On the Manufacture of Salt in India. 247 



In addition to the impurities stated in the table, water either 

 in a free state or in combination with any of the impurities 

 usually contained in common salt, may be considered in itself 

 an impurity, particularly as pure muriate of soda contains 

 very little water of crystallization, and does not attract mois- 

 ture from the atmosphere. Dr. Henry found the quantity of 

 water to be in the better kinds of salt, only from 3to \ per cent. 



The insoluble matters in foreign salt imported into Eng- 

 land, and which is made by solar evaporation, are chiefly 

 argillaceous earth coloured by oxide of iron, together with a 

 small proportion of muriate of lime, probably derived from 

 the pits from which the evaporation is allowed to take place. 

 In salt prepared by rapid evaporation, muriate of magnesia 

 abounds most, and as this salt remains in solution in the 

 mother liquor after the common salt or muriate of soda has 

 formed, it adheres to the surface of the crystals, and requires 

 afterwards to be drained off. The larger the grain, says Dr. 

 Henry, the less is the quantity of this solution which the 

 salt holds suspended, and hence the salt prepared at a low 

 degree of heat, is less debased by the magnesian muriate 

 than salt formed at a boiling temperature. 



Although the quantity of sulphates of magnesia and lime 

 appear to vary from two to five per cent, in some of the 

 varieties of English sea salt, these impurities do not appear 

 to be very conspicuous in Indian salt. Dr. Henry having 

 found the quantity of sulphate of lime in salt bearing the 

 same denomination to differ, he was led to investigate the 

 cause, and found that common salt, formed after two hour's 

 boiling, contained about \\ per cent., after four hour's boil- 

 ing only about one per cent. &c. Hence it appeared that the 

 greater part of this impurity falls during the early stage of 

 boiling, and that different specimens of the same kind of salt 

 may therefore differ in chemical purity. 



As the chemical constituents of the several kinds of salt 

 (setting aside their impurities) are the same, Dr. Henry is of 



