B.— CHEMISTRY 43 



moved will be transferred to the doubly linked oxygen atom, and part will 

 go to diminish the positive charge on the nitrogen. We do not know 

 what amounts will thus be transferred. It would seem probable, however, 

 when we consider the di-pole moment associated with the carbon-oxygen 

 link, that the resultant negative charge of the carbon atom would be more 

 considerably diminished than that of the singly linked oxygen atom. 



There is an important point which this formula does not make clear. 



The anion can combine with the hydrogen ion either to form the aci- 

 modification or to regenerate the true nitro-body, and these processes 

 take place with extraordinarily different velocities. 



The former change is part of an ionic equilibrium and will occur with 

 the great rapidity characteristic of ionic reactions ; the latter takes place 

 so slowly that its course, in favourable cases, can easily be observed. 

 It would thus appear that, in the encounters of the nitro-ion with hydrogen 

 ions, a considerable proportion of the encounters on the nitro group result 

 in combination (since the acidic hydrogen in crystalline phenyl iso- 

 nitromethane, for example, is presumably co-valently linked), whilst an 

 exceedingly minute proportion of the encounters on the negatively charged 

 carbon atom are effective. 



It would, therefore, seem that combination of the hydrogen ion with 

 the tri-co-valent carbon atom can only take place under very special 

 conditions which rarely occur. It is thus probable that it is only when 

 this carbon atom is suitably activated that it is capable of combining with 

 a hydrogen ion. 



One of the most striking features of the stereochemistry of the pyra- 

 midal tri-co-valent atoms is the great variation in configurational stability 

 exhibited by compounds of the different atoms. We may contrast, for 

 example , the permanence of the optical activity of dissymmetric sulphonium 

 ions with the configurational lability of the tertiary amines, which has 

 hitherto prevented the demonstration of their pyramidal configuration 

 by the methods of classical stereochemistry, though it is clearly shown 

 by the considerable molecular di-pole moments which they possess. 



There are certain evident principles which must affect the stability of 

 the molecular configuration determined by the relative valency directions 

 of a central atom. 



It is clear that increase in the atomic radius of the central atom should 

 diminish configurational stability. It is probably in consequence of this 

 that it is so much easier to obtain quaternary ammonium ions in an 

 optically active state than the corresponding phosphonium and arsonium 

 compresses. 



Again, we should expect that compounds in which the normal valency 

 only of the central atom is employed would show greater configurational 

 stability than co-ordination compounds, for the reason that in the latter 

 the nuclear charge of the central atom is relatively less in comparison 

 with the number of electrons it has to control. We may attribute to this 

 cause the optical lability of the compounds of four-co-ordinate beryllium, 

 which contrasts strongly with the configurational stability of the com- 

 pounds of tetravalent carbon. 



