science, the phenomena observed have led to the conclusion that vegetable creation must have taken place in a number of distinctly different centers, 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 discovered 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 but 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 groups 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 strongholds."

The same laws obtain in the distribution of the animal creation. The zoophyte (N. 215), occupying the lowest place in animated nature, is widely scattered through the seas of the torrid zone, each species being confined to the district best fitted to its existence. Shell-fish decrease in size and beauty with their distance from the equator; and as far as is known, each sea has its own kind, and every basin of the ocean 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. 216) tribes of the four quarters of the globe differ in species; and although warm countries abound in venomous snakes, they are specifically different, and decrease both in numbers and in the virulence of their poison with de

crease of temperature. The dispersion of insects necessarily follows that of the vegetables which supply them with food; and in general it is observed, that each kind of plant is peopled by its peculiar inhabitants. Each species of bird has its particular haunt, notwithstanding the locomotive powers of the winged tribes. The emu is confined to Australia, the condor never leaves the Andes, nor the great eagle the Alps; and although some birds are common to every country, they are few in number. Quadrupeds are distributed in the same manner wherever man has not interfered. Such as are indigenous in one continent are not the same with their congeners in another; and with the exception of some kinds of bats, no warm-blooded 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 organized beings that people the surface of the globe, nothing is more remarkable than the distinctions which characterize the different tribes of mankind, from the ebony skin of the torrid zone to the fair and ruddy complexion of Scandinavia 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 civilized 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 of these cases it may be inferred, that the nations speaking the languages in question are 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. The conclusion drawn from the whole investigation is, that although the distribution of organized beings does not follow the direction of the isothermal lines, temperature has a very great influence on their physical development. The heat of the air is so intimately connected with its electrical condition, that electricity must also affect the distribution of plants and animals over the face of the earth, the more so as it seems to have a great share in the functions of animal and vegetable life. It is the sole cause of many atmospheric and terrestrial phenomena, and performs an important part in the economy of nature.

SECTION XXVIII.

Of ordinary Electricity, generally called Electricity of Tension-Methods of exciting Bodies-Transference-Electrics and Non-Electrics-Law of its Intensity-Distribution-Tension-Electric Heat and Light-Atmospheric Electricity-Its Cause-Electric Clouds-Back Stroke-Violent Effects of Lightning-Its Velocity-Phosphorescence-Phosphorescent Action of Solar Spectrum-Aurora,

ELECTRICITY is one of those imponderable agents pervading the earth and all substances, without affecting their volume or temperature, or even giving any visible sign of its existence when in a latent state; but when elicited developing forces capable of producing the most sudden, violent, and destructive effects in some cases, while in others their action, though less energetic, is of indefinite and uninterrupted continuance. These modifications of the electric force, incidentally depending upon the manner in which it is excited, present phenomena of great diversity, but yet so connected as to justify the conclusion that they originate in a common principle.

Electricity may be called into activity by mechanical power, by chemical action, by heat, and by magnetic influence. We are totally ignorant why it is roused from its neutral state by such means, or of the manner of its existence in bodies, whether it be a material agent, vibrations of ether, or merely a property of matter. Various circumstances render it more than probable that, like light and heat, it is a modification or vibration of that subtile ethereal medium which in a highly elastic state pervades all space, and which is capable of moving with various degrees of facility through the pores even of the densest substances. As experience shows that bodies in one electric state attract, and in another repel each other, the hypothesis of two fluids has been adopted by many philosophers; but probably the mutual attraction and repulsion of bodies arise from the redundancy and defect of their electricities, though all the electrical phenomena can be explained on either hypothesis. Bodies having a redundancy of the electric fluid are said to be positively electric, and those in defect negatively. As each kind of electricity has its peculiar properties, the science may be divided into four branches, of which the following notice is intended to convey some idea.

Substances in a neutral state neither attract nor repel. There is a numerous class called electrics, in which the electric equilibrium is destroyed by friction; then the positive and negative electricities are called into action or separated; the positive is impelled in one direction, and the negative in another; or more correctly, the electricity is impelled in one direction at the expense of the other where there is a deficiency of it. Electricities of the same kind repel, whereas those of different kinds attract each other. The attractive power is exactly equal to the repulsive power at equal distances, and when not opposed, they coalesce with great rapidity and violence; producing the electric flash, explosion, and shock: then equilibrium is restored, and the electricity remains latent till again called forth by a new exciting cause. One kind of electricity cannot be evolved without the evolution of an equal quantity of the opposite kind. Thus when a

glass rod is rubbed with a piece of silk, as much positive electricity is elicited in the glass as there is negative in the silk; or in other words there is a redundancy in the glass and a proportional deficiency in the silk. The kind of electricity depends more upon the mechanical condition than on the nature of the surface: for when two plates of glass, one polished and the other rough, are rubbed against each other, the polished surface acquires positive and the rough negative electricity; that is, the one gains and the other loses. The manner in which friction is performed also alters the kind of electricity. Equal lengths of black and white riband applied longitudinally to one another, and drawn between the finger and thumb, so as to rub their surfaces together, become electric. When separated, the white riband is found to have acquired positive electricity, and the black has lost it, or become negative: but if the whole length of the black riband be drawn across the breadth of the white, the black will be positively and the white negatively electric when separate. Electricity may be transferred from one body to another in the same manner as heat is communicated, and like it too, the body loses by the transmission. Although no substance is altogether impervious to the electric fluid, nor is there any that does not oppose some resistance to its passage, yet it moves with much more facility through a certain class of substances called conductors, such as metals, water, the human body, &c., than through atmospheric air, glass, silk, &c., which are therefore called non-conductors. The conducting power is affected both by temperature and moisture.

Bodies surrounded with non-conductors are said to be insulated, because, when charged, the electricity cannot escape. When that is not the case, the electricity is conveyed to the earth, which is formed of conducting matter; consequently it is impossible to accumulate electricity in a conducting substance that is not insulated. There are a great many substances called nonelectrics, in which electricity is not sensibly developed by friction, unless they be insulated, probably because it is carried off by their conducting power as soon as elicited. Metals, for example, which are said to be