reverie that happens to occupy our imagination. It is a problem, the solution of which must be attended with the utmost difficulty. We must know, in the first place, how much oxygen was contained in the air _inspired_. In the second place we must know the quantity of fixed air _expired_. In the third place we must exactly know the proportion of oxygen contained in the fixed air thrown out by the breath. In the fourth place we must determine whether, by the conversion of oxygen into fixed air, any change is made in its bulk. For, if this shall be found to be the case, we should be led to suppose an absorption or augmentation of air when no such thing took place. This point therefore ought to be determined with the utmost accuracy. In the fifth place we must exactly know how much _azote_, _septon_, _phlogisticated air_ is contained in the atmosphere inspired, and likewise in that expired. In the sixth place, we must be assured that there are no other fluids in the atmosphere capable of being absorbed by the lungs, excepting oxygen and azote. Whether there are any others or not, hath not been determined. From an expression of Dr. Fordyce, he would seem to be skeptical on the subject. “The atmosphere (says he) is found to consist of various vapours, of which air, or, as it has been called, pure air, or respirable air, (oxygen air) forms at present about a fourth. Gas (probably fixed air) forms some part;[72] but the greatest part consists of one or more vapours, which, _without any positive quality, but from that indolence which makes mankind in their researches attempt to find a resting place, have been considered by many chemists as one individual species, under the names of phlogisticated air_,” &c. In the seventh place we ought to know what quantity of _pure oxygen_, unconverted into fixed air, or whether any such, is thrown out by the breath. That a quantity of this kind of air is really thrown out, is probable, because we can blow up a fire with our breath, and by a blow-pipe excite a most intense heat, capable of melting the most refractory metals, platina excepted. It is true that the eolipile, by the mere conversion of water into steam, will blow up a fire also; though, if the access of external air be denied, the blast of the eolipile will put the fire out. Probably the breath would do the same; but even this cannot be accounted a decisive proof of the oxygen being totally exhausted; for the moist vapour with which the breath abounds may extinguish the fire, even though some small quantity of oxygen should remain in it. It is not, however, our business at present to enter minutely into such discussions. From what has been already said, it is evident, that the absorption of oxygen by the blood, instead of being indubitably established, is of all things the most uncertain; the requisites for determining it being absolutely beyond the investigation of any person, however accurate. We may indeed, with great labour and