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the latter free transmission. I bring these waves, and consequently cannot be powerinvisible rays to a focus at a distance of sev- fully affected by their heat. eral feet from the electric lamp; the dark The knowledge which we now possess rays form there an image of the source from will enable us to analyze with profit à pracwhich they issue. By proper means this in- tical question. White dresses are worn in visible image may be transformed into a summer because they are found to be cooler visible one of dazzling brightness. I could, than dark ones. The celebrated Benjamin moreover, show you, if time permitted, how Franklin made the following experiment: out of those perfectly dark rays we might He placed bits of cloth•of various colours extract, by a process of transmutation, all upon snow, exposed them to direct sunthe colours of the solar spectrum. I could shine, and found that they sank to different also prove to you that those rays, powerful depths in the snow. The black cloth sank as they are, and sufficient to fuse many deepest, the white did not sink at all. metals, may be permitted to enter the eye Franklin inferred from his experiment that and to break upon the retina without injury black bodies are the best absorbers, and to the eye, without producing the least white ones the worst absorbers, of radiant luminous impression. The dark rays are now heat. Let us test the generality of this collected before you; you see nothing at conclusion. I have here two cards, one of their place of convergence; with a proper which is coated with a very dark powder, thermometer it could be proved that even and the other with a perfectly white one. the air at the focus is just as cold as the sur- I place the powdered surfaces before the fire, rounding air. And mark the conclusion to and leave them there until they have acwhich this leads. It proves the ether at the quired as high a temperature as they can atfocus to be practically detached from the tain in this position. Which of the cards air, and that the most violent ethereal mo- is most highly heated ? It requires no thertion may there exist without the least mometer to answer this question. Simply aerial motion. But though you see it not, pressing the back of the card, on which the there is sufficient heat at that focus to set white powder is strewn, against my cheek London on fire. The heat there at the or forehead, I find it intolerably hot. Plapresent moment is competent to raise iron cing the other card in the same position to a temperature at which it throws off I find it cool. The white powder has abbrilliant scintillations. It can heat platinum sorbed far more heat than the dark one. to whiteness and almost fuse that refractory This simple result abolishes a hundred conmetal. It actually can fuse gold, silver, clusions which have been hastily drawn copper, and aluminium. The moment, more from the experiment of Franklin. Again, over, that wood is placed at the focus it here are suspended two delicate mercurial bursts into a blaze.
thermometers at the same distance from It has been already affirmed that whether a gas-flame. The bulb of one of them is as regards radiation or absorption the ele- covered by a dark substance, the bulb of mentary atoms possess but little power. the other by a white one. Both bulbs have This might be illustrated by a long array of received the radiation from the flame, but facts; and one of the most singular of these the white bulb has absorbed most, and its is furnished by the deportment of that ex- mercury stands much higher than that of tremely combustible substance phosphorus, the other thermometer. I might vary this when placed at this dark focus. It is im- experiment in a hundred ways, and show possible to ignite there a fragment of amor- you that you can draw no safe conclusion phous phosphorus. But ordinary phosphorus from the darkness of a body regarding its is a far quicker combustible, and its de- power of absorption. portment to radiant heat is still more im- The reason of this simply is, that colour pressive. It may be exposed to the intense gives us intelligence of only one portion, radiation of an ordinary fire without burst- and that the smallest one, of the rays iming into flame. It may also be held for twenty pinging on the coloured body. Were the or thirty Seconds at an obscure focus of rays all luminous we might with certainty sufficient power to raise platinum to a white infer from the colour of a body its power heat, without ignition. Notwithstanding of absorption ; but the great mass of the the energy of the ethereal waves here con- radiation from our fire, our gas-flame, and centrated, notwithstanding the extremely in- even from the sun itself, consists of invisiflammable character of the elementary body ble calorific rays, regarding which colour exposed to their action, the atoms of that teaches us nothing. Å body may be highly body refuse to share in the motion of the transparent to one class of rays, and highly
opaque to the other class. Thus the white fore act as a shield to the snow on which it
I would here invite you to consider the fect in its purity, preserve all other condimanner in which we obtain from natural tions constant. Let us then suppose the facts what may be called their intellectual black cloth to be obtained from the dyeing value. Throughout the processes of nature of the white. The cloth itself, without there is interdependence and harmony, reference to the dye, is nearly as good an and the main value of our science, consid- absorber of heat as the snow around it. ered as a mental discipline, consists in the But to the absorption of the dark solar rays tracing of this interdependence and the by the undyed cloth is now added the abdemonstration of this harmony. The out- sorption of the whole of the luminous rays, ward and visible phenomena are with us the and this great additional influx of heat is counters of the intellect; and our science far more than sufficient to turn the balance would not be worthy of its name and fame in favour of the black cloth. The sum of if it halted at facts, however practically its actions on the dark and luminous rays useful, and neglected the music of law exceeds the action of the snow on the dark which accompanies the march of phenome- rays alone. Hence the cloth will sink in na L
us endeavour, then, to extract the snow, and this is the philosophy of from the experiment of Franklin its full in- Franklin's experiment. tellectual value, calling to our aid the knowl- Throughout this discourse the main stress edge which our predecessors have already has been laid on chemical constitution, as stored Let us imagine two pieces of cloth of influencing most powerfully the phenomena the same texture, the one black and the other of radiation and absorption. With regard white, placed upon sunned snow. Fixing our to gases, vapours, and to the liquids from attention on the white piece, let us inquire which these vapours are derived, it had whether there is any reason to expect that been proved by the most varied and conit will sink into the snow at all. There is clusive experiments that the acts of radiaknowledge at hand which enables us to tion and absorption were molecular — that reply at once in the negative. There is, on they depended upon chemical and not upon the contrary, reason to expect that after a mechanical condition. In attempting to sufficient exposure the bit of cloth will be extend this principle to solids I was met by found on an eminence instead of in a hol- a multitude of facts obtained by celebralow; that instead of a depression, we shall ted experimenters, which seemed flatly to have a relative elevation of the bit of cloth. forbid such extension. Melloni, for exFor, as regards the luminous rays of the ample, found the same radiant and absorbsun, the cloth and the snow are alike pow- ent power for chalk and lampblack. MM. erless; the one cannot be warmed, nor the Masson and Courtépée performed a most other melted, by such rays. The cloth is elaborate series of experiments on chemical white and the snow is white, because their precipitates of various kinds, and found that confusedly mingled particles are incompe- they one and all manifested the same power tent to absorb luminous rays. Whether, of radiation. They concluded from their then, the cloth will sink or not depends en- researches, that where bodies are reduced tirely upon the dark rays of the sun. Now to an extremely fine state of division the the substance which of all substances ab- influence of this state is so powerful as ensorbs the dark rays of the sun with the tirely to mask and override whatever influgreatest avidity is ice, -or snow, which is ence may be due to chemical constitution. merely ice in powder. A less amount of But it appears to me that through the heat will be lodged in the cloth than in the whole of these researches a serious oversight surrounding snow. The cloth must there has run, the mere mention of which will
show what caution is essential in the opera- | a series of phenomena, and emerged from tions of experimental philosophy. Let me that series with a purely intellectual constate wherein I suppose this oversight to con- clusion, our duty is to bring that conclusion sist. I have here a metal cube with two of to its crucial test. In this way we fortify its sides brightly polished. I fill the cube our science, sparing no pains, shirking no with boiling water and determine the quan- toil to secure sound materials for the edifice tity of heat emitted by the two bright sur- which it is our privilege to build. If, then, faces. One of them far transcends the oth- our alleged facts be real, and if our inferer as a radiator of heat. Both surfaces ap- ence from these be just, we ought to find: pear to be metallic. What then is the that inference ratified by experiment. For cause of the observed difference in their the purpose of testing it I take two pow. radiative power? Simply this: I have ders of the same physical appearance: one coated one of the surfaces with transparent of them is a compound of mercury and the gum, through which, of course, is seen the other a compound of lead. On two surmetallic lustre behind. Now this varnish, faces of this cube are spread these bright though so perfectly transparent to luminous red powders without varnish of any kind. rays, is as opaque as pitch or lampblack to Filling the cube with boiling water, and denon-luminous ones. It is a powerful emitter termining the radiation from the two surof dark rays; it is also a powerful absorber. faces, one of them is found to emit thirtyWhile, therefore, at the present moment it nine rays, while the other emits seventyis copiously pouring forth radiant heat itself, four. This, surely, is a great difference. it does not allow a single ray from the metal Here, however, is a second cube, harbehind to pass through it. The varnish ing two of its surfaces coated with the then, and not the metal, is the real radiator. same powders, the only difference being
Now Melloni, and Masson, and Courtépée that now the powders are laid on by experimented thus: they mixed their pow- means of a transparent gum. Both surders and precipitates with gum-water, and faces are now absolutely alike in radiative laid them by means of a brush_upon the power. Both of them emit somewhat more surfaces of a cube like this. True they than was emitted by either of the unvarsaw their red powders red, their white ones nished powders, simply because the gum white, and their black ones black, but they employed is a better radiator than either of saw these colours through the coat of var- thein. Excluding all varnish, and comparnish which encircled every particle of their ing white with white, I find vast differences; powders. When, therefore, it was conclud- comparing black with black I find them ed that colour had no influence on radia- also different; and when black and white tion, no chance Lad been given to it of are compared, in some cases the black radiasserting its influence ; when it was found ates far more than the white, while in other that all chemical precipitates radiated alike, cases the white radiates far more than the it was the radiation from a varnish common black. Determining the absorptive power to them all which showed the observed con- of those powders, it is found to go handstaney: Hundreds, perhaps thousands, of in-hand with their radiative power. The experiments on radiant heat have been good itor is a good absorber, and the performel in this way by various inquirers, bad radiator is a bad absorber. From all but I fear the work will have to be done this it is evident that as regards the radiaover again. I am not, indeed, acquainted tion and absorption of non-luminous heat, with an instance in which an oversight of so colour teaches us nothing; and that eventrivial a character has been committed in as regards the radiation of the sun, consuccession by so many able men, and viti- sisting as it does mainly of non-lumiated so large an amount of otherwise ex- nous rays, conclusions as to the influence of cellent work.
colour may be altogether delusive. This is Basing our reasonings then on demon- the strict scientific upshot of our researches. strated facts, we arrive at the extremely But it is not the less true that in the case of probable conclusion that the envelope of wearing apparel — and this for reasons the particles and not the particles them- which I have given in analysing the exselves, was the real radiator in the experi- periment of Franklin black dresses are ments just referred to. To reason thus, more potent than white ones as absorbers and deduce their more or less probable of solar heat. consequences from experimental facts, is Thus, in brief outline, I have brought bean incessant exercise of the student of fore you a few of the results of recent inphysical science. But having thus followed quiry. If you ask me what is the use of for a time the light of reason alone through them, I can hardly answer you, unless you
define the term use. If you meant to ask mechanical skill have made the telegraph me whether those dark rays which clear what it is are deserving of all honour. In away the Alpine snows will ever be applied fact, they have their reward, both in reputato the roasting of turkeys or the driving of tion and in those more substantial benefits steam-engines, while affirming their power which the direct service of the public alto do both, I would frankly confess that I ways carries in its train. But who, I would do not think them capable at present of ask, put the soul into this telegraphic body? competing profitably with coal in these Who snatched from heaven the fire that particulars. Still they may have great uses flashes along the line? This, I am bound unknown to me; and when our coal-fields to say, was done by two men, the one a are exhausted, it is possible that a more dweller in Italy, * the other a dweller in ethereal race than ourselves may cook their England, and therefore not a thousand victuals and perform their work in this miles distant from the spot where I now transcendental way. But is it necessary stand, † who never in their inquiries conthat the student of science should have his sciously set a practical object before them, labours tested by their possible practical whose only stimulus was the fascination applications ? What is the practical value which draws the climber to a never-trodden of Homer's Iliad? You smile, and possibly peak, and would have make Cæsar quit his think that Homer's Iliad is good as a means victories to seek the sources of the Nile. of culture. There's the rub. The people That the knowledge brought us by those who demand of science practical uses, for- prophets, priests, and kings of science is get, or do not know, that it also is great as what the world calls useful knowledge, the a means of culture; that the knowledge of triumphant application of their discoveries this wonderful universe is a thing profitable proves. But science has another function in itself, and requiring no practical appli- to fulfil
, in the storing and the training of cation to justify its pursuit." But while the the human mind; and I would base my apstudent of nature distinctly refuses to have peal to you on the poor specimen which has his labours judged by their practical issues, been brought before you this evening, unless the term practical be made to in- whether any system of education at the clude mental as well as material good, he present day can be deemed even approxiknows full well that the greatest practical mately complete in which the knowledge of triumphs have been episodes in the search nature is neglected or ignored. after natural truth. The electric telegraph
Joun TYNDALL. is the standing wonder of this age, and the
* Volta. whose scientific knowledge and
A WALK FROM LONDON TO THE LAND's this country, and to collect information that END AND Back. — With Illustrations. By might be useful to American farmers ; he has Elihu Burritt. (Sampson Low, Son, and certainly adhered to this design, and in the Marston.). - We can forgive this genial critic chapter on the Wiltshire labourers discusses the some slight inaccuracies in English history, subject of wages in a way that is well worthy and just the least tendency at times to fine of the attention of the employers of labour in writing after the American fashion. If he has those parts; but he by no means confines himadded a crime to King Harry's list, already self to these topics. He writes pleasantly about "long enough to hang a common sinner,” in the old towns and their historical associates the taking off Jane Seymour's head, he has made cathedrals, the baronial mansions and parks ,and compensation by bestowing on the lady“ vir- even the legends, such as that of the Glastonțues enough to make a saint,” which we never bury thorn. As he is satisfied with the people heard of before ; and his ambitious phrases, that he has visited, having“ never experienced such as “tomb-fonts" and "home-stars,” are any disagreeable treatment, nor been subjected few and far between. Generally he writes sen- to any inconvenience worth mentioning," sibly and with just the amount of friendliness though he has “measured the length of the that Englishmen most appreciate, the friendli- island and doubled it half way back on foot," ness that arises from a recognition of relation- so are we with our visitor. We recognize the ship in ancestry, in temperament, and in aim. judgment and the kindliness with which he has The leading motive of this, as of his previous observed us, and we cordially recommend the
Walk,” was, it seems, to see and note the perusal of his volume. — Spectator. agricultural system, aspects, and industries of
MY VIS-A-VIS. That olden lady!- can it be?
Well, well, how seasons slip away!
SIR WILLIAM HAMILTON ON
SHAKESPEARE. “Dear madam, thirty years ago,
When both our hearts were full of glee, Who says that Shakespeare did not know his In many a dance and courtly show
But deem'd that in Time's manifold decay
His memory should die and pass away, “ That pale blue robe, those chestnut curls, And that within the shrine of human thought
That eastern jewel on your wrist, To him no altar should be reared ? O hush! That neck-encircling string of pearls O veil thyself awhile in solemn awe!
Whence hung a cross of amethyst, - Nor dream that all man's mighty spirit-law I see them all, I see the tulle
Thou know'st; how all the hidden fountains Looped up with roses at the knee,
gush Good Lord ! how fresh and beautiful Of the soul's silent prophesying power. Was then your cheek, my vis-à-vis ! For as deep Love, ʼmid all its wayward pain,
Cannot believe but it is loved again, “ I hear the whispered praises yet,
Even so, strong Genius, with its ample dower, The buzz of pleasure when you came,
Of a world-grasping love, from that deep The rushing eagerness to get
feeling Like moths within the fatal flame; Wins of its own wide sway the clear revealing,