280 



Li _ 1-8848 _ 

 Cl ~ 2-4954 ~ 



Na 2-2721 



B7 = 2W = ' 8094 ' 



K 2-8372 



Prof. W. J. Sollas. 



Xa, _ 2-2721 

 CT ~ 2^954 



Br 2-772 



0-977. 



Cl 2-4954 _ 



K = ^83-72 - 8792 ' 



= - 0-7307. 



I 3-1957 



K 2-8372 



Maximum limiting ratio, 1 : 0'72865 



If the arrangement of the atoms in the crystals under consideration 

 be, as I conceive, the actually existing arrangement, it is certainly 

 not the one to which views on close packing would have led us. 

 Nature does not appear to have been at all parsimonious of space, 

 and that atoms should be disposed in such comparatively open order 

 and yet produce structures of great rigidity, almost inevitably 

 suggests the existence of poles or directed forces. The importance 

 of the question renders it necessary to probe the matter deeper, and 

 we may naturally seek for further confirmation of our hypothe- 

 sis. Since the arrangement at which we have arrived is the most 

 open probable, we may expect if any other arrangements exist 

 to find evidence of closer packing. We turn, therefore, to the 

 evidence aiforded by solutions. In a solution the molecules are not, 

 as a rule, constrained to oscillate about fixed positions, they are not 

 built up into a solid architecture, but are free to glide over one 

 another, and to migrate from place to place. In their case open 

 packing is not to be expected; in the absence of arrangement, the 

 closest packing will prevail. Closest packing among equal-sized 

 spheres exists when one is surrounded by twelve others in contact. 

 It is the arrangement met with in triangular piles of round shot. In 

 a stack of this kind the ratio of the volume of the balls to the volume 

 of the pyramid they form, including interstices, is as 1 : 1*35. 



On introducing a crystal of common salt into water the crystalline 

 edifice is destroyed, and the separated molecules become surrounded 

 by those of the solvent in closest packing. As the solution is made 

 very dilute the molecules of the salt are resolved into their ions, 

 which wander about in the solution, still under the influence of 

 internal pressures, adapting themselves to the law of closest packing. 



Under these circumstances what change of volume is naturally to 

 be expected when common salt passes into brine ? Clearly a con- 

 traction, and that to a considerable amount, such, indeed, as is 

 always actually observed whenever the haloid salts of the alkalies are 

 dissolved in water. This general observation affords strong con- 

 firmation of the truth of our hypothetical arrangement, which 

 will be still further strengthened if we pursue the subject into 

 quantitative comparison. The ideas involved in the expression 

 atomic volume are less simple in the case of a liquid than in that of 

 a solid. In any case the atomic volume must be regarded as an 



