Reasoning from common phænomena of sensation, particularly those relating to heat, it is probable that pleasurable feeling is uniformly connected with a moderate increase of nervous action; and that this increase when carried to certain limits, produces mixed emotion or sublime pleasure; and beyond those limits occasions absolute pain. Comparing the facts in the last division, it is likely that individuals possessed of high health and little sensibility, will generally be less pleasurably affected by nitrous oxide than such as have more sensibility, in whom the emotions will sometimes so far enter the limits of pain as to become sublime;[229] whilst the nervous action in such as have exquisite sensibility, will be so much increased as often to produce disagreeable feeling. Modification of the powers of nitrous oxide by mixture of the gas with oxygene or common air, will probably enable the most delicately sensible to respire it without danger, and even with pleasurable effects: heretofore it has been administered to such only in its pure form or mingled with small quantities of atmospheric air, and in its pure form even the most robust are unable to respire it with safety for more than five minutes. [229] Sublime emotion with regard to natural objects, is generally produced by the connection of the pleasure of beauty with the passion of fear. The muscular actions[230] sometimes connected with the feelings produced by nitrous oxide, seem to depend in a great measure upon the particular habits of the individual; they will usually be of that kind which is produced either by common pleasurable feelings or strong emotions. [230] The immortal HARTLEY has demonstrated that all our motions are originally automatic, and generally produced by the action of tangible things on the muscular fibre. The common actions of adults may be distinguished into two kinds; voluntary actions, and mixed automatic actions. The first are produced by ideas, or by ideas connected with passions. The second by impression, or by pleasure and pain. In voluntary action, regular associations of ideas and muscular motions exist: as when a chemist performs a pre-conceived experiment. In mixed automatic actions, the simple motions produced by impression are connected with series of motions formerly voluntary, but now produced without the intervention of ideas: as when a person accustomed to play on the harpsichord, from accidentally striking a key, is induced to perform the series of motions which produce a well-remembered tune. Evidently the muscular actions produced by nitrous oxide are mixed automatic motions. Hysterical affection is occasioned by nitrous oxide, probably only in consequence of the strong emotion produced, which destroys the power of the will, and calls up series of automatic motions formerly connected with a variety of less powerful but similar feelings.