INDIRECT OXIDATION BY HALOGENS. 151 



Sulphurous Water Chlorine Sulphuric Chlorhydric 



H a S0 3 + H a O + Cl a = H a S0 4 + aHCL 



Here, again, I have three white magmas of protoxide of lead, 

 with an excess of dilute alkali, exposed freely to the air ; and 

 this protoxide, although it does not absorb oxygen from the air, 

 yet when treated with chlorine, bromine, and iodine, is at once 

 oxidised more or less completely into the brown peroxide, as you 

 perceive, thus : 



Lead-oxide Water Bromine Lead-perox. Bromhydric 



PbO + H a O + Br a = PbO a + 



Lastly, I have in these three glasses some solution of blue 

 indigo exposed to the air, but unoxidised by the air. On adding 

 chlorine, bromine, and iodine, however, it is at once bleached or 

 oxidised into isatin, thus : 



Indigo Water Iodine Isatin lodhydric 



C 8 H 5 NO + H a O + I, = C 8 H 5 NO a + aHL 



( 1 59.) In all these particulars, then, chlorine, bromine, and iodine, 

 though so different in their medicinal action, resemble one ano- 

 ther to the greatest extent chemically. Now let us see what are 

 their chemical, and, I may add, physical differences. I will first 

 advert to their combining proportions, or the relative weights of 

 each of them, which unite with I part by weight of hydrogen. 

 These are indicated by the numbers 35*5, 80, and 127, which 

 also represent their respective specific gravities-, when in the 

 gaseous state. You will observe that the proportional number 

 of bromine is intermediate between that of chlorine and iodine, 

 and indeed approximates very closely to the true arithmetic 

 mean, ^ = 8o'8, thus : - 



Cl 35*5 Chlorine. 



Br 80 Bromine. 



I 127 Iodine. 



