By J. T. Randall
For the needs of structural investigations many of the kinds of subject may possibly with ease be divided into 4 classes，namely, crystalline solids, amorphous solids, drinks, and gases. Following at the paintings of W. L.Bragg within the years 1912 and 1913 wisdom of the preparations of atoms and molecules within the crystalline country has elevated regularly at a truly swift rate.The first investigations at the diffraction of X-rays by means of amorphous solids and beverages have been released by way of Debye and Scherrer in 1916, however it is just relatively lately that whatever like a formal knowing of the consequences has been received. The theories that have been proposed to provide an explanation for the experimental effects are all eventually. occupied with the measure of constitution within the elements, and the understanding of the information has depended to a wide volume at the result of X-ray crystallography. for instance, in an effort to make certain the measure of atomic association that could be assigned to a bit of silica glass or a drop of water, the constructions of cristobalite and ice has to be appropriately known.Between the really crystalline sturdy and the amorphous sturdy there exists an unlimited zone of microcrystalline elements of significant chemical and organic importance;of those, carbon blacks, cellulose， stretched rubber, and the proteins will be mentioned.In addition there are the ingredients that could be microcrystalline in a single course only;this description applies to a number of floor layers, from these because of chemical motion with the underlying substance to the two-dimensional crystal within the kind of a gasoline layer. the tactic of electron一diffraction is best to the research of constitution in such cages as these.Finally, a similar technique, in addition to that of X-ray diffraction，may be used to check the constitution of unmarried gaseous molecules.