My friend, Mr George Hunter of York, while one day amusing himself with repeating some of these experiments, discovered that by placing one of the metals as high up as possible between the gums and the upper lip, and the other in a similar situation with respect to the under lip, a flash was produced as vivid as that occasioned by passing one of the metals up the nose, and placing the other upon the tongue. It differs, however, from the flash produced in any other way, in the singular circumstance of not being confined to the eye alone, but appearing diffused over the whole of the face. On attending to the concomitant sensations produced by this disposition of the metals, I perceived that a sense of warmth, at the instant they were brought into contact, diffused itself over the whole upper surface of the tongue, proceeding from its root to the point. Dr Rutherford, to whom Mr Hunter had communicated this experiment, remarked, on repeating it, that a flash is produced not only at the instant the metals are brought into contact, but likewise at the instant of their separation. While they remain in contact, no flash is observed. This fact is precisely analogous to one already mentioned of contractions being produced in the leg of a frog, at the instant one of the metals in contact with the other metal is withdrawn from the leg. After this full detail of these curious phenomena, I hardly need remark, that they demonstrate the free communication, which subsists between the several branches of the fifth pair of nerves, and consequently give strong support, if not absolute confirmation, to the well known doctrine of nervous sympathy, or of the reciprocal influence, which different parts exert upon each other, through the medium of nerves. If I might be allowed to hazard a conjecture, where we cannot have recourse to demonstration, I should say that the flash, observed in the above experiments, was the effect of contractions excited in involuntary muscles by the application of a stimulus to their nerves; or, in other words, that the effects of the application of the metals to the nasal branch of the first division of the fifth pair of nerves, had been propagated through the ciliary ganglion, along the ciliary nerves, and to the choroid coat, whose vessels it had excited into instantaneous action; and that their action again (as in the case of action excited by pressure, or a blow upon the eye,) had by stimulating the retina occasioned the sense of light. This supposition is, I think, rendered probable by several considerations. I have already shewn that this influence can excite contractions in involuntary muscles, through the medium of their nerves. And certainly no reason can be assigned, _a priori_, why it should not act equally upon every description of involuntary muscles; upon those which make a part of the minutest vessels in the body, as well as upon the heart, or upon the iris.