32 G. H. KNIBBS. 



not without difficulties, owing to the fact that some elements have 

 more than one valency : an explanation has been attempted by 

 supposing the ordinary atom to consist of a number of smaller or 

 micro-atoms, with links free to form bonds either with the members 

 of the system, or with atoms outside thereof. Thus if nitrogen 

 consist of five trivalent micro-atoms it ought to be monovalent, 

 trivalent, or pentavalent. 1 This theory of linking has given birth 

 to structural chemistry : which may be described as a symbolic 

 system of so representing the constitution of a chemical substance, 

 that the formula shall epitomise the whole story of its characteristic 

 reactions and properties. It was soon evident in organic chemistry, 

 that structural formulae were a necessity, since the physical, 

 chemical, and therapeutical properties of the vast series of organic 

 compounds, depended even more absolutely upon structure than 

 upon the proportion of the elements out of which they were 

 compounded. The structural investigation of an organic compound 

 is summed up in three determinations, viz., (1) the empirical 

 formula or ratio of the constituent elements, (2) the molecular 

 formula or actual number of atoms of each constituent, and (3) 

 the structural formula or way in which the constituents are com- 

 bined. The two first determinations involve nothing more than 

 some degree of expertness in laboratory technique, and frequently 

 the utilization of certain discoveries in chemical physics, as the 

 effect of a substance in lowering the freezing point [De Coppet 

 and Raoult, 1882], or elevating the boiling point of a solvent 

 [Beckmann 1889], or the lowering of solubility [Nernst 1890]. 

 The third determination however, involves not only a comprehen- 

 sive knowledge of the chemical behaviour of organic compounds, 

 but in addition a delicate appreciation of the whole range of avail- 



1 Thus : — Arrange five micro-atoms in the form of a regular pentagon; 

 then denoting the free links by exterior lines the results may be repre- 

 sented : — ! i 



N=N N— N N— N— 



-N< | -N< || • -N / | 



^N=N N— N X N— N— 



I I 



Monovalent. Trivalent. Pentavalent. 



