ANALYSIS OF CRYSTAL-STRUCTURE BY X-RAYS 393 



have been obtained. Nevertheless, it seems likely that the 

 structure is really the same as that of a-naphthol, with four 

 different orientations of molecules, lying mainly along the 

 c-axis, and the cleavage plane still cutting across the /S- 

 junctions. 



Experimental work has been done and is being continued 

 on compounds, such as benzoic and phthalic acids, which 

 contain the single benzene ring onty. The analysis of these, 

 however, is not yet complete. As mentioned above, benzene 

 itself presents exceptional difficulties, and this unfortunate 

 circumstance has material^ retarded the investigation of the 

 single-ring compounds. Still, the work to date has shown 

 that valuable information respecting the linear dimensions of 

 organic crystals is to be gained by the methods of X-ray 

 analysis, and also has hinted at the existence of a certain 

 general principle, viz., that the benzene and naphthalene 

 rings and, in all probability, other ring combinations known 

 to organic chemists have in crystals actual form and dimensions 

 which are nearly, if not quite, the same when they are built 

 into different compounds. And moreover, as Bragg points 

 out, the work agrees extremely well with that of Langmuir 

 on surface films and the results of Adam {Proc. Roy. Soc, 

 vol. xcix, July 1 92 1 ) and several other workers. The forces that 

 link atoms are certainly of more than one kind. Besides 

 the strong valency bonds, there are undoubtedly bonds of a 

 much weaker character, and it is such bonds as these that 

 unite the molecules of organic compounds and lead to the 

 beautiful architecture of their crystal formation. 



It has been a great improvement to the powder method of 

 crystal analysis that recently X-ray tubes have been devised 

 which offer, as their main characteristic, special facilities for 

 bringing the powder and photographic plate close up to the 

 anticathode. At such short distances as are now used, the 

 beam of rays is so intense that powder reflections can be 

 easily examined by the ionisation spectrometer (see below), 

 and, in fact, reflections from good large crystals can even 

 sometimes be detected by a fluorescent screen. Several 

 workers have employed apparatus of similar design. The 

 Swedish physicist, Siegbahn, uses such a tube at Lund for 

 his precision measurements of the high-frequency spectra of 

 the elements (Phil. Mag., June 191 9), and an interesting tube 

 of this type is the one described by A. Muller {Phil. Mag., 

 September 1921). This latter was designed in order to obtain 

 measurements of the L-spectrum from a liquid mercury anti- 

 cathode. The tube is largely of the home-made variety, its body 

 being a bottle with the bottom cut off. The electrodes and 

 joints are water-cooled, since the joints are made gas-tight 



26 



