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almost entirely from those of Suez and the Red Sea. It is observed that shallow seas have a different set of plants from such as are deeper and colder; and, unlike terrestrial vegetation, the algæ are more numerous in the mean latitudes than either towards the equator or the poles. They vary also with the depth: completely different kinds affect different depths, their seeds being of such specific gravity as to remain and germinate where the parent plant grew. The quantity of alge in that accumulation known as the sargassa or grassy sea is so great, that the early navigators, Columbus and Lerius, compared it to extensively inundated meadows: it impeded their ships, and alarmed their sailors. It is in the North Atlantic, a little to the west of the meridian of Fayal, one of the Azores, between the 25th and 36th parallels of latitude. A smaller bank lies between the 22nd and 26th degrees of north latitude, about 80 leagues west of the meridian of the Bahama Islands. These masses chiefly consist of one or two species of sargassa, the most extensive genus of the order Fucoideæ.
Some of the seaweeds grow to enormous lengths, and all are highly coloured, though many of them must grow in deep water. Light, however, may not be the only principle on which the colour of vegetables depends, since Baron Humboldt met with green plants growing in complete darkness in one of the mines at Freyberg.
In the dark and tranquil caves of the ocean, on the shores alternately covered and deserted by the restless waves, on the lofty mountain and extended plain, in the chilly regions of the north, and in the genial warmth of the south, specific diversity is a general law of the vegetable kingdom, which cannot be accounted for by diversity of climate; and yet the similarity, though not identity, of species is such, under the same isothermal lines, that if the number of species belonging to one of the great families of plants be known in any part of the globe, the whole number of the flowering or more perfect plants, and also the number of species composing the other vegetable families, may be estimated with considerable accuracy.
Various opinions have been formed on the original or primitive distribution of plants over the face of the globe; but, since botanical geography has become a science, the phenomena ob
served have led to the conclusion that vegetable creation must have taken place in a number of distinctly different centres, as the islands and continents rose above the ocean, each of which was the original seat of a certain number of peculiar species which at first grew there and nowhere else. Heaths are exclusively confined to the Old World; and no indigenous rose-tree has ever been seen in the New, the whole southern hemisphere being destitute of that beautiful and fragrant plant. But this is still more confirmed by multitudes of particular plants, having an entirely local and insulated existence, growing spontaneously in some particular spot, and in no other place: for example, the cedar of Lebanon, which grows indigenously on that mountain, and in no other part of the world. On the other hand, as there can be no doubt that many races of plants have been extinguished, Sir John Herschel thinks it possible that these solitary instances may be the last surviving remnants of the same group universally disseminated, but in course of extinction, or that perhaps two processes may be going on at the same time:"Some groups may be spreading from their foci, others retreating to their last holds."
The same laws obtain in the distribution of the animal creation. Even the microscopic existences, which seem to be the most widely spread, have their specific localities; and the zoophyte (N. 219), occupying the next lowest place in animated nature, is widely scattered through the seas of the torrid zone, each species being confined to the district and depth best suited to its wants. Mollusks, or the animals of shells, decrease in size and beauty with their distance from the equator; and not only each sea and every basin of the ocean, but each depth, is inhabited by its peculiar tribe of fish. Indeed, MM. Peron and Le Sueur assert that, among the many thousands of marine animals which they had examined, there is not a single animal of the southern regions which is not distinguishable by essential characters from the analogous species in the northern seas.
Reptiles are not exempt from the general law. The saurian (N. 220) tribes of the four quarters of the globe differ in species; and, although warm countries abound in venomous snakes, they are specifically different in different localities, and decrease both in numbers and in the virulence of their poison
with decrease of temperature. The dispersion of insects necessarily follows that of the vegetables which supply their food; and in general it is observed that each kind of plant is peopled by its peculiar inhabitants. Each species of bird has its peculiar haunt, notwithstanding the locomotive powers of the winged tribes. The emu is confined to Australia, the condor to the Andes and their declivities, and the bearded vulture or lemmergeyer to the Alps. Some birds, like the common sparrow, have a wide range; but those met with in every country are few in number. Quadrupeds are distributed in the same manner wherever man has not interfered. Such as are indigenous in one country are not the same with their congeners in another; and, with the exception of some kind of bats, no mammiferous animal is indigenous in the Polynesian Archipelago, nor in any of the islands on the borders of the central part of the Pacific.
In reviewing the infinite variety of organised beings that people the surface of the globe, nothing is more remarkable than the distinctions which characterise the different tribes of mankind, from the ebony skin of the torrid zone to the fair and ruddy complexion of the Scandinavian a difference which existed in the earliest recorded times, since the African is represented in the sacred writings to have been as black as he is at the present day, and the most ancient Egyptian paintings confirm that truth; yet it appears, from a comparison of the principal circumstances relating to the animal economy or physical character of the various tribes of mankind, that the different races are identical in species. Many attempts have been made to trace the various tribes back to a common origin, by collating the numerous languages which are or have been spoken. Some classes of these have few or no words in common, yet exhibit a remarkable analogy in the laws of their grammatical construction. The languages spoken by the native American nations afford examples of these; indeed, the refinement in the grammatical construction of the tongues of the American savages leads to the belief that they must originally have been spoken by a much more civilised class of mankind. Some tongues have little or no resemblance in structure, though they correspond extensively in their vocabularies, as the Syrian dialects. In all these cases it may be inferred that the nations speaking the lan
guages in question descended from the same stock; but the probability of a common origin is much greater in the Indo-European nations, whose languages, such as the Sanscrit, Greek, Latin, German, &c., have an affinity both in structure and correspondence of vocables. In many tongues not the smallest resemblance can be traced; length of time, however, may have obliterated original identity; but so many ages have passed before the subject became a study, and so many languages have worn out of use, that it may be doubted whether any satisfactory result will ever be arrived at with regard to the original speech of mankind.
Terrestrial Heat-Radiation-Transmission -- Melloni's experiments-Heat in Solar Spectrum - Polarization of Heat - Nature of Heat - Absorptions Dew- Rain-Combustion - Expansion-Compensation Pendulum Transmission through Crystals Propagation Dynamic Theory of Heat — Mechanical equivalent of Heat — Latent Heat is the Force of Expansion - Steam- Work performed by Heat- Conservation of Force Mechanical Power in the Tides - Dynamical Power of Light Analogy between Light, Heat, and Sound.
THAT heat producing rays exist independently of those of light is a matter of constant experience in the abundant emission of them from boiling water. They dart in divergent straight lines from flame and from each point in the surfaces of hot bodies, in the same manner as diverging rays of light proceed from every point of those that are luminous. According to the experiments of Sir John Leslie, radiation proceeds not only from the surface of substances, but also from the particles at a minute depth below it. He found that the emission is most abundant in a direction perpendicular to the radiating surface, and that it is more rapid from a rough than from a polished surface: radiation, however, can only take place in air and in vacuo; it is altogether imperceptible when the hot body is enclosed in a solid or liquid. Heated substances, when exposed to the open air, continue to radiate heat till they become nearly of the temperature of the surrounding medium. The radiation is very rapid at first, but diminishes according to a known law with the temperature of the heated body. It appears, also, that the radiating power of a surface is inversely as its reflecting power; and bodies that are most impermeable to heat radiate least. Substances, however, have an elective power, only reflecting heat of a certain refrangibility. Mr. Grove gives paper, snow, and lime as instances, which, although all white, radiate heat of different refrangibilities, while metals, whatever their colour may be, radiate all kinds alike.
Rays of heat, whether they proceed from the sun, from flame, or other terrestrial sources, luminous or non-luminous, are in