94 



METHODS FOR DETERMINING MOLECULAR SIZE AND SHAPE 



There are in fact many possible sets of parallel planes that can be 

 formed by arrays of atoms, as indicated in Fig. 45. Each of these sets of 

 planes will produce reinforcement of incident x-ray beams scattered off 

 them, provided only that the planes are at the proper angle to the inci- 

 dent beam. It should be readily appreciated that some planes have many 

 more atoms than others, and since the over-all intensity obtained depends 

 on the density of atoms in the planes, the more "natural" planes, in 

 practice, yield the most intense reinforcements. 



In general, the orientation of a single crystal will not be such as to 

 produce reinforcement, but there are several means of obtaining the 

 proper orientation. We shall first discuss the so-called powder method. 

 As the name of the method implies, a crystalline material is first smashed 

 into small pieces, and the resultant crystal powder is placed in the path 

 of the x-ray beam. Since there are now very many small crystals, in 

 essentially all possible orientations with respect to the beam, many will 

 chance to be in the precise orientation needed to give ray reinforcement. 

 These correctly oriented crystals will not all be parallel to each other, 

 since a properly oriented crystal can rotate about the beam and still 

 maintain its proper angle. As shown in Fig. 46(a), the result will be a 

 reinforcement sufficient to produce circles of blackening of a photographic 

 film. 



If a strip of film (indicated by the dashed lines in the sketch) is used 

 instead of a very large piece of film, the positions of the reinforcements 

 will be given by the (somewhat curving) lines at various distances from 

 the intense blackening produced by the undeflected x-ray beam. Such a 

 strip is shown in part (b) of the sketch; it is the experimental result 

 frequently presented in research papers. 



(b) 



Figure 46 



