METALLIC ALTERATIVES. 155 



the comparative feebleness of its affinities, or in the loose state of 

 combination with which it is capable of being retained by other 

 bodies ; so that, while the more energetic chlorine acts once for 

 all, the less energetic iodine .is acting and reacting upon every 

 occasion. Accordingly, while it makes all the difference whether 

 we employ free chlorine or chloride of sodium to produce the 

 therapeutic effects of chlorine, it makes very little difference 

 whether we employ free iodine or iodide of potassium to produce 

 the therapeutic effects of iodine. Wherever the element travels, 

 it either oxidises or deoxidises, accordingly as it comes into contact 

 with bodies more or less oxidisable than itself, at that particular 

 moment. It acts, in fact, not only as a converter of inactive or 

 free, into active or combined oxygen, but also as a conveyer of 

 oxygen from wherever it is in excess to wherever it is in defi- 

 ciency. 



(162.) Now, what is true of iodine and its compounds is also 

 true of the compounds of mercury, of arsenic, and of another 

 metal whose alterative action is manifested in almost an opposite 

 fashion, namely, iron. Considered chemically, the compounds 

 of these three metals are, perhaps, the most constantly impression- 

 able of any with which we are acquainted. In the laboratory, 

 and even in the factory, we habitually avail ourselves of mercuric, 

 arsenic, and ferric compounds as oxygenants, and of mercurous, 

 arsenious, and ferrous compounds as deoxygenants or reducants. 

 The relationship between chlorine and iodine is parallel to that 

 between phosphorus and arsenic, the phosphoric and arsenic 

 acids, for instance, though so different in their therapeutic effects, 

 being the strict chemical analogues of one another. Phosphorus is 

 a far more active element than arsenic, and its combinations are 

 far more stable. Phosphate of sodium, once formed, is like common 

 salt; a stable innocuous body; while arseniate of sodium, like iodide 

 of potassium, is an active body, because of its instability because 

 of the liability of its arsenical constituent to affect and be affected 

 by the chemical actions taking place everywhere throughout the 

 body. I am far, of course, from thinking that this susceptibility 

 to oxidation and deoxidation furnishes a complete solution of the 



