The pneumatic system naturally arose from a consideration of the composition of the atmosphere we breathe. Finding this fluid to be composed of two others, the one of which would preserve life for some time at least, and the other instantly destroy it, it became natural to think that diseases might be produced by any considerable variation in the proportion of these ingredients. An instrument was soon invented by which any considerable variation in this respect might be discovered; but upon trial this was found to be of very little use. Dr. Priestley himself tried, by means of this instrument, some very offensive air which had been brought from a manufactory, and could find no remarkable difference between it and that which was accounted pure. Still, however, it was evident that by increasing very much the proportion of one of the ingredients, some considerable alteration might be produced, which could not but be perceptible in the human body; and this led to the application of aerial chemistry to disorders of the lungs. The mixture chosen for this purpose was pure dephlogisticated (_oxygen_) with inflammable air (_hydrogen_;) and, though this has not been known to effect a radical cure, it certainly has given relief in many cases. In fevers also the application of fixed air (carbonic acid) hath been found advantageous; but with regard to oxygen and some others we have not yet a decided instance of their good effects in any case. Dr. Beddoes indeed is of opinion that it would be of service in the sea-scurvy; but in this (whether his conjecture be right or wrong) the theory is certainly erroneous, as shall presently be evinced.