In the present state of science, it will be unnecessary to enter into discussions concerning the importance of investigations relating to the properties of physiological agents, and the changes effected in them during their operation. By means of such investigations, we arrive nearer towards that point from which we shall be able to view what is within the reach of discovery, and what must for ever remain unknown to us, in the phænomena of organic life. They are of immediate utility, by enabling us to extend our analogies so as to investigate the properties of untried substances, with greater accuracy and probability of success. The first Research in this work chiefly relates to the production of nitrous oxide and the analysis of nitrous gas and nitrous acid. In this there is little that can be properly called mine; and if by repeating the experiments of other chemists, I have sometimes been able to make more minute observations concerning phænomena, and to draw different conclusions, it is wholly owing to the use I have made of the instruments of investigation discovered by the illustrious fathers of chemical philosophy,[2] and so successfully applied by them to the discovery of truth. [2] Cavendish, Priestley, Black, Lavoisier, Scheele, Kirwan, Guyton, Berthollet, &c. In the second Research the combinations and composition of nitrous oxide are investigated, and an account given of its decomposition by most of the combustible bodies. The third Research contains observations on the action of nitrous oxide upon animals, and an investigation of the changes effected in it by respiration. In the fourth Research the history of the respirability and extraordinary effects of nitrous oxide is given, with details of experiments on its powers made by different individuals. I cannot close this introduction, without acknowledging my obligations to Dr. Beddoes. In the conception of many of the following experiments, I have been aided by his conversation and advice. They were executed in an Institution which owes its existence to his benevolent and philosophic exertions. _Dowry-Square, Hotwells, Bristol._ _June 25th, 1800._ RESEARCH I. CONCERNING THE ANALYSIS OF NITRIC ACID AND NITROUS GAS AND THE PRODUCTION OF NITROUS OXIDE. [Illustration: _Pl. I. MERCURIAL AIRHOLDER and BREATHING MACHINE_. _Lowry sculpᵗ._] RESEARCH I. INTO THE PRODUCTION AND ANALYSIS OF NITROUS OXIDE, AND THE AËRIFORM FLUIDS RELATED TO IT. DIVISION I. _EXPERIMENTS and OBSERVATIONS on the composition of_ NITRIC ACID, _and on its combinations with_ WATER _and_ NITROUS GAS. I. Though since the commencement of Pneumatic Chemistry, no substance has been more the subject of experiment than Nitrous Acid; yet still the greatest uncertainty exists with regard to the quantities of the principles entering into its composition.