M. "Wilson on the Determination of the Hardness of Water. 519 



of water, which is a modification of Clark's original method. He 

 uses a solution of sulphate of lime prepared by dissolving 1 part 

 CaO SO 3 , 2HO in 2543 parts of water. This corresponds to 

 Clark's standard solution of 16 parts CaO CO 2 in 70,000 parts 

 of water. 



The solution of soap was prepared according to Faisst's method 

 of dissolving 30 grms. of soda oil-soap in alcohol of 56° F., and 

 diluting this solution so that 32 cubic centims. were exactly 

 enough to produce, when shaken with 100 cubic centims. of the 

 normal gypsum solution, a froth which remained for five minutes. 

 By adding 4 cubic centims. of a cold saturated solution of car- 

 bonate of soda the reaction is made more regular, inasmuch as it 

 changes all lime compounds into the carbonate which remains 

 dissolved. 



The experiments were made in the following manner:— By 

 adding to the normal gypsum solution of 16 degrees hardness 

 corresponding quantities of distilled water, sixteen solutions were 

 prepared of from 1 to 16 degrees hardness. Of these solutions 

 100 cubic centims. were placed in a stoppered glass cylinder of 

 400 cubic centims. capacity, with 4 cubic centims. of the cold 

 saturated solution of carbonate of soda, and solution of soap 

 added from a burette until, on agitation, a light froth was 

 formed. The solution was then added very gradually with con- 

 tinual agitation, until after the addition of the last drop a froth 

 was formed lasting five minutes. The experimental results 

 showed that the use of every 2 cubic centims. of solution of soap 

 corresponds to 1 degree of hardness. 



In order to test the hardness of water, 100 cubic centims. are 

 measured off, 4 cubic centims. of saturated solution of carbonate 

 of soda added until a froth is formed which remains standing 

 five minutes. The number of cubic centims. of soap-solution 

 divided by 2, gives the corresponding degree of hardness. 



This method is not applicable to waters of more than 16 

 degrees hardness. With water of 20 degrees hardness, a pre- 

 cipitate of carbonate of lime is formed on the addition of car- 

 bonate of soda. Such waters must be diluted to a proper 

 extent by the addition of distilled water. 



In order to ascertain whether salts of magnesia exhibit the 

 same deportment, a solution of 1 part sulphate of magnesia 

 (MgOS0 3 + 7HO) was made in 1778 parts of water, which 

 corresponds to a gypsum solution of 16 degrees hardness. By 

 corresponding dilution with distilled water, solutions of 1, 4, 8, 

 12 degrees hardness were obtained, and these were estimated, 

 after the addition of carbonate of soda, by means of soap-solu- 

 tion. The same results were obtained as with solutions of lime. 



"Wilson found, as Faisst had previously done, that mixtures of 



