1820.] 31. Stromeyer on Polj/halite. 433 



which are doubtless to be attributed merely to the rock salt 

 always inherent in the mineral. On being saturated with sulphuric 

 acid, and afterwards subjected to a spontaneous evaporation, it 

 >vas entirely converted into crystals of sulphate of potash. 



c. That portion of the mineral which is not dissolved by cold 

 water when treated with hot water produces a solution com- 

 pletely neutral, and perfectly colourless, like pure water. But 

 this solution is without any taste, and when evaporated depo- 

 sits only pointed crystals of sulphate of hme. The same effects 

 are produced by reagents, and nothing but a mere solution of 

 the salt is obtained ; for it is neither precipitated by caustic 

 alkalies, whether fixed or volatile, nor is it at all disturbed by tar- 

 taric acid, muriate of platinum, or by nitrate of silver; yet subcar- 

 bonated alkalies, oxalic acid, and oxalate of potash, likewise 

 muriate of barytes and acetate of lead, produce copious pre- 

 cipitations. 



d. The reddish-brown powder left by the water is scarcely 

 acted upon by dilute nitric acid, without assistance from heat. 

 If heated, it is then dissolved, and affords a reddish-brown solu- 

 tion. From this solution, caustic and carbonated alkalies pre- 

 cipitate a hydrate of oxide of iron. Upon an infusion of galls 

 being mixed with it, it turns black ; and if prussiate of potash 

 and iron be dropped in, it deposits prussian blue ; yet neither 

 muriate of barytes nor nitrate of silver produces the least cloudi- 

 ness ; whence it is evident that this powder consists merely of 

 red oxide of iron. 



By attending to these experiments, we may be satisfied that 

 polyhalite consists, as we have before observed, principally of 

 sulphate of lime, and at the same time contains a small quantity 

 of water with a little red oxide of iron, and muriate of soda. 

 With respect to the two latter there is no doubt but that they 

 are foreign to the original composition of this fossil, and only me- 

 chanically mixed with it. But the water doubtless forms gypsum 

 with a portion of sulphate of lime, as is rendered probable from 

 the efflorescence which the mineral undergoes during calcination. 



2. A more exact Chemical Analysis of Polyhalite. — Having 

 obtained the preceding information, I subsequently, in order to 

 obtain a more complete knowledge of its nature and chemical 

 properties, and to discover the proportion of its principal consti- 

 tuent parts, subjected it to the following more exact analysis : 



(A.) 



In the first place, I endeavoured to ascertain the quantity of 

 water contained in polyhalite, the existence of which is proved 

 by the diminution ni weight which we perceive to take place 

 during calcination. For this purpose I exposed to the fire in 

 the glass tube of a barometer about 10 inches long, and closed 

 at one end, by means of a lamp, a certain weight of the pure 

 mineral divided into small pieces ; then when the water was quite 

 evaporated weighed the anhydrous residuum with great exact- 

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