Indeed, as most gases, though of different specific gravities, when brought in contact with each other, assume some sort of union, it is more than probable, that gas inspired into the lungs, from being placed in contact with the residual gas on such an extensive surface, must instantly mingle with it. Hence, possibly one deep inspiration and compleat expiration of the whole of a quantity of hydrogene, will be sufficient to determine the capacity of the lungs after compleat voluntary exhaustion, and the nature of the residual air. That two inspirations are sufficient, appears probable from the following experiment. E. 3. After a compleat voluntary expiration of common air, I made two deep inspirations of 141 cubic inches of hydrogene. After the compleat expiration, they filled a space equal to rather more than 142 cubic inches, and analised, were found to consist of Carbonic acid 3,1 Oxygene 4,5 Nitrogene 18,8 Hydrogene 115,6 ————— 142,0 Now calculating on the exhausted capacity of my lungs from this experiment, supposing uniform mixture, they would contain after expiration of common air, about 30,7 cubic inches at 58°, equal to 36 at 98°, composed of about Nitrogene 20,9 Oxygene 5,8 Carbonic acid 4,0 ———— 30,7 One should suppose a priori that in this experiment much less of the residual oxygene of the lungs must have been absorbed, than in Expts. 1 and 2; yet there is no very marked difference in the portions evolved. That a tolerably accurate mixture took place, appears from the quantity of nitrogene. The smaller quantity of carbonic acid is an evidence in favour of its evolution from the venous blood. _c._ It is reasonable to suppose that the pressure upon the residual gas of the exhausted lungs, must be nearly equal to that of the atmosphere. But as aqueous vapour is perpetually given out by the exhalents, and perhaps evolved from the moist coats of the pulmonary vessels, it is likely that the residual gas is not only fully saturated with moisture at 98°, but likewise impregnated with uncombined vapor; and hence its volume enlarged beyond the increment of expansion of temperature. Considering all these circumstances, and calculating from the mean of the three experiments on the composition of the residual gas, I concluded, 1st. That the exhausted capacity of my lungs was equal to about 41 cubic inches. 2dly. That the gas contained in my bronchial vessels and fauces, after a compleat expiration of atmospheric air, was equal to about 32 cubic inches, its temperature being reduced to 55°. 3dly. That these 32 cubic inches were composed of about Nitrogene 23,0 Carbonic acid 4,1 Oxygene 4,9