xvii MOLECULAR ARCHITECTURE 441 



3, 98-110; Ann. Chem. Pharm., 1866, 137, 129) recognised 

 the existence of a " closed chain " or " ring " of carbon 

 atoms in the formula of BENZENE ; similar ring-systems are 

 characteristic of nearly all aromatic compounds. Kekule's 

 graphic formula for benzene was of the same type as those 

 shown above, but may be translated into modern symbols 



/~TT /"*T1J\ 



as HO^-TT r-TT^CH. Only three valencies are needed 



to form the ring of carbon-atoms and to hold the six hydro- 



fTT ("'TT 



gen atoms, as shown in the symbol HC/O^rr ^rr^CH ; 



much controversy has therefore arisen as to the way in 

 which the fourth valency is disposed of, but Kekule's 

 formula is universally recognised as the opening of a new 

 chapter in organic chemistry and was made the subject of a 

 public celebration in 1890. 



Position of the atoms in space. Kekule represented 

 the four hydrogen atoms of marsh-gas as lying in a row 

 by the side of the carbon atom. More frequently they 

 have been represented as arranged round the carbon atom 

 in such a way as to occupy the four corners of a square. 

 Van t'HofFs study of the isomerism of carbon compounds, 

 and the crystallographic work of Barlow and Pope, and of 

 Bragg, have shown that they are probably arranged at the 

 corners of an enveloping tetrahedron. This idea, one of 

 the most fertile in modern chemistry, cannot, however, be 

 discussed in the present volume. 



Fixed or variable valency. Is the valency of an atom 

 fixed or variable ? Kekule held strongly to the view that 

 the valency or " atomicity is a fundamental property of the 

 atom, which must be constant and invariable like the weight 

 of the atom itself" (Comptes rendus, 1864, 58, 511). In 

 order to account for the existence of compounds such as 

 ammonium chloride and phosphorus pentachloride in which 

 the valency of nitrogen and phosphorus appears to be in- 

 creased, he assumed the existence of a class of MOLECULAR 



