568 INDUCTION. 



special laws which Liebig has attempted to explain 

 from, and resolve into, more general ones, are, that 

 the blood in passing through the lungs absorbs oxy- 

 gen and gives out carbonic acid gas, changing thereby 

 its colour from a blackish purple to a brilliant red. 

 The absorption and exhalation are evidently chemical 

 phenomena; and the carbon of the carbonic acid must 

 have been derived from the body, that is, must have 

 been absorbed by the blood from the substances with 

 which it came into contact in its passage through the 

 organism. Required to find the intermediate links, 

 the precise nature of the two chemical actions which 

 take place ; first, the absorption of the carbon or of 

 the carbonic acid by the blood, in its circulation 

 through the body; next, the excretion of the carbon, 

 or the exchange of the carbonic acid for oxygen, in its 

 passage through the lungs. 



Dr. Liebig believes himself to have found the 

 solution of this vexata quastio in a class of chemical 

 actions in which scarcely any less acute and accurate 

 inquirer would have thought of looking for it. 



Blood is composed of two parts, the serum and 

 the globules. The serum absorbs and holds in solu- 

 tion carbonic acid in great quantity, but has no 

 tendency either to part with it or to absorb oxygen. 

 The globules, therefore, are concluded to be the por- 

 tion of the blood which is operative in respiration. 

 These globules contain a certain quantity of iron, 

 which from chemical tests is inferred to be in the state 

 of oxide. 



Dr. Liebig recognised, in the known chemical 

 properties of the oxides of iron, laws which, if fol- 

 lowed out deductively, would lead to the prediction 

 of the precise series of phenomena which respiration 

 exhibits. 



