MODERN CHEMISTRY. 447 



may be arranged in groups, so related together, that when 

 similar combinations are formed from elements belonging to 

 two, three, or more of them, such combinations will crystallise 

 in the same geometric forms. Tables of these groups have 

 been formed, and many of the results are exceedingly curious ; 

 especially those which prove this peculiar isomorphic relation 

 between various chemical substances, having in themselves 

 other singular resemblances. Physical properties are thus 

 associated by new relations; and methods suggested for 

 hereafter discovering the common radical of substances now 

 known only as distinct elements; an object ever before the 

 mind of the philosophical chemist. Sulphur and selenium 

 arsenic and phosphorus lime and magnesia chlorine, 

 iodine, and bromine are instances in their various forms 

 of this curious connection of Isomorphism with other phy- 

 sical resemblances. The wide scope of the enquiry has led 

 to various modes of arranging and viewing the results 

 of which the ' law of substitution^ suggested by Dumas, 

 may be considered one of the most important. 



Another topic, closely allied to the law of definite propor- 

 tions, is the theory of that vast class of chemical compounds 

 which we call salts a vague name for a group of objects 

 still imperfectly defined. The great complexity of this 

 theory enhanced by successive changes of doctrine and 

 nomenclature limits us to such mere outline as may suffice 

 to show its nature, and bearing upon other parts of chemical 

 knowledge. 



There was a simplicity and seeming completeness in the 

 old notion of oxygen as the acidifying principle of the 

 alkalies, earths, and metals as elementary bases and of 

 neutral bodies or salts as produced by their union, which 

 made it difficult for chemists to acquiesce in any change of 

 these views. But this became needful, when it was found 

 that the most essential chemical characters of an acid might 



