« VorigeDoorgaan »
it was diffused. It was afterwards found that the dry air then experimented with was not perfectly pure, and that the purer the air became the more it approached the character of a vacuum, and the greater, by comparison, became the action of the aqueous vapour. The vapour was found to act with 30, 40, 50, 60, 70 times the energy of the air in which it was diffused ; and no doubt was entertained that the aqueous vapour of the air which filled the Royal Institution theatre, during the delivery of the discourse, absorbed 90 or 100 times the quantity of radiant heat which was absorbed by the main body of the air of the room.
Looking at the single atoms, for every 200 of oxygen and nitrogen there is about 1 of aqueous vapour. This 1, then, is 80 times more powerful than the 200 ; and hence, comparing a single atom of oxygen or nitrogen with a single atom of aqueous vapour, we may infer that the action of the latter is 16,000 times that of the former. This was a very astonishing result, and it naturally excited opposition, based on the philosophic reluctance to accept a result so grave in consequences before testing it to the uttermost. From such opposition a discovery, if it be worth the name, emerges with its fibre strengthened ; as the human character gathers force from the healthy antagonisms of active life. It was urged, that the result was on the face of it improbable; that there were, moreover, many ways of accounting for it, without ascribing so enormous a comparative action to aqueous vapour. For example, the cylinder which contained the air in which these experiments were made, was stopped at its ends by plates of rocksalt, on account of their transparency to radiant heat. Rocksalt is hygroscopic ; it attracts the moisture of the atmosphere. Thus, a layer of brine readily forms on the surface of a plate of rocksalt; and it is well known that brine is very impervious to the rays of heat. Illuminating a polished plate of salt by the electric lamp, and casting, by means of a lens, a magnified image of the plate upon a screen, the speaker breathed through a tube for a moment on the salt ; brilliant colours of thin plates (soap-bubble colours) flashed forth immediately upon the screen — these being caused by the film of moisture which overspread the salt. Such a film, it was contended, is formed when undried air is sent into the cylinder; it was, therefore, the absorption of a layer of brine which was measured, instead of the absorption of aqueous vapour.
This objection was met in two ways. Firstly, by showing that the plates of salt when subjected to the strictest examination show no trace of a film of moisture. Secondly, by abolishing the plates of salt altogether, and obtaining the same results in a cylinder open at both ends.
It was next surmised, that the effect was due to the impurity of the London air, and the suspended carbon particles were pointed to as the cause of the opacity
to radiant heat. This objection was met by bringing air from Hyde Park, Hampstead Heath, Primrose Hill, Epsom Downs, a field near Newport in the Isle of Wight, St. Catharine’s Down,
and the sea-beach near Black Gang Chine. The aqueous vapour of the air from these localities intercepted at least seventy times the amount of radiant heat absorbed by the air in which the vapour was diffused. Experiments made with smoky air proved that the suspended smoke of the atmosphere of West London, even when an east wind pours over it the smoke of the city, exerts only a fraction of the destructive powers exercised by the transparent and impalpable aqueous vapour diffused in the air.
The cylinder which contained the air through which the calorific rays passed was polished within, and the rays which struck the interior surface were reflected from it to the thermo-electric pile which measured the radiation. The following objection was raised :-You permit moist air to enter your cylinder; a portion of this moisture is condensed as a liquid film upon the interior surface of your tube ; its reflective power is thereby diminished ; less heat therefore reaches the pile, and you incorrectly ascribe to the absorption of aqueous vapour an effect which is really due to diminished reflection of the interor surface of your cylinder.
But why should the aqueous vapour so condense? The tube within is warmer than the air without, and against its inner surface the rays of heat are impinging. There can be no tendency to condensation under such circumstances. Further, let five inches of undried air be sent into the tube—that is, one-sixth of the amount which it can contain. These five inches produce their proportionate absorption. The driest day, on the driest portion of the earth's surface, would make no approach to the dryness of our cylinder when it contains only five inches of air. Make it 10, 15, 20, 25, 30 inches : you obtain an absorption exactly proportional to the quantity of vapour present. It is next to a physical impossibility that this could be the case if the effect were due to condensation. But lest a doubt should linger in the mind, not only were the plates of rock-salt abolished, but the cylinder itself was dispensed with. Humid air was displaced by dry, and dry air by humid in the free atmosphere ; the absorption of the aqueous vapour was here manifest, as in all the other cases.
No doubt, therefore, can exist of the extraordinary opacity of this substance to the rays of obscure heat; and particularly such rays as are emitted by the earth after it has been warmed by the sun. It is perfectly certain that more than ten per cent. of the terrestrial radiation from the soil of England is stopped within ten feet of the surface of the soil. This one fact is sufficient to show the immense influence which this newly-discovered property of aqueous vapours must exert on the phenomena of meteorology.
This aqueous vapour is a blanket more necessary to the vegetable life of England than clothing is to man. Remove for a single summer-night the aqueous vapour from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequited into space, and the sun would
rise upon an island held fast in the iron grip of frost. The aqueous vapour constitutes a local dam, by which the temperature at the earth's surface is deepened: the dam, however, finally overflows, and we give to space all that we receive from the sun.
The sun raises the vapours of the equatorial ocean ; they rise, but for a time a vapour screeu spreads above and around them. But the higher they rise, the more they come into the presence of pure space ; and when, by their levity, they have penetrated the vapour screen, which lies close to the earth's surface, what must occur ?
It has been said that, compared atom for atom, the absorption of an atom of aqueous vapour is 16,000 times that of air. Now the power to absorb and the power to radiate are perfectly reciprocal and proportional. The atom of aqueous vapour will therefore radiate with 16,000 times the energy of an atom of air. Imagine then this powerful radiant in the presence of space, and with no screen above it to check its radiation. Into space it pours its heat, chills itself, condenses, and the tropical torrents are the consequence. The expansion of the air, no doubt, also refrigerates it; but in accounting for those deluges, the chilling of the vapour by its own radiation must play a most important part. The rain quits the ocean as vapour; it returns to it as water. How are the vast stores of heat set free by the change from the vaporous to the liquid condition disposed of? Doubtless in great part they are wasted by radiation into space. Similar remarks apply to the cumuli of our latitudes. The warmed air, charged with vapour, rises in columns, so as to penetrate the vapour screen which hugs the earth ; in the presence of space, the head of each pillar wastes its heat by radiation, condenses to a cumulus, which constitutes the visible capital of an invisible column of saturated air.
Numberless other meteorological phenomena receive their solution, by reference to the radiant and absorbent properties of aqueous vapour. It is the absence of this screen, and the consequent copious waste of heat, that causes mountains to be so much chilled when the sun is withdrawn. Its absence in Central Asia renders the winter there almost unendurable ; in Sahara the dryness of the air is sometimes such, that though during the day “the soil is fire and the wind is flame,” the chill at night is painful to bear. In Australia, also, the thermometric range is enormous, on account of the absence of this qualifying agent. A clear day, and a dry day, moreover, are very different things. The atmosphere may possess great visual clearness, while it is charged with aqueous vapour, and on such occasions great chilling cannot occur by terrestrial radiation. Sir John Leslie and others have been perplexed by the varying indications of their instruments on days equally bright—but all these anomalies are completely accounted for by reference to this newly-discovered property of transparent aqueous vapour. Its presence would check the earth's loss : its absence, without sensibly altering the transparency of the air, would open wide a door for the escape of the earth's heat into infinitude.
WEEKLY EVENING MEETING,
Friday, January 30, 1863.
Sir HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. Vice-President,
in the Chair.
His EMINENCE CARDINAL WISEMAN On the Points of Contact between Science and Art. In his preliminary observations, the Cardinal stated that, in speaking of Science and Art, he wished to extend the meaning of one of these words to the utmost, and to restrict that of the other. By Science he wished to understand whatever knowledge has come to man as the result of his investigations by thought, calculation, and experiment; by Art, he meant not the arts of life--the practical arts—but the Fine Arts, and even these restricted to the Arts of Design. As an example of their union, he referred to our three great museums, where the objects of science are almost always to be found blended or associated with objects of art. The great artist, Leonardo da Vinci, is a “representative man" of this union; þut though so well known as a consummate painter, he is comparatively little acknowledged as a man of science. Yet he finds his place in the history and philosophy of the inductive sciences as a practical reformer, and has left behind thirteen volumes of scientific sketches connected with mechanics and hydraulics. It has been said also of the late Prince Consort, that he never saw Art without Science, and never looked at Science without seeing Art; and that he seized every opportunity of inculcating the necessity of cultivating the two harmoniously and inseparably, yet also independently.
The Arts of Design treated of were Painting, Sculpture, and Architecture.
PAINTING.—The most obvious point of contact between painting and practical science is Perspective, which signifies the art of representing on a flat surface objects which are supposed to be on different planes, or at varying distances, so as to give them, by the gradation of proportion and colour, the appearance to the eye which they would have if they were real substances. This science, by no means old, is of uncertain origin. At the Revival, its principles were observed and acted upon by the great painters by intuition ; but it was long before the rules of perspective can be said to have pervaded the whole art. Attention to scientific and artistic perspective began simultaneously in Belgium and Italy,--in the schools of Van Eyck, in Belgium, and of
Giotto in Italy. In the works of the Van Eycks—(Hubert died in 1426 and John in 1446)-great improvements appear in lineal and aerial perspective, which were much advanced also by the Giotteschi, of Florence, &c. But the true history of Scientific Perspective begins with Michelangelo, in whose works the anticipations of art are verified by science, and reduced to unvarying rule.* Leonardo da Vinci (who died in 1519), and Albert Dürer (who died in 1528), a mathematician as well as painter, were also worthy agents in this great advancement of art. It was not, however, till 1608 that Guido Ubaldo published the first satisfactory treatise on Perspective. In 1642, F. Dubreuil edited his “Prospectiva Practica,' well known as “The Jesuit's Perspective’; and, finally, in 1731 the mathematical theory of perspective was conclusively demonstrated by Brook Taylor. These certain principles gradually became so fixed, that it was impossible afterwards to allow deviation from them. The theorems were converted into practical rules, which are now accepted for all useful purposes, without further proof; and perspective was reduced to scientific principles, which are now popularized and adopted as an essential part of artistic education.
As means of proceeding still further in educating the public eye and mind, attention was called to the greatly increased facilities which the railway, a creation of science, has afforded us for studying the beautiful in nature, and of gratifying our love of landscape in our own country and abroad.
In regard to Colour, there is, most certainly, a want of more contact between Science and Art. Some of the colours on the mural paintings of the ancients are still fresh, after an interval of eighteen hundred years ; and the frescoes of the first periods of modern art, though much decayed, yet promise to remain at least distinguishable for a very much longer period. Surely, with the accurate knowledge which chemistry gives us, we ought to obtain effects not inferior to those produced by the older painters almost by accident. Science must come to the aid of art, and answer the question-" Is there any atmospherical, or other chemical action, in this country, which prevents our carrying out in it such public works of art as exist in other countries ?”– In regard to Mosaics, it was stated, that in the great Vatican studio the progression of new colours and shades are entirely in the hands of a chemist, who commands and directs the laboratories and finances requisite for the work. In the catalogue,' the graduated specimens of colours number upwards of twenty thousand.
In SCULPTURE, one point of contact with science is in pure Mathematics. From the time of Michelangelo, though undoubtedly the feeling is much more ancient, there has been an expression of the thought, that the human figure is perfect in its proportions, and that those proportions must have a law. It has been since shown that the
* For details on this subject, see De Morgan's “ Notes on Perspective," in the Athenaum for October and November, 1861.