The botanic garden. A poem in two parts. Containing the Economy of vegetation; and the loves of the plants; with philosophical notes / / by Erasmus Darwin.

Darwin, Erasmus, 1731-1802.

Date:: 1825

Credit: The botanic garden. A poem in two parts. Containing the Economy of vegetation; and the loves of the plants; with philosophical notes / / by Erasmus Darwin. Source: Wellcome Collection.

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$transparent retina, this would account for the coloured glare of the eyes of dogs or cats and rab- bits in dark corners. NOTE IV—COMETS. Alarm with comet-blaze the sapphire plain, The wan stars glimmering through its silver train. Canto I. 1. 133. There have been many theories invented to account for the tails of comets. Sir Isaac New- ton thinks that they consist of rare vapours rarefied by the sun’s heat as to have their gene- ral gravitation diminished, and that they in consequence ascend opposite to the sun, and from thence reflect the rays of light. Dr. Hal- ley compares the light of the tails of comets to the streams of the aurora borealis* and other electric effluvia. Philos. Trans. No. 347. Dr. Hamilton observes, that thelightof small stars is seen undiminished through both the light of the tails of comets, and of the aurora borealis, and has farther illustrated their elec- tric analogy, and adds that the tails of comets consist of a lucid self-shining substance which has not the power of refracting or reflecting the rays of light. Essays. The tail of the comet of 1744 at one time ap- peared to extend above 16 degrees from its body, and must have thence been above twenty-three millions of miles long. And the comet of 1680, according to the calculations of Dr. Halley on November the 11th, was not above one semi- diameter off the earth, or less than 4000 miles to the northward of the way of the earth ; at which time had the earth been in that part of its orbit, what might have been the consequence! no one would probably have survived to have I registered the tremendous effects. The comet of 1531, 1607, and 1682, having returned in the year 1759, according to Dr Hal- ley’s prediction in the Philos. Trans, for 1705, there seems no reason to doubt that all the other comets will return after their proper periods. Astronomers have in general acquiesced in the conjecture, of Dr. Halley, that the comets of 1532, and 1661, arcane and the same comet, from the similarity of the elements of their or- bits, and were therefore induced to expect its return to its perihelium in 1789. As this comet is liable to be disturbed in its ascent from the sun by the planets Jupiter and Saturn, Dr. Maskelyne expected its return to its perihelium in the beginning of the year 1789, or the latter end of the year 1788, and certainly some time before the 27th of April, 1789, which prediction has not been fulfilled. Philos. Trans. Vol. LXXVI. As the comets are small masses of matter, and pass in their periheliun very near the sun, and become invisible to us on these accounts in a short space of time, their number has not yet been ascertained, and will probably increase with the improvement of our telescopes. M. Bode has given a table of 72 comets, whose orbits are already calculated ; of these 60 pass within the earth’s orbit, and only twelve without it; and most of them appear between the orbits of Ve- nus and Mercury, or nearly midway between the sun and earth ; from whence, and from the planes of their orbits, being inclined to that of the earth and other planets in all possible angles, they are believed to be less liable to Interfere with, or injure each other. M. Bode after- wards inquires into the nearest approach it is possible for each of the known comets to make towards the earth’s orbit. He finds that only three of them can come within a distance equal to two or three times the distance of the moon from it: and then adds the great improbability, that the earth should be in that dangerous point of its oi'bit, at the instant when a comet, which may have been absent some cen- turies, passes so rapidly past it. Historic de l’Academ. Royal. Berlin. 1792. NOTE V SUN’S RAYS. Or give the sun's phlogistic orb to roll. Canto I. 1. 136. The dispute among philosophers about phlo- giston is not concerning the existence of an inflammable principle, but rather whether there be one or more inflammable principles. The disciples of Stahl, which till lately included the whole chemical world, believed in the identity of phlogiston in all bodies which would flame or calcine. The disciples of Lavoisier pay homage to a plurality of phlogistons under the various names of charcoal, sulphur, metals, &c. What- ever will unite with pure air, and thence com- pose an acid, is esteemed in this ingenious tlieoi’y to be a different kind of phlogistic or in- flammable body. At the same time there re- mains a doubt whether these inflammable bodies, as metals, sulphur, charcoal, &c. may not be compounded of the same phlogiston along with some other material yet undiscovered, and thus an unity of phlogiston exist, as in the theory of Stahl, though very differently applied in the ex- plication of chemical phenomena. Some modern philosophers are of opinion, that the sun is the great fountain from .which the earth and other planets derive all the phlogiston which they possess ; and that this is formed by the combination of the solar rays with all opake bodies, but particularly with the leaves of vege- tables, which they suppose to be organs adapted to absorb them. And that as animals receive their nourishment from vegetables they also ob- tain in a secondary manner their phlogiston from the sun. And lastly, as great masses of the mineral kingdom, which have been found in the thin crust of the earth which human labour has penetrated, have evidently been formed from the recrements of animal and vegetable bodies, these also are supposed thus to have derived their phlo- giston from the sun. Another opinion concerning the sun’s rays is, that they are not luminous till they arrive at our atmosphere; and that there uniting with some part of the air they produce combustion, and light is emitted, and that an ethereal acid, yet undiscovered, is formed from this combustion. The more probable opinion is, perhaps, that the sun is a phlogistic mass of matter, whose surface is in a state of combustion, w hich, like other burning bodies, emits light with immense velocity in all directions; that these rays of light act upon all opake bodies, and combining with them either displace or produce their elementary heat, and become chemically combined with the phlogcstic part of them; for light is given out when phlogistic bodies unite with the oxygenous principle of the air, as in combustion, or in the$