The Botanic Garden, a Poem in Two Parts. Part 1: the Economy of Vegetation

by Erasmus Darwin

The more probable opinion is perhaps, that the sun is a phlogistic mass of matter, whose surface is in a state of combustion, which 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 phlogistic 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 reduction of metallic calxes; thus in presenting to the flame of a candle a letter-wafer, (if it be coloured with red- lead,) at the time the red-lead becomes a metallic drop, a flash of light is perceived. Dr. Alexander Wilson very ingeniously endeavours to prove that the sun is only in a state of combustion on its surface, and that the dark spots seen on the disk are excavations or caverns through the luminous crust, some of which are 4000 miles in diameter. Phil. Trans. 1774. Of this I shall have occasion to speak again.

NOTE VI.—CENTRAL FIRES.

Round her still centre tread the burning soil, And watch the billowy Lavas, as they boil.

CANTO I. l. 139.

M. de Mairan in a paper published in the Histoire de l'Academie de Sciences, 1765, has endeavoured to shew that the earth receives but a small part of the heat which it possesses, from the sun's rays, but is principally heated by fires within itself. He thinks the sun is the cause of the vicissitudes of our seasons of summer and winter by a very small quantity of heat in addition to that already residing in the earth, which by emanations from the centre to the circumference renders the surface habitable, and without which, though the sun was constantly to illuminate two thirds of the globe at once, with a heat equal to that at the equator, it would soon become a mass of solid ice. His reasonings and calculations on this subject are too long and too intricate to be inserted here, but are equally curious and ingenious and carry much conviction along with them.

The opinion that the center of the earth consists of a large mass of burning lava, has been espoused by Boyle, Boerhave, and many other philosophers. Some of whom considering its supposed effects on vegetation and the formation of minerals have called it a second sun. There are many arguments in support of this opinion, 1. Because the power of the sun does not extend much beyond ten feet deep into the earth, all below being in winter and summer always of the same degree of heat, viz. 48, which being much warmer than the mildest frost, is supposed to be sustained by some internal distant fire. Add to this however that from experiments made some years ago by Dr. Franklin the spring-water at Philadelphia appeared to be of 52° of heat, which seems further to confirm this opinion, since the climates in North America are supposed to be colder than those of Europe under similar degrees of latitude. 2. Mr. De Luc in going 1359 feet perpendicular into the mines of Hartz on July the 5th, 1778, on a very fine day found the air at the bottom a little warmer than at the top of the shaft. Phil. Trans. Vol. LXIX. p. 488. In the mines in Hungary, which are 500 cubits deep, the heat becomes very troublesome when the miners get below 480 feet depth. Morinus de Locis subter. p. 131. But as some other deep mines as mentioned by Mr. Kirwan are said to possess but the common heat of the earth; and as the crust of the globe thus penetrated by human labour is so thin compared with the whole, no certain deduction can be made from these facts on either side of the question. 3. The warm-springs in many parts of the earth at great distance from any Volcanos seem to originate from the condensation of vapours arising from water which is boiled by subterraneous fires, and cooled again in their passage through a certain length of the colder soil; for the theory of chemical solution will not explain the equality of their heat at all seasons and through so many centuries. See note on Fucus in Vol. II. See a letter on this subject in Mr. Pilkinton's View of Derbyshire from Dr. Darwin. 4. From the situations of volcanos which are always found upon the summit of the highest mountains. For as these mountains have been lifted up and lose several of their uppermost strata as they rise, the lowest strata of the earth yet known appear at the tops of the highest hills; and the beds of the Volcanos upon these hills must in consequence belong to the lowest strata of the earth, consisting perhaps of granite or basaltes, which were produced before the existance of animal or vegetable bodies, and might constitute the original nucleus of the earth, which I have supposed to have been projected from the sun, hence the volcanos themselves appear to be spiracula or chimneys belonging to great central fires. It is probably owing to the escape of the elastic vapours from these spiracula that the modern earthquakes are of such small extent compared with those of remote antiquity, of which the vestiges remain all over the globe. 5. The great size and height of the continents, and the great size and depth of the South-sea, Atlantic, and other oceans, evince that the first earthquakes, which produced these immense changes in the globe, must have been occasioned by central fires. 6. The very distant and expeditious communication of the shocks of some great earthquakes. The earthquake at Lisbon in 1755 was perceived in Scotland, in the Peak of Derbyshire, and in many other distant parts of Europe. The percussions of it travelled with about the velocity of sound, viz. about thirteen miles in a minute. The earthquake in 1693 extended 2600 leagues. (Goldsmith's History.) These phenomena are easily explained if the central parts of the earth consist of a fluid lava, as a percussion on one part of such a fluid mass would be felt on other parts of its confining vault, like a stroke on a fluid contained in a bladder, which however gentle on one side is perceptible to the hand placed on the other; and the velocity with which such a concussion would travel would be that of sound, or thirteen miles in a minute. For further information on this part of the subject the reader is referred to Mr. Michell's excellent Treatise on Earthquakes in the Philos. Trans. Vol. LI. 7. That there is a cavity at the center of the earth is made probable by the late experiments on the attraction of mountains by Mr. Maskerlyne, who supposed from other considerations that the density of the earth near the surface should be five times less than its mean density. Phil. Trans. Vol. LXV. p. 498. But found from the attraction of the mountain Schehallien, that it is probable, the mean density of the earth is but double that of the hill. Ibid. p. 532. Hence if the first supposition be well founded there would appear to be a cavity at the centre of considerable magnitude, from whence the immense beds and mountains of lava, toadstone, basaltes, granite, &c. have been protruded. 8. The variation of the compass can only be accounted for by supposing the central parts of the earth to consist of a fluid mass, and that part of this fluid is iron, which requiring a greater degree of heat to bring it into fusion than glass or other metals, remains a solid, and the vis inertiae of this fluid mass with the iron in it, occasions it to perform fewer revolutions than the crust of solid earth over it, and thus it is gradually left behind, and the place where the floating iron resides is pointed to by the direct or retrograde motions of the magnetic needle. This seems to have been nearly the opinion of Dr. Halley and Mr. Euler.

NOTE VII.—ELEMENTARY HEAT.

Or sphere on sphere in widening waves expand, And glad with genial warmth the incumbent land.

CANTO I. l. 143.

A certain quantity of heat seems to be combined with all bodies besides the sensible quantity which gravitates like the electric fluid amongst them. This combined heat or latent heat of Dr. Black, when set at liberty by fermentation, inflammation, crystallization, freezing, or other chemical attractions producing new combinations, passes as a fluid element into the surrounding bodies. And by thawing, diffusion of neutral salts in water, melting, and other chemical solutions, a portion of heat is attracted from the bodies in vicinity and enters into or becomes combined with the new solutions.