116 ECONOMICAL MINERALOGY. 



cesses, which so easily and completely purify the salt, are interfered with. Instead of allowing 

 time for the gradual subsidence of the insoluble impurities, and the separation of the more 

 soluble ones, the heat is so high that the precipitation of the sulphate and carbonate of lime 

 is more rapid ; and these not being effectually withdrawn by the pans which are used for this 

 purpose, are partly mixed with the salt. The drainage of the salt also is not usually sufficient 

 to remove the deliquescent compounds which the brine contains, and which so greatly injure 

 the character of this salt. 



It is a verj' common practice at the Onondaga works to add lime to the brine contained in 

 the reservoirs, and the same substance is also sometimes thrown into the kettles during the 

 boiling of the salt. This point, among others, was particularly examined by a committee of 

 the Legislature in 1825 ; and I have found no reason to change the opinion which I expressed 

 at that time, viz. that the addition of lime, if properly made, expedites the process of manu- 

 facture, and does not in the least impair the purity of the salt. I should observe, however, 

 that the advantage gained by the use of lime depends upon its purity, and upon the time al- 

 lowed for its precipitation, before the salting commences. When added to the salt during the 

 boiling, it impairs its purity, and may be a fraudulent operation, unless great care is taken in 

 again removing it by the pans with which the kettles are supplied. 



The correctness of these views will be evident, upon a recurrence to the changes which the 

 addition of lime causes in the composition of the brine The excess of carbonic acid which 

 holds in solution the carbonate of lime and oxide of iron in the raw brine, combines with the 

 added lime, and causes a deposit of the whole insoluble carbonate. Moreover, the lime de- 

 composes the chloride of magnesium, and there result chloride of calcium held in solution, 

 and magnesia precipitated. The chloride of calcium thus formed, as well as the amount 

 originally contained in the brine, may be further decomposed by the employment of sulphate 

 of soda, which produces sulphate of lime, presently tlu-own down, and leaves the salt in a 

 nearly pure form. And if this salt, the sulphate of soda, could be obtained at as cheap a rate 

 as it is in some parts of the world, it could no doubt be advantageously employed for this 

 purpose. 



It seems to me, therefore, that there can be no good objection to the use of lime, if it be 

 confined to the resenoirs, as it causes the more speedy separation of the earthy matters, and 

 renders the subsequent salting more rapid and perfect. But I would repeat, that in order to 

 secure its beneficial effect, the lime should be in as caustic or pure a state as it can be con- 

 veniently obtamed. Lime which has been air-slacked, or which has been previously used in 

 a similar operation, caimot be advantageously employed ; indeed, it answers no better purpose 

 than so much sand or clay. And with regrrd to the addition of this substance to the brine 

 while boiling, I can only say, that if the resulting earthy matters be carefully removed, its use 

 may be tolerated ; but it cannot be concealed, that in such cases there is a strong temptation 

 to hurry the process to completion, and thus to mix up the earthy ingredients with the salt. 



In some foreign and American manufactories of salt, blood is added to urine, for the purpose 

 of throwing down its impurities ; and in some arts, net unlike that now under consideration, 

 powdered charcoal is employed with the same intention. Other articles might be used, but 



