CLASSIFICATION OF ELEMENTS. [4;) 



be connected in a probable manner with the above large class, 

 is that of 



Nitrogen Arsenic 



Phosphorus Antimony 



The tendency of discovery is to bring all the elements into one 

 class, either as isomorphous atom to atom, or with the rela- 

 tion to the others which chlorine and sodium exhibit. 



But must not isomorphism be implicitly relied upon in esti- 

 mating atomic weights, and the alterations which it suggests be 

 adopted without hesitation in every case ? Chemists have 

 always been most anxious to possess a simple physical charac- 

 ter by which atoms might be recognised ; and equality of vo- 

 lume in the gaseous state, equality of specific heat, and simi- 

 larity in crystalline form have all in their turn been upheld 

 as affording a certain criterion. The indications of isomor- 

 phism certainly accord much better than those of the other two 

 criteria with views of the constitution of bodies derived 

 from considerations purely chemical, and are indeed invaluable 

 in establishing analogy of composition in a class of bodies, by 

 supplying a precise character which can be expressed in num- 

 bers, instead of that general and ill-defined resemblance 

 between allied bodies, which chemists perceived by an acquired 

 tact rather than by any rule, and which was heretofore their 

 only guide in classification. Admitting that isomorphism is a 

 certain proof of similarity of atomic constitution within a class 

 of elements and their compounds, it may still be doubted 

 whether the relation of the atom to crystalline form is the same 

 without modification throughout the whole series of the elements, 

 or whether all atoms agree exactly in this or any other physi- 

 cal character. 



Crystalline form and the isomorphous relation may prove 

 not to be a reflection of atomic constitution, or immediately 

 and necessarily connected with it, but to arise from some se- 

 condary property of bodies, such as their relation to heat ; in 

 which a simple atom may occasionally resemble a compound 

 body, as we find sulphur isomorphous in one of its forms, 

 with bisulphate of potash. While we find another simple atom, 

 potassium, isomorphous through a long series of compounds 

 with the group of five atoms which constitute ammonium. The 

 occurrence of dimorphism also both in simple and compound 

 bodies gives to crystalline form a secondary character. 



