To persons who have been unaccustomed to breathe through a tube, we have usually given common air till they have learnt to respire with accuracy: and in cases where the form of the mouth has prevented the lips from being accurately closed on the breathing tube, by the advice of Mr. Watt, we have used a tin plate conical mouth-piece fixed to the cheeks, and accurately adapted to the lips; by means of which precautions, all our later trials have been perfectly conclusive. II. _Of the effects of Nitrous Oxide upon persons inclined to hysterical and nervous affections._ The case of Miss—— N. and other cases, detailed by Dr. Beddoes in his Notice, seemed to prove that the action of nitrous oxide was capable of producing hysterical and nervous affections in delicate and irritable constitutions. On this subject, we have lately acquired additional facts. Miss E. a young lady who had been subject to hysteric fits, breathed three quarts of nitrous oxide mingled with much common air, and felt no effects but a slight tendency to fainting. She then breathed four quarts of pure nitrous oxide: her first inspirations were deep, her last very feeble. At the end she dropt the bag from her lips, and continued for some moments motionless. Her pulse which at the beginning of the experiment was strong, appeared to me to be at this time, quicker and weaker. She soon began to move her hands and talked for some minutes incoherently, as if ignorant of what had passed. In less than a quarter of an hour, she had recovered, but could give no account of her sensations. A certain degree of languor continued through the day. A young lady who never had hysterical attacks, wished to breathe the gas. I informed her of the disagreeable effects it had sometimes produced, and advised her if she had the slightest tendency to nervous affection, not to make the trial. She persisted in her resolution. To ascertain the influence of imagination, I first gave her a bag of common air, which she declared produced no effect. I then ordered for her a quart of nitrous oxide mingled with two quarts of common air; but from the mistake of the person who prepared it, three quarts of nitrous oxide were administered with one of common air. She breathed this for near a minute, and after the experiment, described her sensations as unpleasant, and said she felt at the moment as if she was dying. The unpleasant feelings quickly went off, and a few minutes after, she had apparently recovered her former state of mind. In the course of the day, however, a violent head-ache came on, and in the evening after she had taken a medicine which operated violently, hysterical affections were produced, followed by great debility. They occasionally returned for many days, and the continued weak and debilitated for a great length of time.