[234] Supposing the increase or diminution of living action when produced by different agents, uniform, similar and differing only in degree; it would follow, that certain mixtures of hydrocarbonate and nitrous oxide, or hydrogene and nitrous oxide, ought to be capable of supporting the life of animals for a much longer time than pure nitrous oxide. From the experiments in Res. III. Div. I. it appears however, that this is not the case. It would seem, that in life, a variety of different corpuscular changes are capable of producing phænomena apparently similar; so that in the science of living action, we are incapable of reasoning concerning causes from effects. Whenever we attempt to combine our scattered physiological facts, we are stopped by the want of numerous intermediate analogies; and so loosely connected or so independant of each other, are the different series of phænomena, that we are rarely able to make probable conjectures, much less certain predictions concerning the results of new experiments. An immense mass of pneumatological, chemical, and medical information must be collected, before we shall be able to operate with certainty, on the human constitution. Pneumatic chemistry in its application to medicine, is an art in infancy, weak, almost useless, but apparently possessed of capabilities of improvement. To be rendered strong and mature, she must be nourished by facts, strengthened by exercise, and cautiously directed in the application of her powers by rational scepticism. APPENDIX. No. I. _Effects of Nitrous Oxide on Vegetation._ In July 1799, I introduced two small plants of spurge into nitrous oxide, in contact with a little water over mercury; after remaining in it two days, they preserved their healthy appearance, and I could not perceive that any gas had been absorbed. I was prevented by an accident, from keeping them longer in the gas. A small plant of mint introduced into nitrous oxide and exposed to light, in three days became dark olive and spotted with brown; and in about six days was quite dead.—Another similar plant, kept in the dark in nitrous oxide, did not alter in color for five days, and at the end of seven days, was only a little yellower than before. I could not ascertain whether any gas had been absorbed. I introduced into nitrous oxide through water, a healthy budding rose, thinking that its colors might be rendered brighter by the gas. I was disappointed, it very speedily faded and died; possibly injured by the solution of nitrous oxide in water. Of two rows of peas just appearing above ground; I watered one with solution of nitrous oxide in water, and the other with common water daily, for a fortnight. At the end of this time, I could perceive no difference in their growth, and afterwards they continued to grow equally fast.