THE CHEMISTRY OF RESPIRATION. 403 



tion it may readily be secured by arranging, in addition to 

 the registers from which the warmed air reaches the room, 

 proper openings at the opposite side, by which the old air 

 may pass off to make room for the fresh. An open fire in a 

 room will always keep up a current of air through it, and is 

 the healthiest, though nojb the most economical, method of 

 warming an apartment. 



Stoves in a room, unless constantly supplied with fresh 

 air from without, dry its air to an unwholesome extent. If 

 no appliance for providing this supply exists in a room, it 

 can usually be got, without a draught, by fixing a board about 

 four inches wide under the lower sash and shutting the win- 

 dow down on it. Fresh air then comes in by the opening 

 between the two sashes and in a current directed upwards, 

 which gradually diffuses itself over the room without being 

 felt as a draught at any one point. In the method of heating 

 by direct radiation, the apparatus employed provides of itself 

 no means of drawing fresh air into a room, as the draught up 

 the chimney of an open fireplace or of a stove does; and 

 therefore special inlet and outlet openings are very necessary. 

 Since few doors and windows, fortunately, fit quite tight, 

 fresh air gets even into closed rooms, in tolerable abundance 

 for one or two inhabitants, if there be outlets for the air 

 already in them. 



Changes undergone by the Blood in 'the Lungs. These 

 are the exact reverse of those undergone by the breathed air 

 what the air gains the blood loses, and vice versa. Con- 

 'sequently, the blood loses heat, and water, and carbon dioxide 

 in the pulmonary capillaries; and gains oxygen. These 

 gains and losses are accompanied by a change of color from 

 the dark purple which the blood exhibits in the pulmonary 

 artery, to the bright scarlet it possesses in the pulmonary 

 veins. 



The dependence of this color change upon the access of 

 fresh air to the lungs while the blood is flowing through 

 them, can be readily demonstrated. If a rabbit be rendered 

 unconscious by chloroform, and its chest be opened, after a 

 pair of bellows has been connected with its windpipe, it is 

 seen that, so long as the bellows are worked to keep up arti- 

 ficial respiration, the blood in the right side of the heart (as 

 seen through the thin auricle) and that in the pulmonary 

 artery, is dark colored, while that in the pulmonary veins 



