212 Specific Gravity of Soluble Salts by 



whence the gross apparent displacement of the salt in solu- 

 tion is 



v V 1000 = 595-92 grammes, 



and the mean apparent displacement per molecule is 



v 



- = 49-021 grammes. 



In compartment (c) we have the value of for each member 



of the ennead. This expresses, in grammes of water, the average 

 apparent displacement of one gram-molecule of salt in its 

 saturated solution at T. In compartment (d) the same constant 



is expressed in terms of gram-molecules of water ( j . 



Before commenting on the numbers in the table, it is 

 important to form a clear conception of their physical meaning. 

 We shall best arrive at this by returning to our detailed example 

 of chloride of caesium. As the quantity of saturated solution 

 which contains 1000 grammes of water weighs 3048-34 grammes 

 and displaces 1595-92 grammes 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 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 grammes of water. The weight of caesium 

 chloride which has entered the solution is 2048-34 grammes 

 and the weight of water which has left it is 595-92 grammes, 

 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 

 solutions as products 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 



