"reserved air," and there still remains in the chest, "residual air," which no effort can expel. This is roughly estimated at 40 cubic inches, but, like the previous quantities, varies very much. The air which we can expel after the fullest inspiration is said to measure the "vital capacity" of the chest, to which must be added the amount of residual air, if the quantity of air which the chest will hold is required to be known. These terms have been applied by Jeffreys and Hutchinson, the latter of whom has invented the spirometer, an instrument like a gasholder, consisting of two cylinders, one containing water and the other inverted within it, and into which the air is blown through a tube. A small dry gas-meter makes a cheaper and more accurate. spirometer. It determines the vital capacity of the chest, the average of which is 240 cubic inches for men, and scarcely more than half that for women, according to Bourgery. It is influenced almost alone by height, for corpulency without height decreases it. The vital capacity increases 8 cubic inches for every inch in height, from 5 to 6 feet.

Hutchinson's Spirometer.

The Inca Indians, breathing such rare air so high above the sea-level, have an enormous vital capacity. Hutchinson found the maximum to be 464 C. I. in a man 7 feet high, and 3081b in weight; the minimum, 46 C. I., in Don Francisco, the dwarf, whose height was 29 inches, and weight 40fb. Any wide departure from the standard is suspicions of pulmonary disease, or of some abdominal tumour pressing on the chest, and thus the spirometer is of use in examining lives for insurance.

The number of respirations, and quantity of air inspired at each, varying so much, the diurnal amount must also vary; but in the construction of barracks, hospitals, &c., from 600 to 1,200 cubic feet of air space is regarded as a sufficient quantity for each person. Of such hygienic principles I have said much in another work. The average vital capacity for a man 5 feet 8 inches high being 240, it will take 12 inspirations at 20 cubic inches each to change the whole of the air in the lungs. During a year about 9,000,000 respirations occur, consuming about 100,000 cubic feet of air, and arterializing about 3,500 tons of blood. The amount of air inspired is very much lessened by tight-lacing, which sadly interferes with the breathing apparatus. Even man's dress may prevent the full inspiratory action, as a man who can inhale 190 cubic inches when naked, can inspire but 130 when dressed.

Researches on the mechanism of respiration, and the interchange of gases in the lungs, have still to be considered, and afford admirable examples of the application of the principles of physics to physiology. Dalton discovered that if a light gas be placed above a heavy one, it will sink and the heavy one rise, the particles of each finding space between those of the other. This "diffusion of gases" occurs, then, in opposition to gravitation. Graham further demonstrated, that through porous substances gases, if dry and chemically indifferent, will diffuse in inverse proportion to the square roots of their densi

ties.

Animal membranes, films of india-rubber, shellac, soap bubbles, are media which permit diffusion. Draper has shown that a gas held in solution will diffuse with another free, or two gases will diffuse through a liquid although this has no pores-but they will not then obey Graham's law. It will be seen that these principles explain the interchange of the fresh and the deoxygenated air in the tubes and air-cells, the passage of oxygen to the blood, and of carbonic acid out of the blood, and also the exchange of these gases in aquatic respiration. The sugar brought so plentifully to the lungs, besides chemical functions, may promote osmose; water-breathing animals die at once if placed in solution of sugar.

The diffusion quantities of oxygen and carbonic acid are respectively 1,174 and 1,000, and it was supposed that respiration mainly depends on the interchange which these would produce, but the changes during the process are far more complex. The volume of expired air is greater than that of inspired in the ratio of 99.5 to 97.2, which is due to its containing more watery vapor and being more rarified by heat. If the water be removed, and the temperature reduced, the volume of the expired air is less than that of inspired according to the loss of oxygen, for carbonic acid has only the same volume as that of the oxygen in it.

The Chemical Phenomena of respiration will be best understood by considering the changes the air undergoes, and we will therefore first study the average composition of pure atmospheric air, which, excluding mechanical impurities-as particles of dust, sporules of plants, animal matters, and the gases emitted from factories is remarkably uniform; so that the different effects of the bracing country air and that of the close city were for a long time unexplained. In my "Lectures on Public Health," I have drawn attention to gaseous and suspended organic impurities in the air, the ill effects they were capable of producing, and the means by

which they may be each detected and remedied. The composition of atmospheric air, when filtered from suspended particles of animal debris and other organic matters which the cilia probably reject, is as follows:

Such air by respiration becomes (1) warmed; (2) deoxygenated; (3) charged with carbonic acid; and (4)

with water.

1. It usually approaches blood-heat-more or less according to the time it remains in the lungs, and will thus be warmer than the surrounding air, save in very hot climates.

2. The amount of Oxygen taken from the air has been calculated at about 4 per cent., and a corresponding amount of carbonic acid is given in exchange. The oxygen enters into association with the red-cells, which carry it all over the body, as there is no tissue which does not require it for the removal of effete matter, or as its proper excitant. Muscle, for instance, as Liebig has proved, will absorb oxygen and emit carbonic acid as long as irritability remains, and although deprived of blood. Insects have no true blood, yet perform the interchange in their tissues.

The Theory of Respiration propounded by Liebig was, that the venous blood contained carbonic acid as a carbonate of protoxide of iron, and gave it off in the lungs, where, taking more oxygen, the iron became a peroxide.

Ellis believed the venous blood to contain a hydrocarbon, which the oxygen in the lungs converted into carbonic acid and water. Verdeil stated that the evolution of carbonic acid was due to a peculiar acid he described in lung tissue, "the pulmonic," which decomposed the bicarbonate of soda of venous blood. Bernard finds that injection of solution of this salt into the jugular vein kills the animal by the evolution of carbonic acid. Exposure to a rare atmosphere causes the bicarbonate to give off carbonic acid. The theory of respiration suggested in 1791 by Lagrange and Hassenfratz, and supported by Magnus-who has discovered carbonic acid in venous, and oxygen in arterial blood-is, that the oxygen taken in uncombined by the arterial blood, converts the effete products throughout the whole system into carbonic acid, which, returned by the venous blood, is expired from the lungs.

3. The amount of Carbonic Acid varies very much, 4 per cent. of the air expired being a fair average quantity, and will be equivalent to 160 grs. of carbon per hour, or 3,840 grs. (8 ozs. troy) in the 24 hours. Dr. E. Smith found he expired 24.274 ozs. of carbonic acid, and Dr. Frankland 16 42. The amount evolved is influenced by the following circumstances:

As regards the influence of stimulant drinks, the ex