Moreover, holding the breath for a little while before expiration is conducive to good health, a condition, needless to say, which creates confidence and buoyancy in the singer and adds greatly to the efficiency of his voice and the effectiveness of his performance. Proper breathing is a cleaning process for the interior of the body. It cleanses the residual air, the air that remains in the lungs after each respiration; and it does much more. Air enters the lungs as oxygen; it comes out as carbonic acid, an impure gas created by the impurities of the body. The process of breathing dispatches the blood on a cleansing process through the whole body, and, while traveling through this, it collects all the poisonous gases and carries them back to the lungs to be emitted with expiration. By holding the breath we prolong this process, make it more thorough, and correspondingly free the body of more impurities. From the classic ages down physicians have advocated retaining the breath for a little while after inspiration as an aid to general health, and the taking and holding of a full breath has been compared with opening doors and windows of a house for ventilation. Sir Morell Mackenzie emphasizes this purifying function of respiration in his book on the "Hygiene of the Vocal Organs." It consists, as he says, essentially in an exchange of gases between the blood and the air, wherein the former yields up some of the waste matters of the system in the form of carbonic acid, receiving in return a fresh supply of oxygen. It is evident from this how important it is to have a sufficient supply of pure air, air which contains its due proportion of oxygen to renovate the blood. A room in which a number of people are sitting soon becomes close if the windows and doors are kept shut. This indicates that the oxygen in the air is exhausted, its place being taken by carbonic acid exhaled from the lungs of the assembly, so that the purification of the blood must necessarily become more and more imperfect.