increase, or continuance; and the same thing holds in respect of friction, light, and sounds. And as medicinal bodies appear, from observations both philosophical and vulgar, to be endued with more active properties than common aliments; i.e. to be fitted for exciting stronger vibrations; so their tastes and smells are, for the most part, ungrateful; whereas those of common aliments are pleasant. It may be observed also, that some painful sensations, as they decrease by time, or the removal of the cause, pass into positive local pleasures, of the same species as the preceding pain; thus shewing the near alliance between pleasure and pain; and that a mere difference in degree puts on the appearance of one in kind, at a certain point. I suppose it may be referred to this head, that some bitter and acrimonious tastes leave an agreeable relish of the sweet kind upon the tongue after some time.
Secondly, It agrees well with the doctrine of vibrations, that all evident solutions of continuity in the living parts occasion pain, inasmuch as a solution of continuity cannot happen without a violent impression of some sensible object, nor, by consequence, without violent mutual actions between the object, nerves, and æther. The solution of continuity does therefore pre-suppose that degree of violence in the vibrations, which exceeds the limit of pleasure, and is proper to pain, according to the foregoing paragraph.
Thirdly, We may, in like manner, give a reason, from the doctrine of vibrations, both why a moderate degree of distention in the parts is necessary to their growth and pleasurable state; and also, why all great distentions are attended with pain for a considerable time before they are raised to such a pitch as to cause a visible solution of continuity. For a great distention is equivalent to a vigorous impression of a sensible object, being often caused by such; and as the situation of the small particles is changed in great distentions, their mutual actions will be changed also, and so may give rise to more vigorous vibrations; and these increased vibrations may either fall within the limits of pleasure, or go beyond them, according to their degree. We are also to consider, that in all considerable distentions there is an increase of friction between the vessels and circulating fluids, and consequently of heat, i.e. of vibrations.
But besides this, it seems not improbable, that in preternatural and painful distentions, the small particles are perpetually separating themselves from their former cohesions, and running into new ones; so that a minute and invisible solution of continuity is carried on during the whole distention, till such time as this degree of distention becomes familiar to the parts, and the situation and mutual actions of the small particles be accommodated to it. Thus, the cause of the pain in distentions will arise from the solution of continuity, and may be referred to the foregoing head. And conversely it appears, that in manifest
