38 



DISCOVERY 



as it were, of the structure on which the molecules 

 are packed. 



This is only the first step towards finding out the 

 cPt'stal structure. Each molecule is composed of a 

 number of atoms. The positions of the atoms are 

 found by comparing the strength of the reflected beam 

 from any face when the path difference causing inter- 

 ference is one wave-length, two wave-lengths, three 

 wave-lengths, and so on. .\nyonc familiar with the 

 difiraction-grating will know that it gives, for a definite 

 wave-length, several spectra, which arc distinguished 

 as being of the first, second or third order. The 

 relative strengths of these spectra depend on the form 

 of the scratch ruled on the grating by the diamond 

 point when it is being prepared. In the case of a 

 crystal, that which corresponds to the scratch is the 

 molecule itself, and a study of the different orders of 

 reflection tells us the arrangement of the atoms in 

 the molecule, i-ome crystal structures worked out in 

 this way are shown in Fig. 4. 



Knowledge of these structures opens up many 

 possibilities. Now that we know how the atoms 

 pack themselves when they bind together, we may be 

 able to find out the nature of the forces which cause 

 them to do so. These forces must be of the same 

 nature as those causing elements to combine to form 

 chemical compounds. It is interesting to note that, 

 in a structure such as that of rock salt, there is no 

 evidence of the existence of the molecule in the ordinary 

 accepted form. The sodium atom touches sLx neigh- 

 bouring chlorine atoms, to any of which it may be 

 considered as belonging. The same thing is found in 

 all the crystals investigated. The diamond structure 

 is interesting because each carbon atom touches four 

 other atoms around it, agreeing with many other 

 properties of carbon which suggest that carbon has four 

 " bonds " arranged tetrahedron-wise, linking it up 

 to the atoms combined with it. 



We can carry our investigations a stage further. In 

 the case of an element, when all the atoms are alike, 

 that which corresponds to the " scratch " on the 

 grating is the atom, and by examination of the different 

 orders of reflection it may be possible to find out the 

 arrangement of the different parts of the atom. The 

 wave-length of X-rays is so small that we may use them 

 to look into the interior of the atom itself. 



Again, knowledge of the crystal structure tells us 

 the distances between the atoms in a crystal, and if 

 we think of the atoms as small hard spheres packed 

 tightly together, we can get an estimate of the size 

 of these spheres. In Fig. 4c an attempt has been 

 made to represent the sodium and chlorine atoms with 

 their relative sizes as found in this way. 



These examples will perhaps give some idea of the 

 vast interest now attached to the study of crystal 



structure. The diffraction of X-rays by crystals has 

 not only led to a far more complete knowledge of the 

 properties of X-rays, but it also affords one of the 

 most promising lines of attack on the problem of 

 atomic structure. 



X-rays and Crystal Structure, by Professor W. H. Bragg, M.A., 



F.R.S., and Professor W. L. Bragg, MA. Fifth cditjon. 



(London : G. BcU & Sons. los. W. net.) 

 Ions, Electrons, and Ionising Radiations, by J. A. Crowther, 



Sc.D. (Arnold, I2J. 6d. net.) 

 A System of Physical Chemistry, vol. i, by Professor W. C. McC. 



Lewis, D.Sc. (Longmans, 15s. net.) 



The Wars of Greek History 



By W. R. Halliday, B.A., B.Litt. 



Professor 0/ Ancient History in the Vniversitg 0/ Liverpool 



"In one point of view," says Aristotle, "the art of war 



is a natural art of acquisition, for it includes hunting, 

 an art which we ought to practise against wild beasts, 

 and against men who, though intended by nature to be 

 governed, will not submit." ' He does not, however, 

 believe in the militarist State. " Men should not study 

 war with a view to the enslavement of those who do 

 not deserve to be enslaved ; but first of all they should 

 provide against their own enslavement ; and in the 

 second place obtain empire for the good of the governed, 

 and not for the sake of exercising a general despotism ; 

 and in the tliird place they should seek to be masters 

 only over those who deserve to be slaves. Facts, as 

 well as arguments, prove that the legislator should 

 direct all his military and other measures to the pro- 

 vision of leisure and the establishment of peace. For 

 the most of these military States are safe only while 

 they are at war, but fall when they have acquired their 

 empire ; like unused iron they lose their edge in peace 

 time." - Aristotle is always sensible and often 

 profound. 



Successful wars from the seventh century B.C. to a.d. 

 1S70 have in fact provided foundations for national 

 development. At certain stages of a nation's growth, 

 strong government at home, combined wth successful 

 aggression abroad, promotes national solidarity and 

 stimulates national consciousness. There is more 

 than one analogy to be drawn between the work of the 

 Greek tyrants ^ and that of our British Tudors. Further, 

 the spoils of an energetic foreign policy may finance 

 the material progress of the community. The great 

 buildings of Syracuse, for example, or those whose 

 ruins line the cliffs at Girgenti in Sicily, were paid for 

 by indemnities exacted from the Carthaginian, and 

 built by the cheap slave labour with which the over- 



> Politics. I, 8, 12. 2 Politics, VII, 14, 22. 



' In the sixth century B.C. 



