely visible in the air of this room, and it would be still less visible if we could burn the gas in a clean atmosphere. But our atmosphere, even at the summit of Mont Blanc, is dirty; in London it is more than dirty; and the burning dirt gives to this flame the greater portion of its present light. But the heat of the flame is enormous. Cast iron fuses at a temperature of 2,000° Fahr.; the temperature of the oxyhydrogen flame is 6,000° Fahr. A piece of platinum is heated to vivid redness at a distance of two inches beyond the visible termination of the flame. The vapour which produces incandescence is here absolutely dark. In the flame itself the platinum is raised to dazzling whiteness, and is finally pierced by the flame. When this flame impinges on a piece of lime, we have the dazzling Drummond light. But the light is here due to the fact that when it impinges upon the solid body, the vibrations excited by the flame are of periods different from its own.

Thus far we have fixed our attention on atoms and molecules in a state of vibration, and surrounded by a medium which accepts their vibrations, and transmits them through infinite But suppose space. the waves gen

erated by one system of molecules to im- brations. Here are three small gas-flames pinge upon another system, how will the inserted in three glass tubes of different waves be affected? Will they be stopped, lengths. Each of these flames can be caused or will they be permitted to pass? Will to emit a musical note, the pitch of which is they transfer their motion to the molecules determined by the length of the tube suron which they impinge, or will they glide rounding the flame. The shorter the tube round the molecules, through the intermo- the higher is the pitch. The flames are now lecular spaces, and thus escape? The silent within their respective tubes, but each answer to this question depends upon a con- of them can be caused to respond to a propdition which may be beautifully exempli- er note sounded anywhere in this room. I fied by an experiment on sound. These two have here an instrument called a syren, by tuning-forks are tuned absolutely alike. which I can produce a powerful musical They vibrate with the same rapidity, and note. Beginning with a low pitch, and asmounted thus upon their resonant stands, cending gradually to a higher one, I finalyou hear them loudly sounding the same ly reach the note of the flame in the longest musical note. I stop one of the forks, and tube. The moment it is reached, the flame throw the other into strong vibration. I bursts into song. I stop and re-excite the now bring that other near the silent fork, syren, to enable you to hear that its note but not into contact with it. Allowing them and the flame's note are identical. But the to continue in this position for four or five other flames are still silent within their tubes. seconds, I stop the vibrating fork; but the I urge the instrument on to higher notes; sound has not ceased. The second fork has the second flame has now started, and the taken up the vibrations of its neighbour, third alone remains. But a still higher and is now surrounded in its turn. I dis- note starts it also. Thus, as the sound of the mount one of the forks, and permit the syren rises gradually in pitch, it awakens other to remain upon its stand. I throw every flame in passing, by striking it with a the dismounted fork into strong vibration, series of waves whose periods of recurrence but you cannot hear it sound. Detached are similar to its own. from its stand the amount of motion which it can communicate to the air is too small to make itself sensible to the ear at any distance. I now bring the dismounted fork close to the mounted one, but not into actual contact with it. Out of the silence rises a mellow sound. Whence comes it? From the transferred vibrations of the dismounted fork. That the motion should thus transfer itself through the air it is necessary that the two forks should be in perfect unison. If I place on one of the forks a morsel of wax not longer than a pea, it is rendered thereby powerless to affect, or to be affected by, the other. It is easy to understand this experiment. The pulses of the one fork can affect the other, because they are perfectly timed. A single pulse causes the prong of the silent fork to vibrate through an infinitesimal space. But just as it has completed this small vibration, another pulse is ready to strike it. Thus, the small impulses add themselves together. In the five seconds during which the forks were held near each other, the vibrating fork sent 1.280 waves against its neighbour, and those 1,280 shocks, all delivered at the proper moment, all, as I have said, perfectly timed, have given such strength to the vibrations of the mounted fork as to render them audible to you all.

Let me give you one other illustration of the influence of synchronism on musical vi

Let us apply these facts to radiant heat, taking as before the vapour of water as a representative case. The molecules of this vapour have definite periods of vibration to which they are as rigidly bound as a tuningfork is to its periods. Recurring then to our experiment on the mountain top: instead of exposing our hot vapour in the manner described, with nothing above it, let us suppose a stratum of aqueous vapour to be spread out between it and the firmament. The light of the stars is unaffected by this stratum, which I suppose to be true vapour, and, therefore, perfectly transparent. But the case is different as regards the rays issuing from the hot vapour underneath. The molecules of this vapour and of the stratum overhead are, if I may use the expression, tuned to precisely the same note, and instead of the direct transference of the vibratory motion into space, we have it transferred to the molecules of the vapour above. The motion is thus intercepted — in technical language the heat is absorbed. The upper stratum of vapour having thus become warmed, first at its under surface, and then, by a gradual progression, through its entire mass, it would radiate in all directions, returning a portion of the heat communicated to it to the source from which it is derived.. We are here manifestly dealing with that great principle which lies at the basis of spectrum analysis, and which

opaque to the other class. Thus the white powder, which has shown itself so powerful an absorber, has been specially selected on account of its extreme perviousness to the visible rays, and its extreme imperviousness to the invisible ones; while the dark powder was chosen on account of its extreme transparency to the invisible, and its extreme opacity to the visible rays. In the case of the radiation from our fire, about 98 per cent. of the whole emission consists of invisible rays; the body, therefore, which was most opaque to these triumphed as an absorber, though that body was a white one. I would here invite you to consider the manner in which we obtain from natural facts what may be called their intellectual value. Throughout the processes of nature there is interdependence and harmony, and the main value of our science, considered as a mental discipline, consists in the tracing of this interdependence and the demonstration of this harmony. The outward and visible phenomena are with us the counters of the intellect; and our science would not be worthy of its name and fame if it halted at facts, however practically useful, and neglected the music of law which accompanies the march of phenomena. Let us endeavour, then, to extract from the experiment of Franklin its full intellectual value, calling to our aid the knowledge which our predecessors have already stored Let us imagine two pieces of cloth of the same texture, the one black and the other white, placed upon sunned snow. Fixing our attention on the white piece, let us inquire whether there is any reason to expect that it will sink into the snow at all. There is knowledge at hand which enables us to reply at once in the negative. There is, on the contrary, reason to expect that after a sufficient exposure the bit of cloth will be found on an eminence instead of in a hollow; that instead of a depression, we shall have a relative elevation of the bit of cloth. For, as regards the luminous rays of the sun, the cloth and the snow are alike powerless; the one cannot be warmed, nor the other melted, by such rays. The cloth is white and the snow is white, because their confusedly mingled particles are incompetent to absorb luminous rays. Whether, then, the cloth will sink or not depends entirely upon the dark rays of the sun. Now the substance which of all substances absorbs the dark rays of the sun with the greatest avidity is ice, or snow, which is merely ice in powder. A less amount of heat will be lodged in the cloth than in the surrounding snow. The cloth must there

fore act as a shield to the snow on which it rests; and in consequence of the more rapid fusion of the exposed snow, the cloth must in due time be left behind, perched upon an eminence like a glacier-table.

But though the snow transcends the cloth both as a radiator and absorber it does not much transcend it. Cloth is very powerful in both these respects. Let us now turn our attention to the piece of black cloth, the texture and fabric of which I assume to be the same as that of the white. For our object being to compare the effects of colour, we must, in order to study this effect in its purity, preserve all other conditions constant. Let us then suppose the black cloth to be obtained from the dyeing of the white. The cloth itself, without reference to the dye, is nearly as good an absorber of heat as the snow around it. But to the absorption of the dark solar rays by the undyed cloth is now added the absorption of the whole of the luminous rays, and this great additional influx of heat is far more than sufficient to turn the balance in favour of the black cloth. The sum of its actions on the dark and luminous rays exceeds the action of the snow on the dark rays alone. Hence the cloth will sink in the snow, and this is the philosophy of Franklin's experiment.

Throughout this discourse the main stress has been laid on chemical constitution, as influencing most powerfully the phenomena of radiation and absorption. With regard to gases, vapours, and to the liquids from which these vapours are derived, it had been proved by the most varied and conclusive experiments that the acts of radiation and absorption were molecular-that they depended upon chemical and not upon mechanical condition. In attempting to extend this principle to solids I was met by a multitude of facts obtained by celebrated experimenters, which seemed flatly to forbid such extension. Melloni, for example, found the same radiant and absorb ent power for chalk and lampblack. M M. Masson and Courtépée performed a most elaborate series of experiments on chemical precipitates of various kinds, and found that they one and all manifested the same power of radiation. They concluded from their researches, that where bodies are reduced to an extremely fine state of division the influence of this state is so powerful as entirely to mask and override whatever influence may be due to chemical constitution.

But it appears to me that through the whole of these researches a serious oversight has run, the mere mention of which will