of the Elements. 429 



comparable to HCl, the latter being an entire, the former a 

 demi- molecule. HO presents very many of the properties of CI; 

 and for purposes of illustration^ especially when compai'ing chlo- 

 rides with hydrates, it is often convenient to consider HO as 

 taking the place of CI : — 

 Chlorhvdi-ic cacid . . . HCl 



Potassic chloride . . . KCl 



Chloro-sulphuric acid . . SO^ Cl^ 



Chloro-sulphuric acid . . SO" CP 



Chloride of beiizoyle . . C' H'O CI 



H . HO Water. 



K . HO Potassic hydrate. 



SO-(HO)= Sulphuric acid. 



S02 CI . HO Chlorhydro-sulphuric 



C'H^O.HO Benzoic acid. [acid*. 



Oxychloride of phosphorus PO CP | PO^HO)' Phosphoric acid. 



Tellurium represents the metal of this series. Moreover, sele- 

 nium is not altogether destitute of metallic characters. 



The relations of this group to the preceding are not very well 

 marked. Chlorine is by equivalent substitution the representa- 

 tive of hydrogen, but we do not know of any indisputable in- 

 stance of the direct replacement of hydrogen or chlorine by 

 oxygen or sulphur. We are, however, acquainted with a consi- 

 derable number of bodies which may be conceived as arising from 

 such a substitution, two atoms of chlorine or hydrogen being 

 represented by one equivalent of oxygen or sulphur. Thus — 



Phosphorus pentachloride PCPCP PCPO Phosphorus oxychloride. 



Alcohol C= WO C- W 02 Acetic acid. 



Chloride of stibtriethyle Et^ SbCP Et^ SbO Oxide of stibtrietliyle. 



Moreover, we meet with a very great number of corresponding 

 oxides and chlorides which contain respectively the same number 

 of atoms of oxygen and chlorine, but in which the ratio of the 

 other elements is as 2 to 1 : thus — 



The metal manganese serves in a curious manner to associate 

 the members of the first and second groups. We find this metal 

 at one time simulating chlorine, and at another time simulating 

 sulphur. Thus we have, — 



Perchlorate of potash . KCIO^ i K^ SO^ Sulphate of potash. 

 Permanganate of potash KMn^ 0^ | K^ Mn^ O^ Manganate of jjotash. 



Iodine and sulphur both crystallize in the right prismatic 

 system as acute rhombic octahedra. 



Group III. Nitrogen — Phosphorus, Arsenic, Antimony — 

 Bismuth. 



These five elements have each the ])roperty of combining with 

 three atoms of hydrogen, or its representative chlorine,, to form 



* Williamson, Proceedings of the Royal Society, vol. vii. p. 15. 



