15 ANIMAL CHEMISTRY LECTURE VI. 



to the sulphurous acid. It is this chemical mobility of iodine, 

 then, which chiefly distinguishes it from its more active con- 

 geners, chlorine and bromine. The general chemical relationship 

 of these three elements to one another is most striking. With 

 the probable exception of fluorine, they possess the exclusive 

 property of uniting with hydrogen in the proportion of volume 

 to volume, the combinations, moreover, being unattended by 

 any condensation. Again, the resulting compounds namely, 

 the hydrochloric, hydrobromic, and hydriodic acids are all 

 gaseous, all fuming, all soluble in water, and all producible by 

 similar reactions. 



(158.) Another common property by which chlorine, bromine, 

 and iodine are characterised, is their marked activity when in 

 the free state, which very greatly exceeds that of oxygen under 

 similar conditions. In my last lecture I showed you the violent 

 action of chlorine on metallic copper, upon which ordinary 

 oxygen is, as you know, almost without action ; and I have only 

 a few minutes back referred to the little effect exerted by free 

 oxygen upon various oxidisable bodies. But chlorine, bromine, 

 and iodine act upon different metals, pseudo-metals, and com- 

 pounds, with the greatest readiness ; and, indeed, several of the 

 iodides contained in the London and British Pharmacopoeias are 

 directed to be made by treating the respective metals at once 

 with iodine. Lastly, all three elements are capable of acting as 

 oxidising agents in cases where free oxygen is altogether, or 

 almost, impotent. They contain no oxygen, it is true, and are, 

 on the contrary, so far as our present knowledge goes, simple or 

 elementary bodies. Nevertheless, in the presence of water, they 

 act as very powerful oxygenants by uniting with the hydrogen of 

 the water, and so liberating its pre-combined, and consequently 

 active oxygen. Thus, on adding chlorine, bromine, and iodine 

 respectively to the clear mixture of sulphurous acid and chloride 

 of barium contained in these three glasses, we have in each in- 

 stance an immediate precipitate of sulphate of barium from the 

 oxidation of the sulphurous acid just as in our former experi- 

 ments with the peroxides of hydrogen and nitrogen the reaction 

 being as follows : 



