COMPARISON OF IODINE WITH CHLORINE. 153 



oxygen, thus : 2HI + O=H 2 O + 1 2 . Now, it is this feebleness of 

 the force with which iodine enters into certain combinations, and 

 consequent facility with which it is separated from such combi- 

 nations, that give it that peculiar mobility to which I have 

 already adverted. The chief chemical difference, indeed, be- 

 tween chlorine and iodine seems to be that chlorine in the free 

 state is far more active than iodine, and iodine in the combined 

 state far more mobile than chlorine, while in both respects bro- 

 mine occupies an intermediate position. Hence, on adding a 

 little bromine-water to this solution of iodide of potassium, the 

 bromine expels the iodine, KI + Br = KBr + I, which, dissol- 

 ving in the chloroform I had previously introduced, manifests 

 itself to you by the production of a beautiful violet-coloured 

 stratum at the bottom of the tube. Similarly, when I add a 

 little chlorine water to this solution of bromide of potassium, the 

 chlorine expels the bromine, KBr + Cl KCl + Br, which dissolv- 

 ing in the previously introduced ether, floats on the surface as an 

 orange-brown layer ; so that while bromine expels iodine, chlorine 

 expels bromine, and a fortiori iodine, from their respective com- 

 binations. Accordingly, we may regard 35*5 parts of chlorine 

 as an energetic representative of 127 parts of iodine ; but it is 

 this very energy of chlorine, I conceive, which disqualifies it for 

 acting medicinally as an alterative. On account of its intense 

 chemical affinity, it unites more rapidly and forcibly than iodine 

 with the different basyloids it may chance to encounter, but di- 

 rectly it has entered into combination with them, its work is done, 

 its action ceases. The resulting chloride of sodium, or other 

 chloride, is of so stable a nature as to be impressionable only to 

 violent chemical agencies ; whereas, iodine, on the other hand, 

 forming very unstable compounds, is constantly, with every 

 change of circumstance, entering into a fresh state of liberation 

 or combination constantly effecting fresh oxidising or deoxidis- 

 ing actions. 



(161.) I have already given you several illustrations of the 

 oxidising action of free iodine, and ought, perhaps, to mention 

 one or two instances of the correlative deoxidising action of com- 



