230 STUDIES IN ADVANCED PHYSIOLOGY. 



It was stated that ordinary Na H CO 3 is the familiar 

 baking-soda. Baking-soda is used to get the CO 2 gas 

 which is liberated from it when sour milk, or for that 

 matter any acid is poured over it. Everybody is familiar 

 with the fact that if baking-soda be taken and anything 

 sour be added to it, it begins to froth and bubble, and large 

 quantities of CO 2 gas stream out. This is exhibited 

 especially well in the common Seidlitz powder. Here 

 Na H CO 3 and some acid (tartaric, etc.) are mixed to- 

 gether, as a result of which the liquid begins to effervesce 

 very strongly. The addition of some acid to the soda has 

 liberated large quantities of CO 2 gas. Common soda-water 

 (which is but water charged heavily with CO 2 ) derives 

 its name from the fact that formerly this CO 2 gas was de- 

 rived from ordinary soda by pouring acids over it. Now it 

 is highly probable that the chemical substance in venous 

 blood carrying the CO 2 in combination, is soda, and to 

 liberate the CO 2 so contained some acid must be present. 

 The acid in the case of the lung is oxy haemoglobin. While 

 haemoglobin itself is very faintly acid, oxyhaemoglobin is 

 much more markedly acid. Although this oxyhaemoglobin 

 is not acid enough to appear sour to the taste, it is acid 

 enough to act upon the soda dissolved in the plasma and 

 liberate the CO 2 . It is at once apparent that this oxyhaemo- 

 globin, being formed in the lungs, was not present in 

 venous blood. Consequently there was no liberation of the 

 CO 2 in the veins, but arrived at the capillaries of the lungs 

 the haemoglobin becomes converted into oxhyaemoglobin, 

 which, acid in its nature, at once reacts upon the soda in 

 the plasma and liberates from this the CO 2 , which then 

 streams out into the lungs. 



Thus it will be seen that the CO 2 is carried in two 

 ways ; one part of it dissolved in the plasma, which when it 

 arrives at the lungs passes from the plasma into the air of 

 the alveoli in obedience to the general law of gases; the 

 other part united chemically with substances, sodium car- 

 onate (Na 2 CO 3 ) in venous blood to form sodium bicar- 



