I 



149 



;in a simple cubic space-lattice. Now experience shows that tin m- 

 tni-ity of the secondary radiation produced by R on t gen-pulses 

 inj; i ver atoms, is intimately connected with their atomic weight ; 

 UK > i < particularly it appears that the amplitude of the waves reflected by 

 each net-plane is nearly proportional to the total mass of the atoms lying 

 in that net-plane. If the number of particles in two successive net- 

 planes is the same, the ratio of the amplitudes of the waves reflected 

 by them, will be therefore almost the same as that of the individual 

 masses of both net-planes. And because the atomic weights of potas- 

 sium (= 39) and of chlorine (35,5) are only slightly different, it will 

 be understood that 

 these atoms will be- 

 have nearly as if the 

 particles arranged in 

 thr cubic space-lat- 

 tice were really all 

 of the same kind. 



But when sodium- 

 chloride is used in 

 the experiment, ex- 

 perience shows that 

 the character of the 

 reflection at the fa- 

 ces of (100) and (110 

 is exactly analogous 

 to that observed at 



the same faces of the potassium-salt. The reflection at (1 1 1) however, 

 manifests an additional phenomenon in comparison with the corres- 

 ponding reflection in the case of rock-salt, in so far as the maximum 

 intensity now corresponds not only to a glancing angle of 21, as 

 would be expected with regard to the reflection at the potassium- 

 chloride-crystal, but moreover to another angle of about 1030'. The 

 maximum is here somewhat feebler than the first mentioned one. 



The cause of this difference is explained by the fact that the 

 atomic weight, and therefore the power of emission of sodium- and 

 chlorine-atoms, differ much more than those of potassium- and 

 chlorine-atoms. Indeed, if in the simple cubic space-lattice of fig. 124, 

 in which the black dots are the metal-atoms and the white ones the 

 halogen atoms, we make sections parallel to (100) or (110) ,these 

 consecutive sections will all prove to be identical, consisting of equal 



Fig. 124. 



Structure of Potassium-, and Sodium-chloride. 



