498. If one fourth part of the volume of air received by the lungs at one inspiration is decomposed at one "beat" of the heart, it might be supposed that if the expired air be again received into the lungs, one half of the oxygen would be consumed, and, in a similar ratio, if re-breathed four times, all the oxygen would be consumed. But it does not follow, if the air is thus re-breathed, that the same changes will be effected in the lungs. For air that has been inspired does not part with its remaining oxygen as freely as when it contains the proper amount of this life-giving element, and thus the changes in the impure blood are not so completely effected. _Illustration._ In the process of dyeing, each successive article immersed in the dye weakens it; but it does not follow that the dye each time is affected in the same degree, or that the coloring matter by repeated immersions can be wholly extracted. The same principle applies to the exchange of oxygen and carbonic acid gas in the lungs. 499. _If the inspired air is free from moisture and carbonic acid, these substances contained in the blood will be more readily imparted to it._ When the air is loaded with vapor, they are removed more slowly; but if it is saturated with moisture, no vapor will escape from the blood through the agency of the lungs. This may be illustrated by the following experiment: Take two and a half pounds of water, add to it half a pound of common salt, (chloride of sodium,) and it will readily mix with the water; and to this solution add the same quantity of salt, and it will be dissolved more slowly. Again, add more salt, and it will remain undissolved, as the water has become saturated by the pound before dissolved. -=-=-=-=-=-=-=-=-=-=-=-= 498. Does air that is re-breathed freely impart its oxygen? Why? 499. What is the effect on the blood when the air is free from vapor and carbonic acid? When loaded with vapor? When saturated? How is this illustrated? -=-=-=-=-=-=-=-=-=-=-=-= 500. The principle in this experiment is analogous to that of the union between carbonic gas and atmospheric air. Allen and Pepy showed by experiment, that air which had been once breathed, contained eight and a half per cent. of carbonic acid. They likewise showed, that no continuance of the respiration of the same air could make it take up more than ten per cent. This is the point of saturation.