STRUCTURE OF CRYSTALS WYCKOFF. 211 



which are at present the most useful in studying the structure of 

 crystals — will give effects which overlap one another; (4) used by 

 itself this method fails to identify the reflection of any plane with a 

 particular image. "While this last objection is not serious when we 

 are dealing with very simple compounds, it is exceedingly important 

 for all of the more complicated cases. 



The great use of this method lies in its opening up for study a 

 vast number of chemical compounds which do not crystallize well. 

 At present it is not known whether the atoms of man} 7 kinds of 

 substances are arranged in an orderly fashion or not. For instance, 

 are the individual particles of a colloidal substance in reality ex- 

 traordinarily minute crystals or are they more properly to be looked 

 upon as more or less haphazard agglomerations of atoms? Or are 

 many of the naturally occurring minerals, such as chalcedony or 

 the opal, perfectly disordered groupings of atoms, or are they built 

 up of the minutest crystals which stand in disordered relations to 

 one another? This method of obtaining diffraction effects may be 

 expected to answer such questions as these. This field is, however, 

 thus far practically unexplored. 18 Further, are liquid crystals 

 crystalline in the sense that they are made up of a regular arrange- 

 ment of atoms in space? We have here a method of attack for de- 

 ciding it. If the atoms within the molecules of complicated organic 

 compounds stand in a definite orientation to one another, then such 

 compounds should give diffraction effects even m the liquid state. 

 Debye and Scherrer 19 believe that they have obtained diffractions 

 from liquid benzene which prove the existence of the benzene ring; 

 in fact, it is said to be 6.02X10" 8 centimeters on an edge and 1.19X10 -8 

 centimeters thick. This same method finds its use even in the field 

 of chemical analysis. 20 Suppose that we have a substance which on 

 solution gives a test for, let us say, sodium and potassium, chlorine 

 and iodine. Is it a mixture of sodium chloride and potassium iodide, 

 or is it a mixture of sodium iodide and potassium chloride, or are all 

 four present together? The method we have been discussing will 

 decide it, for each of the substances is characterized by its own dis- 

 tance between atoms, and once the pattern of images which each of 

 these substances furnishes is known it is only necessary to search for 

 these same patterns in the photograph obtained from the mixture to 

 be analyzed. 



The method of Laue yhotographs. — The third method of obtain- 

 ing diffraction effects from crystals is the original method of Laue. 

 In this instance X rays are passed through thin sections of indi- 

 vidual crystals in a direction normal or nearly normal to some im- 



18 See, however, S. Kyropoulos, Zeit. anorg. Chem., 00, 197, 1017. 

 M P. Debye u. P. Scherrer, Nacb. Kgl. Ges. Gottingeii, Dec. 17, 1015. 

 10 A. W. Hull, J. Am. Chem. Soc, 41, 1168, 1919. 



