PHARMACY. 



Phosphas calcis, Edin. magnesia usta, 

 Loud, burnt or calcined magnesia. 



Sulphas alumina exsiccutus, Edin. dried 

 sulphate ot'alumine, burnt alum, Load. 



In the Dublin process for making mag- 

 nesia, there is a mutual decomposition of 

 the two salts employed. The potash 

 unites itself to the sulphuric acid, while 

 the carbonic acid combines with the mag- 

 nesia. The large quantity of water used 

 is necessary for the solution of the sul- 

 phate of potash formed; and the boiling 

 is indispensably requisite for the expul- 

 sion of a portion of the carbonic acid, 

 which retains a part of the magnesia in 

 solution. Sulphate of potash may be ob- 

 tained from the liquor which passes 

 through the filter, by evaporation. This 

 is not pure, however, but mixed with un- 

 decomposed carbonate of potash ; for one 

 hundred parts of crystallised carbonate of 

 potash are sufficient for the decomposi- 

 tion oi one hundred and twenty -five parts 

 of sulphate of magnesia; and as the car- 

 bonate of potash of commerce contains a 

 larger proportion of alkali than the crys- 

 tallized carbonate, a still less proportion 

 should be used. From these quantities 

 about forty-five parts of carbonate of mag- 

 nesia are obtained. 



The ablutions should be made with very 

 pure water ; for nicer purposes, distilled 

 water may be used, and soft water is in 

 every case necessary. Hard water for this 

 process is peculiarly inadmissible, as the 

 principle in waters, giving the property 

 called hardness, is generally a salt of 

 lime, which decomposes the carbonate of 

 magnesia, by compound affinity, giving 

 rise to carbonate of lime, while ihe mag- 

 nesia unites itself to the acid of the calca- 

 reous salt, by which the quantity of the 

 carbonate is not only lessened, but is ren- 

 dered impure by the admixture of carbo- 

 nate of lime. Another source of impurity 

 is the silica which the sub-carbonate of 

 potash generally contains. It is most 

 easily got rid of by exposing the alkaline 

 solution to the air for several days before 

 it is used. In proportion as it becomes 

 saturated with carbonic acid, the silica is 

 precipitated, and may be separated by 

 filtration. 



The carbonate of magnesia thus prepar- 

 ed, is a very light, white, opaque sub- 

 stance, without smell or taste, effervescing 

 with acids. It is not, however, saturated 

 with carbonic acid. By decomposing sul- 

 phate of magnesia by an alkaline carbo- 

 nate, without the application of heat, car- 

 bonate of magnesia ia gradually deposited, 



in transparent, brilliant, hexagonal crys- 

 tals, terminated by an oblique hexagonal 

 plane, and soluble in about four hundred 

 and eighty times its weight of water. The 

 crystallized carbonate of magnesia con- 

 sists of fifty acid, twenty-five magnesia, 

 and twenty -five water; the sub carbonate 

 consists of forty-eight acid, forty magne- 

 sia, and twelve water ; and the carbonate 

 of commerce of thirty-four acid, forty -five 

 magnesia, and twenty-one water. It is 

 decomposed by all the acids, potash, so- 

 da, baryte, lime, and strontian, the sul- 

 phate, phosphate, nitrate, and muriate of 

 alumina, and the super-phosphate of lime. 



CLASS IV. SULPHUREA. 



The preparations under this head are 

 few ; we need only enumerate the two fol- 

 lowing : 



Sulphur lotum, Lond. washed flowers 

 of sulphur. 



Sulphur prxcipitatum, Lond. precipi- 

 tated sulphur. 



In preparing this last, instead of dis- 

 solving sulphuret of potash in water, we 

 may gradually add sublimed sulphur to 

 a boiling solution of potash, until it be sa- 

 turated. When the sulphuretted potash 

 is thrown into water, it is entirely dissolv- 

 ed, but not without decomposition, for it 

 is converted into sulphate of potash, hy- 

 droguretted sulphuret of potash, and sul- 

 phureted hydroguret of potash. The two 

 last compounds are again decomposed on 

 the addition of any acid. The acid com- 

 bines with the potash, sulphuretted hydro- 

 gen flies off' in the form of gas, while sul- 

 phur is precipitated. It. is of little conse- 

 quence what acid is employed to precipi- 

 tate the sulphur. The London College 

 order the sulphuric; while the Dublin 

 College use nitrous acid ; probably be- 

 cause the nitrate of potash formed is 

 more easily washed away than sulphate 

 of potash. 



Precipitated sulphur does not differ 

 from well-washed sublimed sulphur, ex- 

 cept in being much dearer. Its paler 

 colour is owing to its more minute divi- 

 sion, or, according to Dr. Thomson, to 

 the presence of a little water ; but from 

 either circumstance it derives no supe- 

 riority to compensate for the disagreea- 

 bleness of its preparation. 



These are all the more simple prepara- 

 tions of sulphur in common use. There 

 are various preparations into which sul- 

 phur enters as an ingredient ; but such as 

 constituting compounds of the general 



