IV. "EFFULGENT MAIDS! YOU round deciduous day,
Tressed with soft beams, your glittering bands array;
175 On Earth's cold bosom, as the Sun retires,
Confine with folds of air the lingering fires;
O'er Eve's pale forms diffuse phosphoric light,
And deck with lambent flames the shrine of Night.
So, warm'd and kindled by meridian skies,
180 And view'd in darkness with dilated eyes,
BOLOGNA'S chalks with faint ignition blaze,
BECCARI'S shells emit prismatic rays.
So to the sacred Sun in MEMNON's fane,
Spontaneous concords quired the matin strain;
185 —Touch'd by his orient beam, responsive rings
The living lyre, and vibrates all it's strings;
Accordant ailes the tender tones prolong,
And holy echoes swell the adoring song.
[Confine with folds of air. l. 176. The air, like all other bad conductors of electricity, is known to be a bad conductor of heat; and thence prevents the heat acquired from the sun's rays by the earth's surface from being so soon dissipated, in the same manner as a blanket, which may be considered as a sponge filled with air, prevents the escape of heat from the person wrapped in it. This seems to be one cause of the great degree of cold on the tops of mountains, where the rarity of the air is greater, and it therefore becomes a better conductor both of heat and electricity. See note on Barometz, Vol. II. of this work.
There is however another cause to which the great coldness of mountains and of the higher regions of the atmosphere is more immediately to be ascribed, explained by Dr. Darwin in the Philos. Trans. Vol. LXXVIII. who has there proved by experiments with the air-gun and air-pump, that when any portion of the atmosphere becomes mechanically expanded, it absorbs heat from the bodies in its vicinity. And as the air which creeps along the plains, expands itself by a part of the pressure being taken off when it ascends the sides of mountains; it at the same time attracts heat from the summits of those mountains, or other bodies which happen to be immersed in it, and thus produces cold. Hence he concludes that the hot air at the bottom of the Andes becomes temperate by its own rarefaction when it ascends to the city of Quito; and by its further rarefaction becomes cooled to the freezing point when it ascends to the snowy regions on the summits of those mountains. To this also he attributes the great degree of cold experienced by the aeronauts in their balloons; and which produces hail in summer at the height of only two or three miles in the atmosphere.]
[Diffuse phosphoric light. l. 177. I have often been induced to believe from observation, that the twilight of the evenings is lighter than that of the mornings at the same distance from noon. Some may ascribe this to the greater height of the atmosphere in the evenings having been rarefied by the sun during the day; but as its density must at the same time be diminished, its power of refraction would continue the same. I should rather suppose that it may be owing to the phosphorescent quality (as it is called) of almost all bodies; that is, when they have been exposed to the sun they continue to emit light for a considerable time afterwards. This is generally believed to arise either from such bodies giving out the light which they had previously absorbed; or to the continuance of a slow combustion which the light they had been previously exposed to had excited. See the next note.]
[Beccari's shells. l. 182. Beccari made many curious experiments on the phosphoric light, as it is called, which becomes visible on bodies brought into a dark room, after having been previously exposed to the sunshine. It appears from these experiments, that almost all inflammable bodies possess this quality in a greater or less degree; white paper or linen thus examined after having been exposed to the sunshine, is luminous to an extraordinary degree; and if a person shut up in a dark room, puts one of his hands out into the sun's light for a short time and then retracts it, he will be able to see that hand distinctly and not the other. These experiments seem to countenance the idea of light being absorbed and again emitted from bodies when they are removed into darkness. But Beccari further pretended, that some calcareous compositions when exposed to red, yellow, or blue light, through coloured glasses, would on their being brought into a dark room emit coloured lights. This mistaken fact of Beccari's, Mr. Wilson decidedly refutes; and among many other curious experiments discovered, that if oyster-shells were thrown into a common fire and calcined for about half an hour, and then brought to a person who had previously been some minutes in a dark room, that many of them would exhibit beautiful irises of prismatic colours, from whence probably arose Beccari's mistake. Mr. Wilson from hence contends, that these kinds of phosphori do not emit the light they had previously received, but that they are set on fire by the sun's rays, and continue for some time a slow combustion after they are withdrawn from the light. Wilson's Experiments on Phosphori. Dodsley, 1775.
The Bolognian stone is a selenite, or gypsum, and has been long celebrated for its phosphorescent quality after having been burnt in a sulphurous fire; and exposed when cold to the sun's light. It may be thus well imitated: Calcine oyster-shells half an hour, pulverize them when cold, and add one third part of flowers of sulphur, press them close into a small crucible, and calcine them for an hour or longer, and keep the powder in a phial close stopped. A part of this powder is to be exposed for a minute or two to the sunbeams, and then brought into a dark room. The calcined Bolognian stone becomes a calcareous hepar of sulphur; but the calcined shells, as they contain the animal acid, may also contain some of the phosphorus of Kunkel.]
[In Memnon's fane. l. 183. See additional notes. No. VIII.]
"YOU with light Gas the lamps nocturnal feed,
190 Which dance and glimmer o'er the marshy mead;
Shine round Calendula at twilight hours,
And tip with silver all her saffron flowers;
Warm on her mossy couch the radiant Worm,
Guard from cold dews her love-illumin'd form,
195 From leaf to leaf conduct the virgin light,
Star of the earth, and diamond of the night.
You bid in air the tropic Beetle burn,
And fill with golden flame his winged urn;
Or gild the surge with insect-sparks, that swarm
200 Round the bright oar, the kindling prow alarm;
Or arm in waves, electric in his ire,
The dread Gymnotus with ethereal fire.—
Onward his course with waving tail he helms,
And mimic lightenings scare the watery realms,
205 So, when with bristling plumes the Bird of JOVE
Vindictive leaves the argent fields above,
Borne on broad wings the guilty world he awes,
And grasps the lightening in his shining claws.
[The lamps nocturnal. l. 189. The ignis fatuus or Jack a lantern, frequently alluded to by poets, is supposed to originate from the inflammable air, or Hydrogene, given up from morasses; which being of a heavier kind from its impurity than that obtained from iron and water, hovers near the surface of the earth, and uniting with common air gives out light by its slow ignition. Perhaps such lights have no existence, and the reflection of a star on watery ground may have deceived the travellers, who have been said to be bewildered by them? if the fact was established it would much contribute to explain the phenomena of northern lights. I have travelled much in the night, in all seasons of the year, and over all kinds of soil, but never saw one of these Will o'wisps.]
[Shine round Calendula. l. 191. See note on Tropaeolum in Vol. II.]