154 ANIMAL CHEMISTRY LECTtRE VI. 



bined iodine. You will remember that succinic acid, which is so 

 to speak, the oxalic acid of the butyric group, is intimately asso- 

 ciated with certain other members of the same group, thus : 



Succinic Sub-group 

 C 4 H 4 4 Fumaric. 



C 4 H 6 4 Succinic. 



C 4 H 6 S Malic. 



C 4 H 6 6 Tartaric. 



Now tartaric acid, when heated with aqueous iodide of hydro- 

 gen or iodhydric acid, is converted into malic acid with liberation 

 of iodine, thus : 



Tartaric Iodhydric Malic Iodine Water 



C 4 H 6 6 + zHI = C 4 H 6 5 + I, + H 3 0. 



Similarly the malic and fumaric acids are reducible into suc- 

 cinic acid by means of iodhydric acid, thus : 



Malic Iodhydric Succinic Iodine Water 



C 4 H 6 5 + aHI = C 4 H 6 4 + I, + H a O. 



Fumaric Iodhydric Succinic Iodine 



C 4 H 4 O 4 + aHI = C 4 H 6 4 + I,. 



Accordingly, we find that while free iodine, acting as an oxi- 

 dising agent, produces iodhydric acid, this same acid, acting as a 

 deoxidising agent, reproduces free iodine. Here, for instance, 

 I have some iodhydric acid, mixed with a little starch to show 

 the results of the experiment. I now add to the iodhydric acid 

 some peroxidised substance, which it immediately reduces, with 

 liberation of iodine, as shown by the blue colour of the liquid. 

 I now add to the free iodine some suboxidised substance, which 

 it immediately oxidises, with reconversion into iodhydric acid, as 

 shown by the disappearance of the blue colour. On now adding 

 some more of the peroxidised substance, I re-liberate the iodine, 

 and, on afterwards adding some more of the suboxidised substance, 

 I reproduce the iodhydric acid, and so on ad infinitum. The 

 characteristic chemical property of iodine consists, therefore, in 



