xvii MOLECULAR ARCHITECTURE 445 



the laws of substitution were obeyed strictly, and mechanical 

 types, in which a part of the hydrogen (existing separately from 

 the rest in the form of water) could be removed without replace- 

 ment by oxygen or chlorine. Laurent, in 1837, found an 

 explanation of the same facts in the theory that an organic 

 compound consists of a nucleus of atoms geometrically arranged, 

 e.g. in the form of a prism ; hydrogen atoms in the prism 

 could only be replaced according to the laws of substitution, 

 but other atoms, forming pyramids on the ends of the prism, 

 could be added, or removed without replacement. Gerhardt, 

 in 1839, regarded substitution as a particular case of the 

 copulation of two residues or radicals. Thus when chlorine 

 acted on oil of bitter almonds, the former lost a chlorine and 

 the latter a hydrogen atom in the form of hydrogen chloride and 

 the product was formed by the union of the two residues, 



C 7 H 6 O!H +"C1JC1 = Hl + C 7 H 5 O 'Cl. 



In this way the idea of substitution could be extended to include 

 the products obtained by the action of sulphuric acid and nitric 

 acid on benzene, of acetic acid on alcohol, of ammonia on 

 benzoyl chloride, etc. Gerhardt regarded his residues, not as 

 real substances, but as expressions of the changes which a 

 compound could undergo ; he therefore distinguished between 

 the radicals H and Cl and the gases H 2 and C1 2 . He also halved 

 the formulae of nearly all organic compounds and used simple 

 formulas based on Avogadro's hypothesis. 



F. SIMPLE INORGANIC TYPES. 



The discovery by Wurtz, in 1849, of the primary amines, 

 methylamine, CH 3 'NH 2 , and ethylamine, C 2 H 6 'NH 2 , and by 

 Hofmann, in 1850, of the secondary and tertiary amines, led 

 to the recognition of ammonia as a simple inorganic type^ 

 from which derivatives could be obtained by replacing one or 

 more of the three hydrogen atoms by organic radicals. The 

 experiments of Williamson, in 1852, on etherification, showed 

 that alcohol and ether could be derived from the water type by 

 replacing one or both of the two hydrogen atoms by ethyl. 

 Gerhardt, in 1856, added the hydrogen type and the hydrochloric 

 acid type. The marsh gas type, CH 4 , in which four hydrogen 



