141 



theoretical values, appears to be only an approximate one : 

 )r magnesium it is: 1 : 1,6242; for beryllium: 1 : 1,5802; for 

 rscnic: 1 : 1,4025, while for most of them those values oscillate 



1 : 1,33. 



11 ic agreement is better in the case of the oxides and sulphides 

 lent elements as zinc, cadmium, beryllium, etc., where evidently 

 H -amc conditions must exist. A few examples will make this clear '): 



Zinc-oxide a : c \ : 1,6077. 



Zinc-sulphide a : c = 1 : 0,8175 



Beryllium-oxide a : c = \ : \ ,6305. 



Cadmium-sulphide a : c = \ : 0,8109. 



etc. 



18. Notwithstanding this obvious, at least partial, agreement 

 itween measurements and calculations, we shall see that the struc- 

 ires following from the above mentioned theory agree only occa- 

 >nally with the result of the recent investigations of some of 

 lese substances by means of Rontgen-rays. It will certainly there- 

 >re be necessary to postpone a final judgement of the various results 

 )tained, until full certainty has been obtained about the real value 

 the conclusions drawn from the experimental results of the newly 

 :overed methods. 



There is however one point in which all these different investigators 

 jree : it is the conviction of the soundness of the contention that 

 mstitutive atoms of a molecule preserve their individuality, and 

 some extent also their autonomy, as the component particles 

 a crystalline structure. The theory of Sohncke, that a crystal 

 lay be regarded as an interpenetration of regular point-systems, 

 id the view maintained by Groth and by Barlow and Pope, 

 lat the structural units of these systems and space-lattices are the 

 sparate atoms of the chemical molecule, have been supported by 

 lern experience, and their correctness seems to become more 

 id more certain. It will therefore be of interest to consider these 

 lew methods of research here more in detail. 



20. In the preceding paragraphs we have learned to consider 

 the crystal as a discontinuous system of atoms and molecules regu- 



i) C. Friedel, Compt. rend, de 1'Acad. d. Sciences Paris, 52. 983. (1861); 

 O. Miigge, Jahrb. f. Miner. 2. 1. (1882); E. Mallard, Ann., des Mines, 12. 

 427. (1887); H. Traube, Jahrb. f. Miner. Beil. Bnd. 9. 147. (1894). 



