Hesults of Crystal Analysis. 403 



are projected on a plane perpendicular to the molecular 

 axis. If we choose the position of one hydrogen atom, the 

 symmetry conditions will give seven other positions where 

 an H atom must be placed. 



If we choose one position quite arbitrarily, we shall get 

 eight atoms. If the number is to be reduced to four, we 

 must place the H atom on one of the symmetry planes 

 through the molecular axis either in position (1) or in 

 position (2) (fig. 2 b) . 



From these considerations it follows that the most 

 general arrangement of the atoms of a molecule involves 

 the following parameters : — 



(1) The distance I between the I and the N atoms, or the 



length of the molecular axis. 



(2) The distance l x from a carbon atom to the molecular 



axis. 



(3) The distance V from the N atom to the point where 



the plane of the C atoms intersects the molecular 

 axis. 



(4) To arrange the twelve H atoms we may either divide 



them into three groups of four, or into one group 

 of eight and one group of four as in fig. 2 b. The 

 first possibility means that all H atoms are situated 

 in the symmetry planes. As the determination of 

 a point in a plane requires two parameters, this 

 arrangement would involve six arbitrary parameters. 

 The second possibility gives three parameters for 

 the determination of the group of eight, and two 

 parameters for the determination of the group o£ 

 four. 



A complete determination of the molecular element would 

 require the knowledge of eight or nine parameters ; and as 

 one parameter is necessary to fix the position of the two 

 groups of molecules, a complete knowledge of the structure 

 of N(CH 3 )I should involve the determination of nine or ten 

 parameters. 



We shall see, however, that some of these parameters 

 have very simple values, which will considerably simplify 

 the lattice. 



This type of lattice which we have sketched is a con- 

 sequence of the assumption expressed in equation (4) and 

 the symmetry properties of the crystal. If the equation (4) 

 is right, the lattice must be able to explain the observed 

 reflexion max ma. ' First of all, the lattice must give the 



