SULPHUR COMPOUNDS 63 



joined to other poles of attraction. Thus H is a 

 monad ion in all circumstances ; oxygen is always a 

 dyad ; and carbon is a tetrad in all its relations to 

 hydrogen. The halogens, however, are only monads 

 in their combinations with ions more electro-positive 

 or electro-negative than themselves. Towards 

 ions they may present one, three, five, or even seven 

 valencies or poles of attraction, according to the 

 circumstances under which they combine. Sulphur 

 is similar to them. It is a divalent ion or dyad 

 towards electro-positive ions, but a tetrad or even 

 hexad in the presence of electro-negative ions. 



In its electro-negative character, therefore, sulphur 

 is a dyad. It resembles oxygen in its chemical 

 behaviour and can replace it in its many compounds. 

 Thus H 2 becomes H 2 S or sulphuretted hydrogen. 

 All the compounds in which is replaced by S are 

 called thio-compounds. They are numerous, and 

 include thio-alcohols, thio-ethers, thio-aldehydes, 

 and thio-acids ; and in each case the oxygen is re- 

 placed by sulphur by the action of its ions. Thus 

 C 2 H 6 + H 2 S = H 2 + C 2 H 6 S or ethyl-hydrosulphide, 

 a substance which causes the peculiar smell in the 

 urine after eating asparagus ; and CH 4 + H 2 S = 

 H 2 + CH 4 S or methyl-hydrosulphide, one of the 

 intestinal gases. 



As a tetrad sulphur can unite with oxygen by 

 two valencies forming SO or oxy-sulphine, a dyad 

 or divalent ion. One valency of this ion may unite 

 with C and the other with an OH ion, and from such 

 a combination many salts and compound ethers are 

 derived. 



As a hexad, sulphur unites with two atoms of 



