agreeably to the hypothesis concerning pain, above proposed. For the strong vibrations excited by great heat must put the small parts beyond the spheres of each other’s attraction, and so produce the solution of continuity. But neither does it seem inconceivable, that cold may have a like effect, agreeably to the doctrine of vibrations, though the process be different. For great cold, by checking the vibrations in the external part, to which it is applied, will alter the situation and distance of the small medullary particles there, and so must excite vigorous vibrations in the ascending nerves, and the corresponding region of the brain, which is just the effect occasioned by heat, applied to the same external part. Hence, if we are touched by a very hot or a very cold body, inadvertently, and without seeing it, it ought to be difficult to distinguish which it is, agreeably to the fact. The conflict between the diminished vibrations, in the external part to which cold is applied, and the previous ones subsisting in the corresponding part of the brain, may exalt these previous ones, as much as heat does, so as to render the first simple impression of cold similar to that of heat.
Fifthly, The continued impression of heat makes us more sensible of cold. For when heat has rarefied the parts, and adapted them to a peculiar strength and frequency of vibrations, differing from the usual standard, the cold, whose difference from the usual standard lies on the other side, must raise a greater conflict, and produce a greater change, than if the parts had remained at the usual standard. The continued impression of cold must for the same reasons make us more sensible of heat. This explication will perhaps suit with other theories of sensation, as well as with that drawn from vibrations. However, the mere consistency of any phænomena with the doctrine of vibrations is worthy of some attention in this inquiry.
Sixthly, When the calf of the leg is affected with the cramp, setting the foot upon a cold marble will afford immediate relief. For the cold may check the violent vibrations in the fibres of the gastrocnemii and soleus, directly and immediately; or it may do it by exciting vigorous vibrations of a different kind, which extend to their antagonist muscles, as well as to the fore-mentioned ones. But I judge the first account to be more probable.
Seventhly, If a limb, that has been much chilled with cold, be brought to a fire suddenly, it will first be much pained, and then mortify. For the vibrations excited by the fire, though moderate in respect of the usual standard, are yet excessive in respect of those which the cold has introduced, also in respect of that sphere of attraction, which it has now fixed upon the parts: there will arise therefore a violent conflict, solution of continuity, and consequent pain; and the parts will be agitated so much more than their present spheres of attraction will permit, that they cannot return to it any more, but must be entirely disunited, and run into different combinations, i.e. the limb must
