248 PROGRESS IN CHEMISTRY. 



also that an atom of hydrog-en can be replaced by a group of atoms, or 

 radical, behaving for the occason like the atom of an element. It is 

 to Laurent and Gerhardt that we owe the deiinition of an atom — the 

 smallest quantit}- of an element which can be present in a compound; 

 an equivalent — that weight of an element which combines with or 

 replaces one part by weight of hydrogen; and a molecule — the smallest 

 quantitv which can exist in a free state, whether of an element or a 

 compound. They recognized, too, that a molecule of hydrogen, 

 chlorine, etc., consists of two atoms. 



POWERS OF COMBIXATION. 



In 1849 Wurtz, professor in Paris (1S1T-18S4), and Ilofmann, then 

 professor in'the College of Chemistry in London, afterwards at Berlin 

 (1818-1892), discovered series of compounds allied to ammonia, NH3, 

 in which one or more atoms of hydrogen were replaced by a group, 

 or radical, such as methyl (CH3), ethyl (CjH:^), or phenyl (CgH.). 

 Wurtz referred such compounds to the ammonia "tvpe." They all 

 resemble ammonia in their physical properties, smell, taste, etc., as 

 well as in their powc of uniting with acids to form salts resembling 

 anmionium chloride (NH^Cl) and other ammonium compounds. 

 Shortly afterwards Williamson, professor at University College, Lon- 

 don (1855-1887), added the ''water type,'' in consequence of his 

 researches on "mixed ethers" — bodies in which the hydrogen of water 

 might be regarded as replaced by organic radicals. Thus we have the 

 series, H.O.H: CH3O.H; CHg.O.CH,; NH3NH,.CH3; NH (CH3),, and 

 N (CH3)3, the tirst representing compounds following the water type, 

 the latter the ammonia type. This suggestion had })een previously 

 made by Laurent in 1846. But Williamson extended his A'icws to 

 inorganic compounds; thus, sulphuric acid was represented as con- 

 structed on the double water type — HO.SO2.OH, being derived from 

 H.O.(HH).0. H, the two hydrogen atoms inclosed in brackets being- 

 replaced by the radical SO.,. To these t^-pes Gerhardt added the 

 hydrogen and hydrogen chloride types, H.H. and H.Cl; and later 

 Kekule, professor in Bonn (1829), added the marsh gas type CH^. 

 The next important step was taken b\^ Frankland, professor in the 

 Koyal School of Mines, London. His work, however, had been antici- 

 pated by Crum Brown (professor at Edinburgh University) in a pam- 

 phlet even yet little known. It was to attribute to elements one or 

 more powers of combination. To these he gave the name '"valency,"" 

 and the capacity of possessing valency was called "quanti valence." 

 Thus h3'drogen was taken as a '"monad" or monovalent. Chlorine, 

 because it unites with hydrogen atom to atom, is also a monad. Oxy- 

 gen, having the power to combine with two atoms of hydrogen, was 

 termed a dyad, or divalent; nitrogen a triad, or trivalent; carbon a 



