SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 211 



On examining the values of D, we see that they increase with those of ME, 

 but the increase is not continuous, it is remittent. It takes place triadwise ; and 

 this holds whether we take the triads in column or in line. Comparing salts in the 

 same line, we see that replacing Rb by Cs causes a rise of specific gravity which is 

 twice as great as that caused by the substitution of Eb for K. Comparing salts in 

 the same column, the replacement of CI by Br causes more than double the rise caused 

 by the substitution of I for Br. However we regard it, we see that the specific gravity 

 of the salts is a periodic function of their molecular iveight, within the ennead. 



In compartment {j) we have the values of :^ or the displacement of one gram- 



molecule (ME) of salt stated in grams of water, and in compartment (m) the same 



constant is stated in gram-molecules of water f — = ). In dealing with the specific 



gravities, we saw that, whether we follow the columns or the lines, they increase 

 with increase of molecular weight. In the case of the molecular displacements this 

 holds for the columns but not for the lines. In these the salts of rubidium have 

 the greatest molecular displacement, the potassium salts have the least, and the 

 caesium salts occupy an intermediate position. As we shall see later, this irregularity 

 is due to a specific peculiarity of the caesium salts. Meantime it may be noted 



that the values of -^^, which may be called the volumetric equivalent of one gram- 

 1 oU 



molecule of any of the salts of the ennead, varies from 2124 HgO to 3 "2 04 H2O, the 



iodides having the highest and the chlorides the lowest equivalents. The average 



difference between the volumetric equivalents of the iodides and bromides is 0'563 



H2O, and that between those of the bromides and chlorides is 0'343 HgO. 



§ 128. TJie Mother-liquor. — The values of T are the same for the mother-liquor as 

 for the crystals, and are presented in (&). In (e) we have the values of m or the 

 molecular concentration of the mother-liquor. This is expressed in gram-molecules salt 

 per 1000 grams of water, its equivalent w in grams is given in sub-table {d), and the total 

 weight in grams (W) of the solution is given in sub-table {/). The concentration, 

 m, of the mother-liquor represents with great exactness the molecular solubility of 

 the salt at T, and we shall consider it for a moment from this point of view. 



The least soluble, molecularly, of the nine salts is caesium iodide, which has the 

 highest molecular weight, and potassium chloride, which has the lowest molecular 

 weight, comes next to it. Next to caesium iodide, in molecular weight and in solubility, 

 we Jiave caesium bromide ; and, similarly, next to potassium chloride, in molecular 

 weight and in solubility, we have potassium bromide. In the latter case the solubility 

 increases with the molecular weight, while in the former it decreases with it. But, if 

 sub-table (c) be referred to, it will be observed that, as regards molecular weight, KCl 

 and Csl occupy singular positions in the ennead. On the other hand, KBr (119-1) 

 and EbCl (121) have almost identical molecular weights, as have also CsBr (213) 



