146' ISOMORPHISM. 



compounds exhibit isomorphism. These four elements all form 

 gaseous compounds with three atoms of hydrogen, namely 

 ammonia, phosphuretted, arsenuretted and antimonuretted 

 hydrogen. The hydriodates of ammonia and of phosphuretted 

 hydrogen are also isomorphous ; so are arsenious acid and the 

 oxide of antimony, both of which contain three atoms of oxygen 

 to one of metal. Arsenious acid also is capable of replacing 

 oxide of antimony in tartrate of antimony and potash or tartar 

 emetic, without change of form, and arsenic often substitutes 

 antimony in its native sulphuret. The nitrous acid (NO 3 ) 

 which corresponds with arsenious acid and oxide of antimony, 

 likewise acts occasionally as a base, as in the crystalline com- 

 pound with sulphuric acid of the leaden chambers used in the 

 manufacture of the latter acid. The complete isomorphism of 

 the arseniates and phosphates has already been noticed. But 

 phosphoric acid forms two other classes of salts, the pyrophos- 

 phates and metaphosphates, to which arsenic acid supplies 

 no parallels. At present this class of elements can be con- 

 nected by means of an isomorphous link with no other. It 

 approaches most nearly to the fifth class, nitrogen and chlorine 

 both forming a powerful acid with five proportions of oxygen, 

 nitric acid and chloric acid ; but of the many nitrates and chlo- 

 rates which can be compared, no two have proved isomorphous. 

 Nor do the metaphosphates appear at all like the nitrates, 

 although their formulae correspond. 



Seventh class. Tin, titanium. Connected by the isomor- 

 phism of titanic acid and peroxide of tin (Ti O 2 and Sn O 2 ). 

 Titanium is connected in a curious manner with iron and the 

 second class. Titanic acid occurs in ilmenite and other va- 

 rieties of titanic iron, in combination with protoxide of iron, 

 and in the crystalline form of the peroxide of that metal; 

 namely, that of specular iron, and also of corundum (alumina). 

 Hence, 



Fe 2 O 3 , or (Fe + Fe)+ O 3 , and 

 Fe O+Ti O 2 , or (Fe+Ti) + O 3 



are isomorphous. Now it is to be remarked that peroxide of 

 iron and titaniate of iron, although they agree in the number 

 of their elements, each containing three of oxygen and two of 

 metal, are yet not certainly analogous in proximate composi- 

 tion. If an acid of iron, consisting, like titanic acid, of one 

 of metal and two of oxygen were known, which might be called 



