ISOMERIC CHANGE 629 



cyanic and cyanic acids must be represented by equations instead 

 of by simple formulae, and may be shown as 



H.C = N r> H.N = C = 

 < — 



Liquid prussic acid 



HO.C-N r> HN = C = 

 — <— 



Liquid cyanic acid 



This simple method of formulation serves admirably to 

 express the essential features of the theory of dynamic iso- 

 merism, and brings out with special clearness its analogy with 

 reversible actions of other kinds. Thus, just as the equilibrium 



2KNO3 + CaCL ^> 2KCI + Ca(N0 3 ) 2 



may be carried right over in one direction, by substituting 

 silver for potassium nitrate, 



2AgN0 3 + CaCl, -> 2AgCl + Ca(N0 3 ) 2 



and in the other direction by substituting lead for calcium 



nitrate, 



2KNO, + PbCl 3 <- 2KCI + Pb(N0 3 ) a 



as a consequence of the sparing solubility of the chlorides of 

 lead and silver, so the equilibrium of prussic acid may be 

 pictured as being overturned in one direction by the action of 

 caustic potash, and in the other direction by the addition of a 

 silver salt, 



K . C ■ N <r- HC : N ^> H . N : C : -> AgNC 



Potassium cyanide Prussic acid Silver r'aocyanide 



Again, it is equally easy to express the reconversion of urea 

 into a cyanate in presence of silver nitrate by the symbols 



AgNO, + CO(NHj) ^> NH 4 CNO+AgN0 3 -> AgCNO + NH 4 N0 3 



since the change is one which depends mainly on the insolu- 

 bility of the silver cyanate for the reversal of an equilibrium 

 which normally acts in favour of the formation of urea at the 

 expense of ammonium cyanate. 



Crystallisation of Dynamic Isomerides. — It is a noteworthy 

 fact that Butlerow, clinging to past experience as a safe guide 

 in exploring the unknown phenomena of dynamic isomerism, not 

 merely selected two liquid acids as types of reversible isomeric 

 change, but deliberately restricted his consideration of equi- 

 librium between isomers to liquids and gases. This limitation, 



