Chapter XIX: Conclusion (2)

[44] A few _flowers_ have also the property of emitting light at ordinary temperatures. Among these may be cited the _tube-rose_, _nasturtium_, and _marigold_, which occasionally give out flashes of light on a warm summer’s evening. This, probably, arises from the combination of the oxygen of the atmosphere with the vapour of the volatile oils upon which the perfumes of flowers are known to depend. Again, other substances emit light during the act of _crystallization_. This phenomenon is most distinctly observed during the gradual deposit of arsenious acid, when dissolved in hot hydrochloric acid (spirits of salts). In a dark place each crystal, as it is formed, may be seen to emit a spark of light; and on shaking the flask, so many crystals are sometimes suddenly produced, that vivid flashes become perceptible. The cause of this phenomenon is probably dependent on the fixation of oxygen by the arsenic at the moment of precipitation.

[45] The blue colour of the oxy-hydrogen flame is due, probably, to the particles of water formed by the combustion of the two gases.

[46] Dr. Paris’s “Life of Sir Humphry Davy,” p. 352.

[47] “Edinburgh Review,” January, 1816.

[48] “It will hereafter be scarcely credited,” says Dr. Paris, “that an invention so eminently philosophic, and which could never have been derived but from the sterling treasury of science, should have been claimed in behalf of an enginewright of Killingworth colliery, of the name of Stephenson; _a person not even professing a knowledge of the elements of chemistry_.” The “enginewright” here sneered at for his ignorance, ultimately rose to be the great George Stephenson, the inventor of the locomotive. How would the fashionable physician speak of the quondam enginewright of Killingworth colliery now! Stephenson’s lamp was formed on the principle of admitting the fire-damp by narrow tubes, and “in such small detached portions, that it would be consumed by combustion.” The two lamps were, doubtlessly, distinct inventions, though Davy, in all justice, appears to be entitled to precedence—not only in point of date, but as regards the long chain of inductive reasoning concerning the nature of flame by which his result was arrived at.

[49] “Davy’s passion for angling betrayed itself upon all occasions,” says Dr. Paris; “whenever I had the honour of dining at his table, the conversation, however it might have commenced, invariably ended on fishing; and when a brother of the angle happened to be present, you had the pleasure of hearing all his encounters with the finny tribe—how he had lured them by his treachery, and vanquished them by his perseverance. He would occasionally strike into a most eloquent and impassioned strain upon some subject which warmed his fancy; such, for example, as the beauties of mountain scenery: but before you could fully enjoy the prospect which his imagination had pictured, down he carried you into some sparkling stream, or rapid current, to flounder for the next half-hour with a hooked salmon.... Nothing irritated him so much as to find that his companions had caught more fish than himself, and if, during conversation, a brother fisherman surpassed him in the relation of his success, he betrayed similar impatience.”

[50] See Dr. Paris’s description of Davy’s ordinary fishing costume, p. 189.

[51] “This favourite dog is well remembered in Penzance,” says Davy’s brother. “My sister writes,” he adds, “that ‘on his first return from Bristol, after an absence of about twelve months, Chloe did not remember him, till he called her by name, and then she was in a transport of joy.’ Her descendants are now numerous in the Mount’s Bay, and prized for good qualities.” Vol. i. p. 54.

[52] See Dr. Paris’s “Life of Davy,” p. 12.

[53] See Dr. Paris’s account of Davy’s first interview with Gregory Watt, p. 35.

[54] For the sake of simplicity, the nitric acid is here made to consist (according to the new theory) of NO₆+H (instead of NO₅+HO), and so to combine directly with the metal as NO₆+Ag (instead of with the oxide as NO₅+OAg).

[55] The range of the eye, or diameter of the field of vision, is 110°; consequently, this is the largest angle under which an object can be seen. The largest angle, however, is here made 120°, for the simplification of the numbers. The range of vision is from 110° to 1′.

[56] See illustration at p. 378.

[57] The arrangement of lenses, above described, constitutes the principle of what is termed the “_astronomical_ telescope;” for this makes the objects appear upside down. But, though the inversion of a star or planet is a matter of no moment in astronomical observation, such an effect is most disagreeable when applied to terrestrial objects. The ordinary telescope for _land_ purposes, therefore—or the “_day_ telescope,” as it is usually styled—has two other lenses behind the eye-lens. These lenses have both the same focus as the eye-lens itself, and are placed at a fixed distance from each other, such distance being equal to the sum of their focal lengths: that is to say, if the eye-lens have a focus of 2 inches, then each of the two other glasses should have the same length of focus, and be placed at 4 (2 + 2) inches apart from one another. The magnifying power of the day telescope may be calculated in the same manner as that of the astronomical one above explained; for the two additional lenses in the day instruments, having the same focal length as the eye-lens itself, produce no further enlargement of the objects, but serve only to cross the rays a second time, and so to render the image _erect_ instead of _inverted_.

[58] Within the last 30 years the diamond has been used for the purpose of microscopic lenses; for, owing to the refractive power of this precious stone being greater than almost any other known substance, and nearly double that of glass, lenses can be produced from it of a great degree of magnifying power, and that with a comparatively small curvature, so that increased distinctness is obtained; while the lens itself, being nearly “_achromatic_,” the image produced by it is untinged by prismatic colours. Mr. Pritchard constructed the first diamond microscope in 1826. The diamond lens of this was double convex, and had a focus of ¹⁄₃₀th of an inch, so that its magnifying power was 150 times. Dr. Goring, an eminent authority on the subject, says—“I conceive diamond lenses to constitute the ultimatum of perfection in the single microscope.” The sapphire has also been used for the construction of microscopic lenses with considerable advantage, its magnifying power being much greater than that of glass. Mr. Pritchard says, that the sapphire, next to the diamond, possesses all qualities requisite for the formation of a perfect magnifier, and presents less difficulties in the construction.

INDEX.

Air, currents of, how caused, 186.

Air-pump, experiments with, 236.

Animal mechanism, wonders of, 88.

Argand-burners, why superior in brilliancy, 300.

Artificial light dependent on heat, 297.

Bath wells, temperature of the, 125.

Blow-pipe, cause of the increased heat produced by the, 300.

Brocken, spectre of the, in the Hartz mountains, 380;
philosophy of, 383.

Camera, images produced in, 342;
why reversed, 372;
how the lens intensifies images in the, 377;
cause of variation in the size of objects in, 378;
pictures must be projected on an opaque body, 380.

Cellini, Benvenuto, and the necromancer, 417.

Chlorine, experiments with, 279.

Coal, power concentrated in, 110.

Coal-mine, destructive explosion in, 94.

Colours, curious result from mixing in certain proportions, 425.

Combustion, laws of, 113;
phenomena of, 245;
nature of, 250;
experiments in, 247;
philosophy of, 284.

Combustion, spontaneous, 262, 272.

_Corpse candles_, philosophy of, 261.

Creation, the wondrous story of, 116.

Daguerreotype process of photography, 436.

Dew, deposition of, 144;
less plentiful in cloudy weather, 146;
its laws, 147.

Drummond light, the, 296.

Earthquakes, 126.

Electric light, 296.

Ether, its powers of vaporization, 224.

Explosive substances, 261, 264, 266.

Eye, the, wonderful construction of, 87.

Fire-damp not inflammable by red heat, 273;
rapidly explodes at white heat, 274;
explosible only when mixed with atmospheric air, 303.

Flame, subterranean, 125.

Flame, nature of, 285, 293;
of a candle hollow, 298;
experiments with, 301.

Fluids, expansive power of, 202.

Fulminates of the precious metals, 267;
gold, 268;
mercury, 268;
silver, 269;
platinum, 270.

Gas, its first application to illumination, 109.

Glass _absorbs artificial_, but transmits _solar_ heat, 159.

Hastings, French coast sometimes visible from, by refraction, 358.

Heat, natural sources of, 116;
celestial, 118;
subterraneous, 120-124;
mechanical production of, 129;
chemical, 130;
combustion, 132;
respiration, 131;
communication of, 134;
radiation, 137;
reflexion, 148;
difference between _solar_ and _terrestrial_, 158;
transmission of, 158-161;
absorption of, 163;
degrees of, in the spectrum, 162;
relative absorbing and radiating powers of surfaces, 165;
radiation by different colours, 166;
_solar_ more powerful reflected than direct, 168;
conduction, 169;
wonderful effects of, 193;
expansive power of, 196;
latent, 212;
white, 244;
artificial, curious changes in its character at high rates of
temperature, 244;
then assumes all the properties of solar, 244.

Hot-springs and wells, 125.

_Ignes fatui_, how produced, 260.

_Jack-o’-lanterns_, causes of, 260.

Land’s End described, 41.

Lens, magnifying power of the, how determined, 391.

Light, electric, 296;
artificial, dependent on heat, 297;
rays of, travel in straight lines, 344, 419;
refraction of, 350, 357;
experiments, 351;
rays of, assume the colour of objects from which they are
reflected, 367;
reflexion of, 407;
compound nature of a ray of, 422;
composed of seven colours, 423.

Lightning, varieties of, 128.

Liquids imperfect conductors of heat, 177;
merely solids whose particles are kept apart by heat, 212.

Lucifer-matches, why so readily inflammable, 272.

Luminosity, temperature at which bodies assume, 243.

Magician’s mirror, the, explained, 412.

Mail-coaches, the first, 109.

Metals, their relative power of conducting heat, 172;
expansion and contraction of, 197;
practical application of this power, 198;
cooling of, 239.

Microscope, principle of the, 395, 401;
the single, 397;
the compound, 400.

_Mirage_, an optical illusion caused by refraction, 357.

Mirrors, concave, experiments with, 148;
wonders produced by, 413;
different effects produced by metallic and glass, 154.

Mont Blanc, ascent of, 110;
ebullition on summit of, 230.

Objects, why they diminish in size in proportion to distance, 386;
magnifying of, by lenses, 388.

Oceanic currents, direction of the, 190.

Oils, lamp, philosophy of the combustion of, 275, 295.

Palissy, Bernard, his discoveries in pottery, 333.

Pendulum, compensation, principle of the, 200.

Phlogiston, an imaginary principle, 245.

Photography, first experiments in, 335, 433;
practice of, 435.

Prism, the spectrum produced by, merely an oblong figure of the sun,
421;
its colours the decomposition of the sunbeam into its elementary
tints, 422.

Pyramids of Egypt, size of the, 110.

Pyrophorus, how produced, 261.

Radiation, power of, in different substances, 142.

Ramsgate, Dover Castle rendered visible from, by refraction, 360.

Refraction of light, curious property of, 357;
illusions caused by extraordinary instances of, 358, 360.

Respiration, philosophy of, 86.

Safety-lamp, first glimmer of the, 93;
experiments with, 285;
completion of the first, 305;
its perfected form, 311;
value and importance of the invention, 303.

St. Michael’s Mount, Cornwall, 73.

Science, true, its nature, 87.

Scoresby’s, Captain, observation of a distant ship by refraction, 365.

Secretion, marvels of, 85.

Spectrum, proportions of prismatic tints in the, 426;
circular, 427.

Spontaneous combustion, 262, 272.

Steam, difference of heat in high and low pressure, 178.

Steam-boat, the first, 107.

Sun-pictures, the earliest, 335.

Talbot, Mr. Fox, his curious experiment of photographing a rapidly
revolving wheel, 443.

Talbotype process of photography, 436.

Telescope, principle of the, 393, 400.

Temperature, rate of increase below the surface of the earth, 122.

Vaporizable liquids readily explosible, 274.

Vision, range of, 386.

Volcanoes, 126.

_Will-o’-the-wisp_, philosophy of, 260.

Wire-gauze, its power of resisting flame, 310.

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