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The primary motions of matter may be divided into three classes, those belonging to gravitation, to chemistry, and to life; and each class has its peculiar laws. Though these three classes include the motions of solid, liquid, and aerial bodies; there is nevertheless a fourth division of motions; I mean those of the supposed ethereal fluids of magnetism, electricity, heat, and light; whose properties are not so well investigated as to be classed with sufficient accuracy.
lso. The gravitating motions include the annual and diurnal rotation of the earth and planets, the flux and reflux of the ocean, the descent of heavy bodies, and other phaenomena of gravitation. The unparalleled sagacity of the great Newton has deduced the laws of this class of motions from the simple principle of the general attraction of matter: to which should perhaps be added the general repulsion of matter; which seems to have caused the projection of the planets from the sun, and to prevent their relapse into one chaotic mass. These motions are distinguished by their tendency to or from the centres of the fun or planets.
2d. The chemical class of motions includes all the various appearances of chemistry. Many of the facts, which belong to these branches of science, are nicely ascertained, and elegantly classed; but their laws have not yet been developed from such simple principles as those above mentioned; though it is probable, that they depend on the specific attractions belonging to the particles of bodies, or to the difference of the quantity of attraction belonging to the sides and angles of those particles, to which should perhaps be added the specific repulsions belonging to the particles of bodies. When these repulsions prevail over the attractions, they may cause the diffusions of light and of odours, the explosions of some bodies, and the flower decomposition of others, and occasion our ideas of fluidity; when the attractions prevail over the repulsions, they may cause the strićter combinations and cohesions of matter, as in crystallization or cooling, and give rise to our ideas of solidity; and when these two causes of motion are in active equilibrium, they may produce the vibrations of the particles of bodies, and occasion our ideas of sound. The chemical motions are distinguished by their being generally attended with an evident decomposition or new combination of the active materials.
3d. The third class includes all the motions of the animal and vegetable world; as well those of the vessels, which circulate their juices, and of the muscles, which perform their locomotion, as those of the organs offense, which constitute
their ideas. This last class of motion is the subjećt of the following pages; which, though conscious of B 2. their
the neck and back, and which is erroneously called the spinal marrow. The ultimate fibrils of these nerves terminate in the immediate organs of sense and muscular fibres, and is a ligature be put on any part of their passage from the head or spine, all motion and perception cease in the parts beneath the ligature. 2. The longitudinal muscular fibres compose the locomotive muscles, whose contračtions move the bones of the limbs and trunk, to which their extremities are attached. The annular or spiral muscular fibres compose the vascular muscles, which constitute the intestinal canal, the arteries, veins, glands, and absorbent vessels. 3. The immediate organs of sense, as the retina of the eye, probably consist of moving fibrils, with a power of contračtion fimilar to that of the larger muscles above described. 4. The cellular membrane consists of cells, which resemble those of a sponge, communicating with each other, and connecting together all the other parts of the body. 5. The arterial system consists of the aorta and the pulmonary artery, which are attended through their whole course with their correspondent veins. The pulmonary artery receives the blood from the right chamber of the heart, and carries it to the minute extensive ramifications of the lungs, where it is exposed to the action of the air on a surface equal to that of the whole external skin,