1856.] and the Means of Determining its Amount. 245 



Why do we want to take in oxygen ? and why do we want to get 

 rid of carbonic acid ? 



Shortly, we want oxygen, because of its chemical energy. It is 

 the main spring of our life. On it the production of animal heat 

 depends, and the vital powers — sensation and motion, no less than 

 nutrition and secretion, are directly influenced by its action. 



Why do we want to get rid of surrounding carbonic acid ? 

 Literally, because the carbonic acid stops the way, and prevents the 

 escape of newly formed carbonic acid from within. If we were 

 placed in an atmosphere containing as much carbonic acid as exists 

 in the lungs, the carbonic acid of the atmosphere would not pass 

 from the lungs to the blood and act as a poison, but that carbonic 

 acid which was passing out from the blood would stop in the lungs 

 and prevent more from escaping out of the blood, and that carbonic 

 acid which was formed in the body would act as a narcotic poison. 

 From experiments on animals it appears, that the air must contain 

 20 per cent, of carbonic acid before absorption of that gas by the 

 blood is observed. Moreover the escape of gases from the blood 

 affects the circulation of the blood. In sudden death from suffoca- 

 tion, the side of the heart which throws the blood to the lungs is 

 found distended, whilst the side which throws the blood from the 

 heart is empty, there has been an obstruction to the flow. By stop- 

 ping respiration and causing pressure we can stop the pulse and 

 the heart's sounds and impulse when we please. This experiment is 

 easy to make. There can be no doubt that this is more the result of 

 pressure than of any arrest of escape of carbonic acid. I mention it 

 only as a striking evidence how suddenly the action of the heart 

 may be influenced by the respiration. When the escape of carbonic 

 acid from the blood is retarded or prevetited, the want of ventilation 

 of the blood causes more or less stoppage of the blood in the vessels, 

 and makes the blood a narcotic poison to all the tissues with which 

 it is in contact. 



We may consider oxygen as our most necessary food, and car- 

 bonic acid as the refuse which passes into our sewers. We have all 

 probably come to the full belief that a house badly drained causes 

 disease and death ; but we hardly yet fully admit to ourselves that 

 a house or body without good means of ventilation is a house or 

 body badly drained. At present our chimneys are our chief aerial 

 drains, which almost cease to act as soon as the temperature outside 

 and inside the house is the same ; and even when these drains are in 

 action, we are unwilling to think that that fire which so cheer- 

 fully ministers to our warmth, like most human contrivances for 

 doing two things at once, does neither well. 



[H. B. J.] 



