152 ANIMAL CHEMISTRY LECTUKE VI. 



The circumstance of these numbers also expressing the chemical 

 equivalents of the several elements leads to some curious medico- 

 chemical considerations. Thus, bearing in mind that sodium and 

 potassium are related to one another in much the same way as 

 are chlorine and iodine, though not perhaps quite so intimately, 

 it will follow that 58*5 parts of common salt or chloride of sodium 

 and 74-5 parts of chloride of potassium are the chemical equiva- 

 lents of 1 66 parts of iodide of potassium, thus : 



NaCl 23 + 35-5 = 58-5 



KC1 39 + 35-5 = 74-5 



KI = 39 + 127 = 166-0 



Chemically, then, 58*5 parts of chloride of sodium are just as 

 efficient as 1 66 parts of iodide of potassium ; but while most of 

 us, I suppose, are in the habit of taking fifty or sixty grains of 

 common salt twice or three times a day, none of us, I conceive, 

 would like to take 166 grains of iodide of potassium even once 

 in the same period. 



(160.) The gradational difference subsisting between the 

 atomic weights or volume-weights of chlorine, bromine, and 

 iodine is typical of all their chemical and physical differences. 

 Thus their usual states of aggregation, gaseous, liquid, and solid, 

 and the colours of their respective gases or vapours, green, 

 orange, and violet, are sequential to one another ; while, in a 

 chemical point of view, we notice a successive decrease of the 

 force with which they respectively enter into and remain in com- 

 bination with most other bodies. Thus chlorine combines 

 directly with hydrogen upon simple exposure of the mixed 

 gases to ordinary daylight, and the resulting chlorhydric acid is 

 an extremely stable body. But the direct combination of bro- 

 mine with hydrogen takes place very imperfectly, and then only 

 at a red heat, while the bromine of the resulting compound is 

 liberated with comparative ease. Further, the direct combina- 

 tion of iodine with hydrogen is almost impracticable, while the 

 iodhydric acid resulting from the indirect combination of the two 

 elements is readily decomposed even by the action of atmospheric 



