I remain, Yours, &c. P. ROGET. To MR. DAVY. VI. _Letter from_ MR. JAMES THOMSON. The first time I respired nitrous oxide, the experiment was made under a strong impression of fear, and the quantity I breathed not sufficient, as you informed me, to produce the usual effect. I did not note very accurately my sensations. I remember I experienced a slight degree of vertigo after the third or fourth inspiration; and breathed with increased vigor, my inspirations being much deeper and more vehement than ordinary. I was enabled the next time I made the experiment, to attend more accurately to my sensations, and you have the observations I made on them at the time. After the fourth inspiration, I experienced the same increased action of the lungs, as in the former case. My inspirations became uncommonly full and strong, attended with a thrilling sensation about the chest, highly pleasurable, which increased to such a degree as to induce a fit of involuntary laughter, which I in vain endeavoured to repress. I felt a slight giddiness which lasted for a few moments only. My inspirations now became more vehement and frequent; and I inhaled the air with an avidity strongly indicative of the pleasure I received. That peculiar thrill which I had at first experienced at the chest, now pervaded my whole frame; and during the two or three last inspirations, was attended with a remarkable tingling in my fingers and toes. My feelings at this moment are not to be described: I felt a high, an extraordinary degree of pleasure, different from that produced by wine, being divested of all its gross accompaniments, and yet approaching nearer to it than to any other sensation I am acquainted with. I am certain that my muscular strength was for a time much increased. My disposition to exert it was such as I could not repress, and the satisfaction I felt in any violent exertion of my legs and arms is hardly to be conceived. These vivid sensations were not of long duration; they diminished insensibly, and in little more than a quarter of an hour I could perceive no difference between the state I was then in, and that previous to the respiration of the air. The observations I made on repeating the experiment, do not differ from the preceding, except in the circumstance of the involuntary laughter, which I never afterwards experienced, though I breathed the air several times; and in the following curious fact, which, as it was dependent on circumstances, did not always occur.