324 SALTS SULPHATES. 



(e.) Found in the ashes of old wood, and in some plants, particu- 

 larly tamarisk.* 



(/.) In large proportion in the Glauberite of Spain. 



3. PREPARATION. By saturating a solution of soda or its carbo- 

 nate with sulphuric acid, but the quantity produced in the manufac- 

 ture of muriatic acid, and chlorine, and that can be made from sea 

 water, is much greater than can be consumed. 



4. PROPERTIES. 



(a.) Crystallizes in transparent six sided prisms, with dihedral 

 summits, usually striated at the edges, and often very irregular. 



(b.) Taste bitter, and dissolves easily in the mouth ; suffers readily, 

 the watery fusion ; then dries and melts, with the true igneous fusion. 



(c.) Effloresces in the air loses half its weight, and thus becomes, 

 as a medicine, twice as strong ; by a high heat, a part of the acid is 

 driven off. 



(d.) Soluble in 2.67 of water at 60, and in .8, at 212 ;f in this 

 respect, strongly contrasted with sulphate of potassa. The hot solu- 

 tion of sulphate of soda, crystallizes by cooling, { and when the quan- 

 tity is great, the crystals are very large, sometimes half a yard in 

 length, and several inches in diameter. 



5. COMPOSITION. When anhydrous, 



Acid, 55.55 or 1 proportion 40 

 Soda, 44.45 or 1 " 32 



100. 72 its representative number. 



* Four. Ill, 42. 



t In judging of the solubility of a salt, we must not put the salt into water, and 

 expose that water directly to heat, but immerse the vessel containing the salt in a 

 water bath, in which the thermometer is placed. 



t At 70, water dissolves nearly half its weight, twice its weight at 88, and 3.2 of 

 its weight at 106, at any higher degree, some of the salt is deposited in opake anhy- 

 drous crystals, so that it grows less soluble with more heat. Turner. 



If the saturated boiling solution of this salt be made with care in a matrass or 

 flask, and free from agitation, it may be reduced to the temperature of the air with- 

 out crystallizing. Close the vessel by a stop cock at the top, or a good cork, the in- 

 stant before it is withdrawn from the fire, and while still boiling. Sometimes on open- 

 ing or on agitating the solution, and always on throwing in a crystal, (any crystal or 

 solid will do, but better one of the same salt,) nearly the whole fluid will rapidly crys- 

 tallize, and the temperature will rise considerably. The balance offerees between 

 cohesion and repulsion is disturbed by agitation, or by a crystal affording a nucleus. 

 The pressure of the atmosphere acts only as a disturbing force, and any other disturb- 

 ing forces produce the concretion ; for it happens in vacuo if a crystal be dropped 

 in. Mr. Graham, (Phil. Mag. New Series, Vol. IV, p. 215,) has discovered that 

 a saturated solution of sulphate of soda, placed over mercury, previously heated 

 to 110 or 120, will cool without crystallizing, but that if a bubble of air, or 

 of any gas, especially of those that are soluble in water, or a portion of any fluid 

 that attracts water, as alcohol, be thrown up into the solution, it will immediately 

 crystallize. Hence it is concluded that the influence of air in causing the crystal- 

 lization in this well known experiment, is owing to the solution of a portion of it, 

 which thus deprives the salt of a part of its water, and causes the crystallization to 

 begin. 



