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a pair. This results in the entire suppression of the second 

 order spectrum in the reflection from this plane, and of the first 

 order spectrum in the reflection from the (ioo) plane, through 

 interference. 



Of other interesting cubic crystals, calcium fluoride or fluor- 

 spar, CaF 2 , gives X-ray spectra analogous to those given by the 

 diamond. Here the weight of the two fluorine atoms is very 

 nearly the same as the weight of the single calcium atom. Its 

 ^structure is that of zinc sulphide, with the difference that there 

 is a fluorine atom in each of the eight small cubes of the figure. 

 In iron sulphide or pyrites, FeS 2 , another cubic crystal, the 

 structure is similar to that of fluorspar, except that the sulphur 

 atoms no longer occupy the centre of the small cubes, but a 

 position on the diagonal joining one of the iron atoms with the 



W 



Fig. 5. 



unoccupied corner of the small cube, approximately four times 

 as far from the iron atom as from the unoccupied corner. In 

 fig- 5> 7 ) represents the arrangement for fluorspar and (b) for 

 pyrites. 



The authors then go on to the consideration of more complex 

 crystals, of which the examination has already been begun, but 

 these examples must suffice. To do full justice to such a subject 

 would require a mastery of many branches of science not yet 

 interlinked. " We have refrained from the discussion of a 

 number of interesting points of contact with other sciences " — it 

 is stated in the preface — " and with older work, such as for 

 example the remarkable investigations of Pope and Barlow." 

 Tutton's work on isomorphous crystals, and his clear distinction 

 between eutropic series and isomorphous series, is recalled by 

 the following passage : " It is interesting to notice that ammonium 

 chloride is in no sense isomorphous with the other alkali haloids, 



