38 J. Y. Buchanan — Specific Gravity of Soluble Salts 



We shall best arrive at this by returning to onr detailed 

 example of chloride of caesium. As the quantity of saturated 

 solution which contains 1000 grams of water weighs 3018'34: 

 grams and displaces 1595*92 grams of water, we may imagine 

 it to have been prepared in the following way: — 1595 - 92 

 grams of water are taken and caesium chloride is dissolved in 

 it so that each portion, as it is added, forms a saturated solu- 

 tion with the exact quantity of water which it requires for 

 this purpose and the remainder of the water remains uncon- 

 taminated. 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*92 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 substitu- 

 tion. It is therefore permissible to regard solutions as pro- 

 ducts of substitution. If we give to the above numbers 

 their molecular interpretation, we see that the mean apparent 

 displacement of one molecule of caesium chloride in its satu- 

 rated solution at 23*1° is equal to that of 2*723 gram mole- 

 cules of water, and therefore, that, in these conditions, CsCl is 

 volumetrically equivalent to % m 7&3 H n O. 



If we study Table Y (d), we see that the average molecular 

 displacement of the salts increases with their molecular weight, 

 whether we follow the columns or the lines. The only excep- 

 tion is furnished by caesium bromide, the displacement of 

 which is very slightly lower than that of caesium chloride. 

 The greatest molecular displacement is that of caesium iodide, 

 which has the highest molecular weight; and the least molec- 

 ular displacement is that of potassium chloride, which has the 

 lowest molecular weight. The pair, potassium bromide and 

 rubidium chloride, which have almost equal molecular weights, 

 have also almost equal molecular displacements.. The same is 

 true of the pair, potassium iodide and caesium chloride, but 

 rubidium bromide has a markedly lower displacement. 

 Finally, the pair, rubidium iodide and caesium bromide, which 

 have almost identical molecular weights, present no resem- 

 blance in their apparent molecular displacements. 



Comparison of the Displacement of the Salt in Crystal and 



in Mother-liquor. — The molecular displacement ^j— of the 



salts in crystal is given in Table TV (c) in terms of grams of 

 water ; that of the salts in mother-liquor is similarly given in 

 Table V (c) . 



