VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. 663 



with as much coinmon water as is sufficient to dissolve them, and allow of a 

 gentle evaporation, till they are again ready to be poured into the coolers for 

 crystallization. This generally proves to be the pure sal catharticum, thoroughly 

 freed, as far as the experiments I have tried can be convincive, from either a 

 sea salt, or the third salt. The liquor decanted from this shooting, may be 

 boiled down again, for a second shooting, and after that a third; but as the 

 liquors from these shootings are boiled away more or less, so you will sooner or 

 later meet again with the pungent liquor, which contains the third salt, as in 

 the former shootings from the bittern, which the pure sal catharticum is as 

 necessarily required to be freed from, as from the common salt ; a proof of 

 which cannot be better determined than by one of the experiments to be taken 

 notice of hereafter, viz. that with the ol. vitriol, which will certainly ferment 

 with this salt, if the sea salt has not been well separated from it, or if it still 

 holds some of the third salt. And when any of the crystallizations will not 

 stand the test of this experiment, they ought to be dissolved and shot again, as 

 before, by which means the pure salt is to be obtained. I do not mention this 

 as a trial made use of at the salt-works, but what I have by experience found to 

 be true. And the same experiment will serve to distinguish a sal mirabile made 

 at these works, from that made with ol. vitrioli and common salt. The account 

 they give of it is this. They take any quantity of coarser grained crystals, 

 boiled from the bittern, which when dissolved and evaporated, more than they 

 would otherwise do for making the sal catharticum, they throw into a wooden 

 bowl, with some oil of vitriol, where it stands for 10 days, and shoots into 

 large crystals, transparent, and like the sal mirabile : but as this salt, by this 

 method, is not sufficiently satiated with the ol. vitriol, if they use any, so it is 

 easily discovered by the ol. vitriol, which will readily ferment with it ; whereas 

 it has no effect on the other sal mirabile made as above.* 



* The liquor, termed bittern, which remains after the crystallization of common salt from sea- 

 water, consists, for the most part, of muriate of magnesia. It is therefore necessary to add the sul- 

 phuric acid in some form or other to the bittern, in order to obtain from it the sal catharticum 

 amarum (Epsom salt) which is a sulphate of magnesia. But this circumstance is not duly noticed in 

 Mr. Brown's communication. The late Dr. Donald Monro (Treatise on Chemistry, Vol. i. p, 200) 

 mentions that at his desire a gentleman inquired of the manufacturers of this salt at Lymington, 

 '* whether they added any of the vitriolic [sulphuric] acid, or any substance containing it, to the 

 bittern from which it was prepared. They said that in general they put none, though they owned 

 that they sometimes did; but they did not mention to him the circumstances which at times induced 

 them to make that addition." In all probability these manufacturers did not choose to explain the 

 whole of their process. 



