﻿396 Royal Institution : — 



duals, while others cannot ; so some compounds can unite with ele- 

 ments, while others have not this capability. 



The Theory of Atomicity regards chemical phenomena from an 

 altogether different point of view. In it the various substances are 

 considered as modifications of one another rather than as compounds. 

 The rise of this mode of viewing chemical phenomena maybe traced 

 from the early papers by Dumas, and by Laurent, on Substitution. 

 It appears more prominently in the position given to double de- 

 composition as the representative of all chemical action, by Laurent 

 and Gerhardt, in the types of Gerhardt and Williamson, in Frank- 

 land's theory of the organo-metallic bodies, and in its extension by 

 Kolbe to the compounds of carbon. It was reserved, however, for 

 Kekule to combine these ideas into a consistent theory*. The 

 theory has been further elaborated by Butlerow (to whom we owe 

 the name "Chemical Structure"), by Erlenmeyer, and by many 

 others ; and it has been adopted and applied with slight modifications 

 by almost all chemists engaged in organic research. 

 | According to this theory the typical form of chemical action is 

 what we may call the chemical exchange. To illustrate this idea we 

 may consider the simplest case, that of double decomposition, 

 where two molecules act on one another to produce two new 

 molecules. 



Chloride of sodium, for instance, acts on nitrate of silver, produ- 

 cing chloride of silver and nitrate of sodium. Comparing chloride 

 of sodium and chloride of silver, we at once see that, while there are 

 important respects in which the sodium and the silver differ as to 

 the nature of their union with chlorine (thus the amount of work 

 required to separate the metal from the chlorine is very different in 

 the two cases), still from one point of view (and that is the point of 

 view taken by the atomicity theory) the silver may be said to re- 

 place or to be substituted for the sodium. In the same way, a cup 

 filled with mercury is very different from the same cup filled with 

 water ; and the relation of the mercury to the cup differs in many 

 respects (such as pressure and adhesion) from the relation of the 

 water to the cup ; but they agree in this, that the cup is Jilted in 

 both cases. In the same way, the chlorine is said to be saturated 

 by the sodium or the silver, although the intimacy or firmness of the 

 combination is not the same in the two cases. 



We may also consider this double decomposition from the other 

 side. As the silver and sodium have changed places, so the chlo- 

 rine has changed place with the rest of the nitrate of silver, with 

 what in the nitrate of silver is not silver ; or, representing the action 

 in symbols (NaCl + AgN0 3 = AgCl + NaN0 3 ), CI and N0 3 have 

 changed places. 



In this example we have one atom or group replacing one other 

 atom or group ; but ail cases of double decomposition are not of 

 so simple a kind. Thus, when water is treated with pentachloride 



* It is right to observe that although Kekule has used this theory with the 

 most eminent success, both in the explanation of facts already known, and in the 

 discovery of new chemical relations, lie does not exclude the possibility of the 

 union of compounds with each other to form compounds of a second order. 



