British and Foreign Salt. 1 ?3 



the ammoniaco-magnesian phosphate, it is easy, therefore* 

 to infer the proportion of sulphate of magnesia in any mix- 

 ture of the two earthy sulphates. 



(D.d.) It was possible, however, that in addition to the 

 sulphates of lime and of magnesia, the quantity of which 

 had been determined by the foregoing process, the speci- 

 men of salt under examination might contain also an alka- 

 line sulphate. To decide this point, it was necessary to 

 compare the amount of the acid, deducible from the weight 

 of the sulphate of barytes (C. b.), with that which ought 

 to exist in the sulphate of lime and sulphate of magnesia 

 actually found by experiment. But to make this com- 

 parison, some collateral experiments were previously ne- 

 cessary. 



(D. e.) By these experiments, I found that sulphate of 

 lime prepared by double decomposition, then calcined in a 

 low red heat, and afterwards dissolved in a large quantity 

 of boiling distilled water, yields, when precipitated by a 

 barytic salt, in the proportion of 175*9 grains of sulphate 

 of barytes from 100 of the calcareous sulphate*. The 

 same quantity of ignited sulphate of lime (=128 grains 

 dried at 160° Fahrenheit), precipitated by super-oxalate of 

 potash, gives 102*3 of oxalate of lime; or, precipitated by 

 sub -carbonate of potash at a boiling heat, 74*3 grains of 

 carbonate of limef. One hundred grains of crystallized 

 sulphate of magnesia ( = 56 desiccated) afford, when preci- 

 pitated by muriate of barytes, 111 or 112 of the barytic 

 sulphate. 



(E.) By a comparison of the above proportions with 

 those obtained in the analysis of any specimen of com- 

 mon salt, we may learn whether it contain other sulphates 

 beside those with earthy bases. For example, if the pre- 

 cipitate (D.) consist of carbonate of lime only, and bear to 



about one half the weight of the salt. Mr. Kirwan states the water of cry- 

 stallization to be 53*6 in 100 grains; but this, I believe, a little exceeds the 

 truth. ^ 



* This result corresponds, within a fraction of a grain, with one obtained 

 in a somewhat different way by Dr. Marcet, and ve-y nearly with an ex- 

 periment of my friend Mr. James Thomson, who found the barytic sul- 

 phate, precipitated from 100 grains of sulphate of lime by nitrate of barytes, 

 to weigh 173 grains. 



f On reversing this experiment, I found that 100 grains of carbonate of 

 lime, saturated with sulphuric acid, and calcined in a low red reat, afford 

 135 of sulphate of lime. A similar experiment of Mr. Thomson gave 134*6 

 grains. Dr. Marcet also informs me, that from 9355 grains of pure marble 

 he obtained 1 25*95 grains of sulphate of lime, proportions which exactly 

 coincide with those of Mr. Thomson. 



the 



