THE CHEMISTRY OF THE ANIMAL BODY. 961 



mals poisoned with CO die from want of oxygen, since the latter cannot dis- 

 place the carbon monoxide from combination with haemoglobin. 



Carbon Dioxide, CO 2 . This is the highest oxidation compound of carbon, 

 the product of its complete combustion. It is present in the air to the extent 

 of 0.04 per cent. It is formed in all living cells, and in higher animals is 

 collected by the blood and brought to the lungs and skin for excretion ; it is 

 also a product of putrefaction ; it gives an acid reaction to herbivorous urine. 

 It is found dissolved in all natural waters, and is present combined in sea 

 shells. It is found in the blood principally combined with sodium in the 

 serum, and is likewise combined with calcium and magnesium in the bones. 



Preparation. (1) By burning carbon or a carbon-containing substance, 

 C 6 H 12 O 6 + 12O = 6CO 2 + 6H 2 O. 



Sugar. 



(2) By heating a carbonate, 



(3) By the action of an acid on a carbonate, 



Na 2 C0 3 + 2HC1 = 2NaCl + CO 2 + H 2 O. 



In the blood, haemoglobin and, to a less extent, serum-albumin and primary 

 sodium phosphate act like acids. If the gases be extracted from fresh defib- 

 rinated blood in a vacuum, all the CO 2 is removed. If sodium carbonate be 

 added to blood, the carbonic acid belonging to this is likewise given up in a 

 vacuum, while a simple aqueous solution of sodium carbonate is not affected. 

 If serum be extracted in vacua, only a little more than half the carbonic acid 

 contained in it is dissociated from combination, indicating that in the previous 

 experiment haemoglobin had acted like an acid. If a solution of bicarbonate 

 of sodium (NaHCO 3 ) be exhausted under the air-pump, just one-half of the 

 CO 2 is given off, sodium carbonate (Na 2 CO 3 ) remaining. In the serum more 

 than one-half of the CO 2 is obtained in vacuo, because the serum-albumin, 

 like the haemoglobin, though less effectively, acts like an acid in fixing the 

 alkali and liberating the gas. There is likewise present the action of pri- 

 mary phosphate on the acid carbonate, 



JSTaH 2 P0 4 + NaHC0 3 = Na 2 HPO 4 + H 2 O + CO 2 . 



Through these agencies the tension of carbonic acid is kept high in the blood, 

 and its escape through the walls of the alveolar capillary is not unlike the 

 escape of gas on uncorking a bottle of carbonated water. 



After drinking a carbonated water, carbonic oxide may be detected dissolved 

 in the urine. 



Properties. A colorless, odorless gas. It is poisonous, its accumulation at 

 first stimulating and afterwards paralyzing the nervous centres. It affects the 

 irritability not, however, the conducting power of the nerves. A solution 

 of carbonic oxide in water forms carbonic acid, H 2 CO 3 , and from this are derived 

 two series of salts, primary or acid salts, MHCO 3 , and secondary or neutral 

 salts, M 2 CO 3 . 



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