278 



Prof. W. J. Sollas. 



gravity of the different salts, but that they are real and of signifi- 

 cance. The difference in the coefficients of expansion for the different 

 haloids suggest a certain "want of constancy in volume. Thus, 

 Fizeau obtained for the linear coefficient of expansion of 



NaCl 0-00004039 



KC1 0-00003803 



KBr 0-00004201 



KI 0-00004265 



4-49 



5-15 



9-78 



16-76 



and since it is improbable that the component atoms of these salts all 

 possess the same coefficient of expansion, it would seem that an 

 absolute constancy of relative volume is in a high degree impro- 

 bable. It is sufficient, however, for our purposes if it can be shown 

 that the deviation from constancy is confined to narrow limits, as 

 indeed, from the results so far obtained, appears to be the case. 



The importance of these results for our inquiry is, however, this, 

 that they cannot be obtained by any other method of packing; other 

 methods, instead of minimising the differenced between the gross 

 volumes, would, on the contrary, exaggerate them. 



The relative dimensions of the atoms already considered may be 

 tabulated as follows : 



Element. Diameter. Volume. 



Li 1-8848 3-5059 



Na 2-2721 6-1408 



(inNal) 2-325 6'6659 



K 2-8372 11-960 



Element. Diameter. Volume. 



Cl 2-4954 8-1274 



Br (in KBr) 2772 H'1526 



(in NaBr) 2'807 H'5804 



I.. 3-1957 17-1276 



In the next table the sum of the volumes of the atoms in a molecule 

 is compared with the gross volume of the salt, and exhibited in 

 the form of a ratio. 



Salt. I. 



LiCl 11-6333 



NaCl 14-2682 



KC1 20-0874 



NaBr 17*7212 



KBr 23-1126 



Nal 23-7935 



KI 29-0876 



I. Sum of the volumes of the atoms. 

 II. Gross volumes of the salts. 

 III. Ratio of I to II. 



