114 Analysis of several Varieties of 



consist of absolutely pure muriate of soda, attract much 

 moisture from a humid atmosphere. 



IV. The sulphate of magnesia and the sulphate of lime 

 both enter into the composition of all the varieties of salt 

 prepaTed from sea water; but the sulphate of lime alone is 

 found in Cheshire salt. The proportion of sulphate of 

 magnesia is greatest in that variety of sea salt which has 

 been formed by rapid evaporation. In foreign bay salt its- 

 quantity is very insignificant. 



From the table \\ may be seen, that the proportion of 

 sulphate of lime is greater in foreign bay salt than in any 

 variety of British salt, even than in those which are pre- 

 pared from sea water with a boiling heat. The only ex- 

 planation of this fact, that oceurs to me, is, that during 

 the rapid evaporation of sea water a considerable part of 

 the calcareous sulphate is precipitated at an early stage of 

 the process, and is partly removed in clearing the boiler, a 

 process which can scarcely be performed during the for- 

 mation of bay salt, in pits whose sides are composed of 

 moist clay. The remainder of the selenite, thus precipi- 

 tated by the rapid evaporation of sea water, enters into the 

 composition of the pan-scale. 



In the course of this inquiry I was induced to repeat the 

 same experiments several times, on various specimens of 

 salt bearing the same designation; and was surprised to 

 find that the results by no means corresponded. In one 

 instance, for example, fishery salt was found in 1000 parts 

 to contain no less then 16 parts of sulphate of lime; while 

 another specimen, nominally the same, contained only ll± 

 parts of selenite in the same quantity; and a third only 5±. 

 At length it occurred to me that these differences were 

 probably owing to the circumstance of the salt having been 

 taken from the boiler at different periods of the evaporation. 

 I requested, therefore, to be furnished with specimens of 

 salt, drawn at different stages of the process from a given 

 portion of brine, evaporated in the same boiler. These 

 were submitted to analysis; and the results are shown in 

 the following table. 



Common salt drawn from the boiler two f •£ - 

 hours after the first application of heat . . } | 5 



Salt drawn four hours after do ] | g 



Salt drawn six hours after do L 2 2 J 2>\ 



Hence it appears that there was a gradually increasing 



purity in the salt from sulphate of lime, as the process of 



evaporation 



