MOLECULAR ARCHITECTURE 697 



crystals as regards symmetry, cleavage and gliding planes, modes 

 of twinning, etc. From an examination of a large number of 

 trihalogen compounds of the metals potassium, rubidium and 

 caesium, it is concluded that the sphere of influence of the 

 metallic atom in these compounds has the same volume as in the 

 monohalogen compounds. This is in accordance with the theory 

 of multivalency already put forward. It is interesting too to 

 note that thallic iodide is almost identical crystallographically 

 with rubidium triiodide. It is well known of course that thallous 

 salts are isomorphous with potassium salts and the existence of 

 crystallographic similarity between TII3 and Rbis serves but to 

 emphasise the relationship between thallium and the alkali 

 metals. It seems indeed that thallium is out of place in the 

 third group of the periodic system and that its fundamental 

 valency is unity. 



The halogen salts of ammonium crystallise in forms belonging 

 to the cubic system and although the structure of these salts is 

 obviously more complex than that of the corresponding salts of 

 the alkali metals, yet it has been found possible to account satis- 

 factorily for their crystalline structure. The deduction of the 

 crystalline structure of these compounds depends upon the 

 geometrical properties of close-packed assemblages already 

 referred to. 



So far only inorganic substances have been discussed here 

 from the Barlow-Pope standpoint ; but as a matter of fact, the 

 first paper published by these authors dealt principally with the 

 application of the theory to organic compounds, the work on 

 the elements and binary compounds appearing later. Inasmuch, 

 however, as the latter work is simpler and more elementary, it 

 has been introduced here first. In view of the overwhelming 

 influence which the study of organic chemistry has had upon 

 the development of chemical theory, the application of this new 

 theory of the atomic structure of crystals to organic compounds 

 is however of great importance. Many of the most fruitful ideas 

 which have found their way into chemistry have had their origin 

 in the study of organic chemistry. The idea of valency itself 

 was not first developed, as might have been expected, from the 

 study of simple inorganic substances but grew in the mind of 

 Frankland from consideration of more complex organic bodies. 

 During recent years too, a great amount of time has been spent 

 upon the study of the relationships existing between the chemical 



