SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 213 



of water, we may imagine it to have been prepared in the following way: — 1595*90 

 grams of water are taken, and caesium chloride is dissolved in it so that each portion, 

 as it is added, forms a saturated solution with the exact quantity of water which, it 

 requires for this purpose, and the remainder of the water remains uncontaminated. 

 Parallel with the dissolution of the salt, pure water is removed at such a rate as to 

 keep the displacement or bulk of the liquid always the same. When no more salt will 

 dissolve, we have a saturated solution which contains 1000 grams of water. The weight 

 of caesium chloride which has entered the solution is 2048 "34 grams, and the weight 

 of water which has left it is 595 "90 grams, whilst the displacement of the liquid is the 

 same at the end of the operation as it was at the beginning. In thus describing the 

 preparation of the saturated solution, we have described an operation of substitution. 

 It is therefore permissible to regard saturated solutions as products of substitution. 

 If we give to the above numbers their molecular interpretation, we see that the mean 

 increment of displacement produced by the presence of one molecule of caesium chloride 

 in its saturated solution at 23*1° is equal to that of 2723 gram-molecules of free 

 water, and therefore, that, in these conditions, CsCl is, in a sense, volumetrically 

 equivalent to '2 '7 23 H^O. 



If we study sub-table (/), we see that the average molecular increment of dis- 

 placement produced by the salts increases with their molecular weight, whether we 

 follow the columns or the lines. The only exception is furnished by caesium bromide, 

 the increment produced by which is very slightly lower than that of caesium chloride. 

 The greatest increment is that due to caesium iodide, which has the highest molecular 

 weight ; and the least increment is that due to potassium chloride, which has the 

 lowest molecular weight. The pair, potassium bromide and rubidium chloride, which 

 have almost equal molecular weights, cause also almost equal molecular increments 

 of displacement. The same is true of the pair, potassium iodide and caesium chloride, 

 but rubidium bromide causes a markedly lower increment of displacement. Finally, 

 the pair, rubidium iodide and caesium bromide, which have almost identical molecular 

 weights, present no resemblance in the increment of displacement which they produce. 



§ 130. Comparison of the Displacement of the Salt in Crystal and the Increment 

 of Displacement ivhich it produces in the Water of its Mother -Liquor. — The molecular 



displacement .j= of the salts in crystal is given in sub-table (_;* ) in terms of grams 



of water ; that of — , the salts in mother-liquor, is similarly given in sub-table {I). 



If we compare these two tables, we find the remarkable result that while in the case 

 of the potassium and the rubidium salts the numbers for the displacement in crystal 

 are greater than those for the increment of displacement in mother-liquor, in the case 

 of the caesium salts the reverse is the case. 



In sub-table in) we have the diiference ( ^^^ ) of the molecular displacement of 



V D mj 



