14 PROCEEDINGS OF THE ACADEMY OF 



incrustation which is so vorv near chemically pure calcic sulphate, 

 and none in which tliis is so perfectly anhydrous. AVe know tiiat 

 calcic snlpiiate occurs in nature in two forms. In one it is com- 

 bined with two equivalents of water, viz.: 



Calcic sulphate = 79.07 

 Water = 20.93 



crystallizing in oblique rhombic prisms, and is called gypsum. 

 Tiien it is found without water, cr^^stallized in orthorhombic 

 prisms, and is named anhydrite. 



We know fuither that gypsum begins to lose water not much 

 above the boiling point of water, and can be rendered anhydrous 

 by prolonged heating at about 200 centigrade. Still it seemed 

 of interest to ascertain what changes would take place in a satu- 

 rated solution of gyi)sum Avhen evaporated /inder the atmospheric 

 pressure at the boiling point, and also under a higher pressure. 



A saturated solution of gypsum was kept at the boiling point 

 until an ample amount of precipitate had formed. This precipi- 

 tate consisted of minute scales with a marked silky lustre. Under 

 a magnifying power of GO diameters the scales proved to possess 

 the characteristic tabular forms of gypsum with the oblique basis. 

 They were perfectly transparent, and many were twins, those 

 swallow-tail shaped forms so well known. 



Upon ignition 20.7 per cent, of water was found instead of 

 20.93, as required by the formula. 



A saturated solution of gypsum was now sealed up into a glass 

 tube, and kept in an oil bath for fourteen hours at a temperature 

 of 148 centigrade. This temperature is equal to a pressure of 

 4.4 atmospheres, or 66 pounds to the square inch. 



A slight granular precipitate was found on the glass after re- 

 moving the tube from the bath and drawing olf the mother liquor. 

 Under a magnifying power of 1 20 diameters the apparent granules 

 dissolved into stellate groups of needle-shaped crystals intermixed 

 witli single needles of a larger size. Most of the crystals had the 

 oblique rhombic basal termination of gypsum; but some showed 

 an orthorhombic basis. All the crystals had become opaque, 

 apparently b}'^ innumerable fissures, as a network could be dis- 

 cerned in many individuals. The presence of prismatic protu- 

 berances on some of the cr3'stals standing at right angles to the 

 princii)al axis of the main crystal seemed very curious. 



The preci}/itate was now removed from the tube and carefully 

 washed, then dried over sulphuric acid. After ignition a loss was 

 ])roduced of 3.1 per cent. Taking into consideration that not all 

 of the mother licpior was washed out and crystallizeil as gyi)- 

 snm, this result may be looked upon as coniirmatory that the 

 whole of the precipitate is anhydrite. Professor Genth is of 

 opinion that tlie opaque stellate crystals are pseudomorphs of 

 anhydrite after gy|)sum, and there does not seem at present any 

 reason to the contrary. 



