His situation in the "Medical Pneumatic Institution" in 1799, imposing upon him the duty of investigating the physiological effects of such aëriform fluids as held out any promise of useful agency, he resumed the investigation; a considerable period, however, elapsed, before he succeeded in procuring _nitrous oxide_ in a state of purity; he was therefore obliged to breathe it in mixture with oxygen gas, or common air; but as no just conclusion could be deduced from the action of an impure gas, he commenced an enquiry for the purpose of discovering a process by which it might be procured in an uncontaminated condition; when, after a most laborious investigation concerning its composition, properties, and combinations, enquiries which were necessarily extended to the different bodies connected with nitrous oxide, such as _nitrous gas_, _nitrous acid_, and _ammonia_, he was enabled, by a series of intermediate and comparative experiments, to reconcile apparent anomalies, and thus, by removing the greater number of those difficulties which had previously obscured this branch of science, to present to the chemical world the first satisfactory history of the COMBINATIONS OF OXYGEN AND NITROGEN. Thus prepared, he proceeded to examine the action of nitrous oxide upon living beings, and to compare it with the effects of other gases upon man; and in this manner he completed its physiological, as he had already done its chemical history. These interesting results were published in a distinct volume, in the year 1800, entitled, "Researches Chemical and Philosophical, chiefly concerning Nitrous Oxide, and its Respiration. By Humphry Davy, Superintendent of the Medical Institution." It may be observed in passing, that the merits of this work could never have been inferred from the title-page, which its most sanguine admirers must admit to be as clumsy and unpromising an invitation as an author ever addressed to his scientific brethren. Amongst Davy's letters to Mr. Gilbert, I find one written on a proof sheet of the chapter of contents of the above work, and which may not be uninteresting in this place. TO DAVIES GIDDY, ESQ. July 3, 1800. That our feelings, as well as our actions, are rendered stronger and more vivid by habit, is probable from many facts, and from no one more so than that of procrastination. My much respected friend, two months after my return,[26] I had formed the resolution of writing to you; week after week this resolution was renewed and put off to a future day, with the hope that this day, by presenting something new, would enable me to make my letter more interesting. In vain! the feeling of procrastination, thus increased by association, at length became so strong as to prevent me from writing at all.[27] I have received your letter; it has awakened my duties, and has been doubly welcome, as being unexpected and undeserved.