Part 4

Although, according to Bessel, 25,000 cubic miles of water flow in every six hours from one quarter of the earth to another, and the temperature is augmented by the ebb and flow of every tide, all that we know with certainty is, that the _resultant effect_ of all the thermal agencies to which the earth is exposed has undergone no perceptible change within the historic period. We owe this fine deduction to Arago. In order that the _date palm_ should ripen its fruit, the mean temperature of the place must exceed 70 deg. Fahr.; and, on the other hand, the _vine_ cannot be cultivated successfully when the temperature is 72 deg. or upwards. Hence the mean temperature of any place at which these two plants flourished and bore fruit must lie between these narrow limits, _i. e._ could not differ from 71 deg. Fahr. by more than a single degree. Now from the Bible we learn that both plants were _simultaneously_ cultivated in the central valleys of Palestine in the time of Moses; and its then temperature is thus definitively determined. It is the same at the present time; so that the mean temperature of this portion of the globe has not sensibly altered in the course of thirty-three centuries.

THEORY OF CRYSTALLISATION.

Professor Plücker has ascertained that certain crystals, in particular the cyanite, “point very well to the north by the magnetic power of the earth only. It is a true compass-needle; and more than that, you may obtain its declination.” Upon this Mr. Hunt remarks: “We must remember that this crystal, the cyanite, is a compound of silica and alumina only. This is the amount of experimental evidence which science has afforded in explanation of the conditions under which nature pursues her wondrous work of crystal formation. We see just sufficient of the operation to be convinced that the luminous star which shines in the brightness of heaven, and the cavern-secreted gem, are equally the result of forces which are known to us in only a few of their modifications.”--_Poetry of Science._

Gay Lussac first made the remark, that a crystal of potash-alum, transferred to a solution of ammonia-alum, continued to increase without its form being modified, and might thus be covered with alternate layers of the two alums, preserving its regularity and proper crystalline figure. M. Beudant afterwards observed that other bodies, such as the sulphates of iron and copper, might present themselves in crystals of the same form and angles, although the form was not a simple one, like that of alum. But M. Mitscherlich first recognised this correspondence in a sufficient number of cases to prove that it was a general consequence of similarity of composition in different bodies.--_Graham’s Elements of Chemistry._

IMMENSE CRYSTALS.

Crystals are found in the most microscopic character, and of an exceedingly large size. A crystal of quartz at Milan is three feet and a quarter long, and five feet and a half in circumference: its weight is 870 pounds. Beryls have been found in New Hampshire measuring four feet in length.--_Dana._

VISIBLE CRYSTALLISATION.

Professor Tyndall, in a lecture delivered by him at the Royal Institution, London, on the properties of Ice, gave the following interesting illustration of crystalline force. By perfectly cleaning a piece of glass, and placing on it a film of a solution of chloride of ammonium or sal ammoniac, the action of crystallisation was shown to the whole audience. The glass slide was placed in a microscope, and the electric light passing through it was concentrated on a white disc. The image of the crystals, as they started into existence, and shot across the disc in exquisite arborescent and symmetrical forms, excited the admiration of every one. The lecturer explained that the heat, causing the film of moisture to evaporate, brought the particles of salt sufficiently near to exercise the crystalline force, the result being the beautiful structure built up with such marvellous rapidity.

UNION OF MINERALOGY AND GEOMETRY.

It is a peculiar characteristic of minerals, that while plants and animals differ in various regions of the earth, mineral matter of the same character may be discovered in any part of the world,--at the Equator or towards the Poles; at the summit of the loftiest mountains, and in works far beneath the level of the sea. The granite of Australia does not necessarily differ from that of the British islands; and ores of the same metals (the proper geological conditions prevailing) may be found of the same general character in all regions. Climate and geographical position have no influence on the composition of mineral substances.

This uniformity may, in some measure, have induced philosophers to seek its extension to the forms of crystallography. About 1760 (says Mr. Buckle, in his _History of Civilization_), Romé de Lisle set the first example of studying crystals, according to a scheme so large as to include all the varieties of their primary forms, and to account for their irregularities and the apparent caprice with which they were arranged. In this investigation he was guided by the fundamental assumption, that what is called an irregularity is in truth perfectly regular, and that the operations of nature are invariable. Haüy applied this great idea to the almost innumerable forms in which minerals crystallise. He thus achieved a complete union between mineralogy and geometry; and, bringing the laws of space to bear on the molecular arrangements of matter, he was able to penetrate into the intimate structure of crystals. By this means he proved that the secondary forms of all crystals are derived from their primary forms by a regular process of decrement; and that when a substance is passing from a liquid to a solid state, its particles cohere, according to a scheme which provides for every possible change, since it includes even those subsequent layers which alter the ordinary type of the crystal, by disturbing its natural symmetry. To ascertain that such violations of symmetry are susceptible of mathematical calculation, was to make a vast addition to our knowledge; and, by proving that even the most uncouth and singular forms are the natural results of their antecedents, Haüy laid the foundation of what may be called the pathology of the inorganic world. However paradoxical such a notion may appear, it is certain that symmetry is to crystals what health is to animals; so that an irregularity of shape in the first corresponds with an appearance of disease in the second.--See _Hist. Civilization_, vol. i.

REPRODUCTIVE CRYSTALLISATION.

The general belief that only organic beings have the power of reproducing lost parts has been disproved by the experiments of Jordan on crystals. An octohedral crystal of alum was fractured; it was then replaced in a solution, and after a few days its injury was seen to be repaired. The whole crystal had of course increased in size; but the increase on the broken surface had been so much greater that a perfect octohedral form was regained.--_G. H. Lewes._

This remarkable power possessed by crystals, in common with animals, of repairing their own injuries had, however, been thus previously referred to by Paget, in his _Pathology_, confirming the experiments of Jordan on this curious subject: “The ability to repair the damages sustained by injury ... is not an exclusive property of living beings; for even crystals will repair themselves when, after pieces have been broken from them, they are placed in the same conditions in which they were first formed.”

GLASS BROKEN BY SAND.

In some glass-houses the workmen show glass which has been cooled in the open air; on this they let fall leaden bullets without breaking the glass. They afterwards desire you to let a few grains of sand fall upon the glass, by which it is broken into a thousand pieces. The reason of this is, that the lead does not scratch the surface of the glass; whereas the sand, being sharp and angular, scratches it sufficiently to produce the above effect.

Sound and Light.

SOUNDING SAND.

Mr. Hugh Miller, the geologist, when in the island of Eigg, in the Hebrides, observed that a musical sound was produced when he walked over the white dry sand of the beach. At each step the sand was driven from his footprint, and the noise was simultaneous with the scattering of the sand; the cause being either the accumulated vibrations of the air when struck by the driven sand, or the accumulated sounds occasioned by the mutual impact of the particles of sand against each other. If a musket-ball passing through the air emits a whistling note, each individual particle of sand must do the same, however faint be the note which it yields; and the accumulation of these infinitesimal vibrations must constitute an audible sound, varying with the number and velocity of the moving particles. In like manner, if two plates of silex or quartz, which are but crystals of sand, give out a musical sound when mutually struck, the impact or collision of two minute crystals or particles of sand must do the same, in however inferior a degree; and the union of all these sounds, though singly imperceptible, may constitute the musical notes of “the Mountain of the Bell” in Arabia Petræa, or the lesser sounds of the trodden sea-beach of Eigg.--_North-British Review_, No. 5.

INTENSITY OF SOUND IN RAREFIED AIR.

The experiences during ascents of the highest mountains are contradictory. Saussure describes the sounds on the top of Mont Blanc as remarkably weak: a pistol-shot made no more noise than an ordinary Chinese cracker, and the popping of a bottle of champagne was scarcely audible. Yet Martius, in the same situation, was able to distinguish the voices of the guides at a distance of 1340 feet, and to hear the tapping of a lead pencil upon a metallic surface at a distance of from 75 to 100 feet.

MM Wertheim and Breguet have propagated sound over the wire of an electric telegraph at the rate of 11,454 feet per second.

DISTANCE AT WHICH THE HUMAN VOICE MAY BE HEARD.

Experience shows that the human voice, under favourable circumstances, is capable of filling a larger space than was ever probably enclosed within the walls of a single room. Lieutenant Foster, on Parry’s third Arctic expedition, found that he could converse with a man across the harbour of Port Bowen, a distance of 6696 feet, or about one mile and a quarter. Dr. Young records that at Gibraltar the human voice has been heard at a distance of ten miles. If sound be prevented from spreading and losing itself in the air, either by a pipe or an extensive flat surface, as a wall or still water, it may be conveyed to a great distance. Biot heard a flute clearly through a tube of cast-iron (the water-pipes of Paris) 3120 feet long: the lowest whisper was distinctly heard; indeed, the only way not to be heard was not to speak at all.

THE ROAR OF NIAGARA.

The very nature of the sound of running water pronounces its origin to be the bursting of bubbles: the impact of water against water is a comparatively subordinate cause, and could never of itself occasion the murmur of a brook; whereas, in streams which Dr. Tyndall has examined, he, in all cases where a ripple was heard, discovered bubbles caused by the broken column of water. Now, were Niagara continuous, and without lateral vibration, it would be as silent as a cataract of ice. In all probability, it has its “contracted sections,” after passing which it is broken into detached masses, which, plunging successively upon the air-bladders formed by their precursors, suddenly liberate their contents, and thus create _the thunder of the waterfall_.

FIGURES PRODUCED BY SOUND.

Stretch a sheet of wet paper over the mouth of a glass tumbler which has a footstalk, and glue or paste the paper at the edges. When the paper is dry, strew dry sand thinly upon its surface. Place the tumbler on a table, and hold immediately above it, and parallel to the paper, a plate of glass, which you also strew with sand, having previously rubbed the edges smooth with emery powder. Draw a violin-bow along any part of the edges; and as the sand upon the glass is made to vibrate, it will form various figures, which will be accurately imitated by the sand upon the paper; or if a violin or flute be played within a few inches of the paper, they will cause the sand upon its surface to form regular lines and figures.

THE TUNING-FORK A FLUTE-PLAYER.

Take a common tuning-fork, and on one of its branches fasten with sealing-wax a circular piece of card of the size of a small wafer, or sufficient nearly to cover the aperture of a pipe, as the sliding of the upper end of a flute with the mouth stopped: it may be tuned in unison with the loaded tuning-fork by means of the movable stopper or card, or the fork may be loaded till the unison is perfect. Then set the fork in vibration by a blow on the unloaded branch, and hold the card closely over the mouth of the pipe, as in the engraving, when a note of surprising clearness and strength will be heard. Indeed a flute may be made to “speak” perfectly well, by holding close to the opening a vibrating tuning-fork, while the fingering proper to the note of the fork is at the same time performed.

THEORY OF THE JEW’S HARP.

If you cause the tongue of this little instrument to vibrate, it will produce a very low sound; but if you place it before a cavity (as the mouth) containing a column of air, which vibrates much faster, but in the proportion of any simple multiple, it will then produce other higher sounds, dependent upon the reciprocation of that portion of the air. Now the bulk of air in the mouth can be altered in its form, size, and other circumstances, so as to produce by reciprocation many different sounds; and these are the sounds belonging to the Jew’s Harp.

A proof of this fact has been given by Mr. Eulenstein, who fitted into a long metallic tube a piston, which being moved, could be made to lengthen or shorten the efficient column of air within at pleasure. A Jew’s Harp was then so fixed that it could be made to vibrate before the mouth of the tube, and it was found that the column of air produced a series of sounds, according as it was lengthened or shortened; a sound being produced whenever the length of the column was such that its vibrations were a multiple of those of the Jew’s Harp.

SOLAR AND ARTIFICIAL LIGHT COMPARED.

The most intensely ignited solid (produced by the flame of Lieutenant Drummond’s oxy-hydrogen lamp directed against a surface of chalk) appears only as black spots on the disc of the sun, when held between it and the eye; or in other words, Drummond’s light is to the light of the sun’s disc as 1 to 146. Hence we are doubly struck by the felicity with which Galileo, as early as 1612, by a series of conclusions on the smallness of the distance from the sun at which the disc of Venus was no longer visible to the naked eye, arrived at the result that the blackest nucleus of the sun’s spots was more luminous than the brightest portions of the full moon. (See “The Sun’s Light compared with Terrestrial Lights,” in _Things not generally Known_, pp. 4, 5.)

SOURCE OF LIGHT.

Mr. Robert Hunt, in a lecture delivered by him at the Russell Institution, “On the Physics of a Sunbeam,” mentions some experiments by Lord Brougham on the sunbeam, in which, by placing the edge of a sharp knife just within the limit of the light, the ray was inflected from its previous direction, and coloured red; and when another knife was placed on the opposite side, it was deflected, and the colour was blue. These experiments (says Mr. Hunt) seem to confirm Sir Isaac Newton’s theory, that light is a fluid emitted from the sun.

THE UNDULATORY SCALE OF LIGHT.

The white light of the sun is well known to be composed of several coloured rays; or rather, according to the theory of undulations, when the rate at which a ray vibrates is altered, a different sensation is produced upon the optic nerve. The analytical examination of this question shows that to produce a red colour the ray of light must give 37,640 undulations in an inch, and 458,000,000,000,000 in a second. Yellow light requires 44,000 undulations in an inch, and 535,000,000,000,000 in a second; whilst the effect of blue results from 51,110 undulations within an inch, and 622,000,000,000,000 of waves in a second of time.--_Hunt’s Poetry of Science._

VISIBILITY OF OBJECTS.

In terrestrial objects, the form, no less than the modes of illumination, determines the magnitude of the smallest angle of vision for the naked eye. Adams very correctly observed that a long and slender staff can be seen at a much greater distance than a square whose sides are equal to the diameter of the staff. A stripe may be distinguished at a greater distance than a spot, even when both are of the same diameter.

The _minimum_ optical visual angle at which terrestrial objects can be recognised by the naked eye has been gradually estimated lower and lower, from the time when Robert Hooke fixed it exactly at a full minute, and Tobias Meyer required 34″ to perceive a black speck on white paper, to the period of Leuwenhoeck’s experiments with spiders’ threads, which are visible to ordinary sight at an angle of 4″·7. In Hueck’s most accurate experiments on the problem of the movement of the crystalline lens, white lines on a black ground were seen at an angle of 1″·2; a spider’s thread at 0″·6; and a fine glistening wire at scarcely 0″·2.

Humboldt, when at Chillo, near Quito, where the crests of the volcano of Pichincha lay at a horizontal distance of 90,000 feet, was much struck by the circumstance that the Indians standing near distinguished the figure of Bonpland (then on an expedition to the volcano), as a white point moving on the black basaltic sides of the rock, sooner than Humboldt could discover him with a telescope. Bonpland was enveloped in a white cotton poncho: assuming the breadth across the shoulders to vary from three to five feet, according as the mantle clung to the figure or fluttered in the breeze, and judging from the known distance, the angle at which the moving object could be distinctly seen varied from 7″ to 12″. White objects on a black ground are, according to Hueck, distinguished at a greater distance than black objects on a white ground.

Gauss’s heliotrope light has been seen with the naked eye reflected from the Brocken on Hobenhagen at a distance of about 227,000 feet, or more than 42 miles; being frequently visible at points in which the apparent breadth of a three-inch mirror was only 0″·43.

THE SMALLEST BRIGHT BODIES.

Ehrenberg has found from experiments on the dust of diamonds, that a diamond superficies of 1/100th of a line in diameter presents a much more vivid light to the naked eye than one of quicksilver of the same diameter. On pressing small globules of quicksilver on a glass micrometer, he easily obtained smaller globules of the 1/100th to the 1/2000th of a line in diameter. In the sunshine he could only discern the reflection of light, and the existence of such globules as were 1/300th of a line in diameter, with the naked eye. Smaller ones did not affect his eye; but he remarked that the actual bright part of the globule did not amount to more than 1/900th of a line in diameter. Spider threads of 1/2000th in diameter were still discernible from their lustre. Ehrenberg concludes that there are in organic bodies magnitudes capable of direct proof which are in diameter 1/100000 of a line; and others, that can be indirectly proved, which may be less than a six-millionth part of a Parisian line in diameter.

VELOCITY OF LIGHT.

It is scarcely possible so to strain the imagination as to conceive the Velocity with which Light travels. “What mere assertion will make any man believe,” asks Sir John Herschel, “that in one second of time, in one beat of the pendulum of a clock, a ray of light travels over 192,000 miles; and would therefore perform the tour of the world in about the same time that it requires to wink with our eyelids, and in much less time than a swift runner occupies in taking a single stride?” Were a cannon-ball shot directly towards the sun, and were it to maintain its full speed, it would be twenty years in reaching it; and yet light travels through this space in seven or eight minutes.

The result given in the _Annuaire_ for 1842 for the velocity of light in a second is 77,000 leagues, which corresponds to 215,834 miles; while that obtained at the Pulkowa Observatory is 189,746 miles. William Richardson gives as the result of the passage of light from the sun to the earth 8´ 19″·28, from which we obtain a velocity of 215,392 miles in a second.--_Memoirs of the Astronomical Society_, vol. iv.

In other words, light travels a distance equal to eight times the circumference of the earth between two beats of a clock. This is a prodigious velocity; but the measure of it is very certain.--_Professor Airy._

The navigator who has measured the earth’s circuit by his hourly progress, or the astronomer who has paced a degree of the meridian, can alone form a clear idea of velocity, when we tell him that light moves through a space equal to the circumference of the earth in _the eighth part of a second_--in the twinkling of an eye.

Could an observer, placed in the centre of the earth, see this moving light, as it describes the earth’s circumference, it would appear a luminous ring; that is, the impression of the light at the commencement of its journey would continue on the retina till the light had completed its circuit. Nay, since the impression of light continues longer than the _fourth_ part of a second, _two_ luminous rings would be seen, provided the light made _two_ rounds of the earth, and in paths not coincident.

APPARATUS FOR THE MEASUREMENT OF LIGHT.

Humboldt enumerates the following different methods adopted for the Measurement of Light: a comparison of the shadows of artificial lights, differing in numbers and distance; diaphragms; plane-glasses of different thickness and colour; artificial stars formed by reflection on glass spheres; the juxtaposition of two seven-feet telescopes, separated by a distance which the observer could pass in about a second; reflecting instruments in which two stars can be simultaneously seen and compared, when the telescope has been so adjusted that the star gives two images of like intensity; an apparatus having (in front of the object-glass) a mirror and diaphragms, whose rotation is measured on a ring; telescopes with divided object-glasses, on either half of which the stellar light is received through a prism; astrometers, in which a prism reflects the image of the moon or Jupiter, and concentrates it through a lens at different distances into a star more or less bright.--_Cosmos_, vol. iii.

HOW FIZEAU MEASURED THE VELOCITY OF LIGHT.

This distinguished physicist has submitted the Velocity of Light to terrestrial measurement by means of an ingeniously constructed apparatus, in which artificial light (resembling stellar light), generated from oxygen and hydrogen, is made to pass back, by means of a mirror, over a distance of 28,321 feet to the same point from which it emanated. A disc, having 720 teeth, which made 12·6 rotations in a second, alternately obscured the ray of light and allowed it to be seen between the teeth on the margin. It was supposed, from the marking of a counter, that the artificial light traversed 56,642 feet, or the distance to and from the stations, in 1/1800th part of a second, whence we obtain a velocity of 191,460 miles in a second.[12] This result approximates most closely to Delambre’s (which was 189,173 miles), as obtained from Jupiter’s satellites.