On the Intimate Structure of Crystals. 



215 



With the exception of the densities of sodium and potassium 

 chlorides, which have been very exactly determined by Retgers, the 

 densities given above cannot be regarded as more than approxima- 

 tions, not to be trusted in the second place of decimals, not always 

 in the first. If one-half of the volumes of the metals given in the 

 first table be subtracted from the volumes of the salts given in the 

 second, we obtain the following results : 





It will be observed that the difference between the gross volume of 

 the salts containing the same haloid element and the gross volume of 

 the metal they contain is approximately equal in all cases; thus, 

 the common difference in the case of the chlorides varies from 14'4 

 to 15-27 ; in the case of the bromides, from 20'487 to 20-032 ; in that 

 of the iodides, from 32'23 to 29"299. It was assumed by Kopp that 

 these differences corresponded to the volume of the haloid element. 

 We make the same assumption, and by treating the subject from 

 another point of view obtain a more consistent agreement between 

 the differences which represent the volumes of the haloids. 



The system in which lithium bromide crystallises is not known ; 

 the same is true of lithium iodide, and these salts are in consequence 

 necessarily excluded from our inquiry. Neglecting them, it will be 

 seen by inspection that the greater the difference between the gross 

 volume of metal and the haloid with which it is combined, the 

 smaller is the value found for the volame of the haloid ; thus, in the 

 case of the chlorides, the smallest value found for chlorine is 

 14'4, and the disparity between the volumes of lithium and chlo- 

 rine is the greatest in the series. In the same way the bromine in 

 sodium bromide has a less volume than that in the corresponding 

 potassium salt, and the volumes of sodium and bromine are farther 

 removed from equality than those of potassium and bromine. This 

 is a necessary consequence of such an arrangement of spheres as 



