56 KEi'oiiT — 1869. 



Eighteen j-ears ago I'rofessor Williamson read before the members of this Asso- 

 ciation a remarkable paper which contained the germ of our modern chemical views, 

 and was the cause of many important discoveries. He proposed to regard three 

 large classes of bodies, acids, bases, and salts from the same point of view, and to 

 compare their chemical properties ^\ith those of a single selected substance. For 

 this term of comparison he chose water. Now water is composed of three atoms, 

 two of hycb'ogen and one of oxygen. Williamson showed that all oxygen acids, all 

 oxygen bases, and the salts resulting from a combination of the two can, like water, 

 be considered to be composed of three parts or radicals, two of the radicals playing 

 the part of the hydrogen atoms in water, and the third that of the atom of oxygen ; 

 thus : — ■ 



ll\o 



Hi -^ Hi" „ K (" 



Water, Potassic hydrate. Hydric nitrite. Potassic nitrite. 



Potassic hydrate is water which has one of its atoms of hydrogen replaced by an 

 atom of potassium, hydric nitrite is water which has one atom of hydrogen replaced 

 by nitric oxide, and potassic nitrite is water with one of its hydrogen atoms re- 

 placed by nitric oxide and the other by potassium. This speculation, as every 

 chemist knows, is well supported by experiments ; it embraces three large classes 

 of bodies which till then had been considered as distinct. M. CTcrhardt, in 1853, 

 extended Williamson's views by distinguishing two other types of molecidar 

 structure, represented respectively hj hydrogen and ammonia, and succeeded, by 

 help of the radical theory, in arranging the niaiority of the then known substances 

 under one or the other of the three t^^-pes alreacly mentioned. 



Like everj' theory which is in harmony with experience, the above considerations 

 led to results of unexpected importance ; for it soon became apparent that the ra- 

 dicals whicli thus replace hydrogen in water are not all of the same chemical 

 value. If wo place together the formulfc of hydric nitrite and carbonic acid 



g^[o C0}0 



Hydric nitrite. Carbonic acid. 



we perceive at once that the atomic group N has replaced one atom of hydrogen 

 in one molecule of water, and carbonic oxide, C 0, two atoms of hydrogen in one 

 molecule of water. Nitric oxide (N 0) is therefore said to be equivalent to one 

 atom of hydrogen, and carbonic oxide (C 0) equi-^alent to two atoms of hydrogen. 

 The radical of phosphoric acid [P 0] is found to be equivalent to three atoms of 

 hydrogen. Professor Odling was one of the first to observe this ditlerence in the 

 equivalence of atoms and gi-oups of atoms, or compound radicals, as they are tenned, 

 a difference which he marks as shown in the following examples : — 



Radicals. 



Equivalent to one atom of hydrogen. Equivalent to two atoms of hydrogen. 



[C 0]'"' 



c.n,]" 



-Nitric oxide (N 0)' Carbonic oxide 



Methvl (CH,)' IMethylene 



Ethyl (C,HJ' Ethylene 



The notion of equivalence enabled Professor Kekul^ to form most interesting 

 speculations on the constitution of organic bodies, and to explain the relation be- 

 tween composition and equivalence of such radicals as methyl, C H3, ethyl, C, Hj, 

 methylene, C H^, ethylene, C'^Hj, and acetylene, CoHj. 



If from one molecule of marsh-gas, C H^, one atom of hydrogen is abstracted, 

 the residue, C Il.„ called methyl, can combine with an atom of hydrogen again, and 

 produce the original marsh-gas molecide. But methyl, inistead of combining with 

 an atom of hydrogen, can unite with an atom of chlorine, or an atom of bromine, 

 that is to sav, the place of the atom of hydrogen cau be taken by an atom of chlo- 

 rine or bromine. Methyl being thus equivalent to an atom of hydrogen is said to 

 be monovalent. If from a molecule of marsh-gas two atoms of hydrogen are re- 

 moved, the residue, C Hj, called methylene, can again unite with two atoms of hy_ 



