510 AGRICULTURAL ANALYSIS 



that when concentrated, the more soluble magnesium salt would 

 first be deposited. Thus, the actual composition of the salt undex" 

 observation must be taken into consideration when applying the 

 theory of the deposition of salt solutions. A study of the order 

 in which these solutions have been deposited leads to a knowl- 

 edge not only of the relative degree of saturation of the original 

 salt in the solution but also to the conception of the temperature 

 under which the deposits were laid down. 



If,, in the theoretic consideration of this problem some definite 

 temperature, pressure and method of crystallization are assumed, 

 it is easy to pass to a knowledge of the relative degree of con- 

 centration in the different constituents under those conditions. 

 In sea water, naturally the amount of sodium chlorid pres- 

 ent is of the first consideration, because that is always the most 

 abundant of the mineral substances in solution. The next most 

 important mineral constituents are the salts of magnesium and po- 

 tassium, especially the chlorids and sulfates thereof, and the next 

 series of salts to be considered are those of calcium. Van't Hoff, 

 in order to give a clearer idea of this theory, expresses the propor- 

 tion of the constituents in sea water, which are found to be, with 

 the exception of calcium, the same everywhere, as follows : 100 

 NaCl+2.2KCl+7.8MgCl 2 -f 3.8MgSO 4 . Van't Hoff states that in 

 a problem of this kind if only one salt is present the situation 

 is very simple. When over two salts are present it is important 

 to determine which will crystallize first and then when will the 

 second salt begin to be deposited. This is illustrated by con- 

 sidering two common salts ; namely, potassium and sodium chlorid 

 in solution at 25. If potassium chlorid be near the point of satura- 

 tion it will be the first to separate into salt form and by further 

 concentrating the solution the content of the sodium chlorid is 

 gradually increased until the saturation point is reached and then 

 it begins to come down. Further concentration does not change 

 the present relation of the two salts in solution. They are both 

 deposited at the same rate when the saturation point is reached 

 and the evaporation continues, with a gradual deposit of the two 

 salts until the water is all gone. If the experiment be started in 

 the opposite condition of the sodium chlorid at saturation point 



