THE ARRANGEMENT OF THE MOLECULES 



IN A CRYSTAL. 



ALTHOUGH we have little or no certain knowledge 

 about the molecular structure of solids in general, 

 some very interesting investigations have recently been 

 made into the nature of crystalline structure. Little has 

 been published on the subject in England, and it is more 

 than probable that these speculations have escaped the 

 notice of many even of those who are both familiar with 

 the theory of gases and take an interest in recent specula- 

 tions on the nature of liquids. 



With the object therefore of briefly summarising, at 

 any rate some of the more important points which have 

 been established in this fascinating subject : I propose first 

 to indicate what are the views to which crystallographers 

 have been led by more purely mathematical reasoning, and 

 in a subsequent article to criticise the applications of the 

 geometrical theories of crystalline structure which have been 

 made by those who have attempted to give them a physical 

 meaning. To this category belong the interesting structures 

 devised by Lord Kelvin, Liveing, Barlow and Beckenkamp. 



In order to be of any practical use, such a theory should 

 enable us to say something about the arrangement of the 

 particles which constitute a crystal, or about their symmetry, 

 sufficient to explain the form and properties of a given sub- 

 stance in a consistent manner. The attempt to define the 

 actual arrangement of the crystalline particles has as yet 

 been made for very few compounds, but that it has been 

 made, and with a certain show of success, will be evident 

 from a few instances. 



In the first place, Sohncke in 1888 (1) devised an arrange- 

 ment which should accord with the physical and morpho- 

 logical features of quartz, by supposing a spiral line drawn 

 upon the surface of a circular cylinder, and placing particles 

 at equal intervals along this line, so that the fourth comes 

 vertically above the first, the fifth above the second, and so 

 on, the particles thus lying upon the thread of a screw. 



