to vibrate is general, or even universal, in the bigger orders of particles; and therefore makes it more easy to conceive, that there may be a like disposition in the lesser orders, i.e. in the infinitesimal medullary particles, considered in this and the foregoing proposition.
5. The mutual attractions and repulsions which seem to intercede between all small particles, concur to the same purpose. For when the attractions and repulsions are changed, by changing the distances of the particles, these must oscillate to and fro for some time, before they can gain their former equilibrium.
6. Elasticity seems to result from mutual attractions and repulsions of some kind, and is evidently the cause of vibrations in musical strings, and many other bodies. It seems also, that there is scarce any body entirely devoid of elasticity. And thus elasticity is connected with the doctrine of vibrations in different ways.
7. The effluvia of electric bodies seem to have vibrating motions. For they are excited by friction, patting, and heat; and excite light, sound, and a pricking sensation. They have also a repulsive power in respect of each other, as the particles of air have; and therefore must, like them, be easily susceptible of vibrations. Their motions along hempen strings resemble the motions along the nerves in sensation and muscular contraction; and their attractive powers, at the end of such strings, resemble the powers of the sensations over the muscles for contracting them. So that electricity is also connected in various ways with the doctrine of vibrations.
Lastly, To sum up in one what has been remarked in the last five paragraphs: as the attractions of gravitation, electricity, magnetism, and cohesion, with the repulsions which attend upon the three last, intimate to us the general tenor of nature in this respect; viz. that many of its phænomena are carried on by attractions and repulsions; and that these may be expected to take place in the small descending orders of particles, as well as in gross bodies, and in the biggest component particles; so the pulses of the air, the tremors of sounding bodies, the propagation of sounds both through the air, and along contiguous solid bodies, the oscillations of elastic bodies, and the phænomena of electricity, may, in like manner, serve as a clew and guide to the invention, and afford a presumption, that other reciprocal motions or vibrations have a great share in the production of natural phænomena.
Nor is it an objection to this, but rather a confirmation of it, that these principles of attraction and repulsion of the several kinds, and of vibrations, are dependent upon and involved within each other, since this also is agreeable to the tenor of nature, as it is observed in the body, in the mind, in science in general, and in the several branches of each science in particular. Each part, faculty, principle, &c. when considered and pursued
