62 THE THEORY OF IONS 



which can displace hydrogen from very many of its 

 combinations and occupy the same position in the 

 molecule. A large number of halogen compounds 

 derived from the hydrocarbons are known to 

 pharmacology. Thus CH 4 by displacement of H 

 may become successively methyl-chloride, CH 3 C1, 

 methylene chloride, CH 2 C1 2 , and chloroform, CHC1 3 . 

 Other alcohol radicals may likewise have their 

 hydrogen displaced, thus C 2 H 6 becomes Ethyl 

 bromide, C 2 H 5 Br, and Ethylene Iodide, C 2 H 4 I 2 . The 

 hydrogen may likewise be displaced by a halogen- 

 compound ; thus C 2 H 6 may become C 2 H 5 HgCl, or 

 Mercury-ethyl-chloride. Aldehydes and acids also 

 may have their hydrogen displaced by Cl ions, thus : 



Acetic aldehyde, C 2 H 4 + 3C1 = 3H + C 2 HC1 3 0, or 



chloral. 

 Acetic acid, C 2 H 4 2 + 3C1 = 3H + C 2 HC1 3 2 , or 



Tri-chlor-acetic acid. 



Sulphur, like chlorine and phosphorus, enters the 

 organism in the form of an ion derived from organic 

 or inorganic compounds. The salts of the soil enter 

 the root-hairs of plants chiefly as ions in the dilute 

 solution which surrounds them, and they exist 

 mainly in form of ions in the circulating fluids of 

 the plants. 



Sulphur behaves towards other elements differ- 

 ently from C, H, and 0. The latter have a certain 

 number of poles or valencies, no more nor less. Such 

 valencies represent the demand or craving for union 

 with other atoms. Atoms are active ions until each 

 valency is satisfied ; and the body is not at rest or 

 the compound in a stable form until such poles are 



