GRIMSLEY. | Technology of Gypsum, 85 
gypsum near Monte Marte, in the neighborhood of Paris. This 
is the locality which has given the name plaster of Paris to the 
calcined plaster, which holds all over the world. ‘This analysis 
gave : 7 
NAVE HUS IRS 8 SRRAN oaNe Gis oe eRe CHORE, SEER a OER ER 7.10 per cent. 
Sones Cre Whhnys ood gue Gocod domaobacpeooes 92.56 66 
Carbonatesotehimenner mae ene astios ae 32 ah 
Sill Cea een eon Ieee tru eii Uepe TAPAS Cora Un arnt Jai 3 .02 oC 
100. 
Other analyses were made and the theoretical percentages of 
the components of a pure gypsum were computed, which have 
been slightly revised from time to time; but now these per- 
centages may be given as 46.6 per cent. of sulphuric acid, 32.5 
per cent. of lime, and 20.9 per cent. of water. The chemical 
formula for gypsum would be CaSO,+2H.,0; and when the 
mineral is deprived of the water it is known as anhydrite. 
Physical Properties of Gypsum. 
In its physical properties gypsum is one of the softest min- 
erals, and in the old inaccurate Mohs’ scale it ranked No. 2 
in the scale of 10, with a specific gravity when pure of 2.32. 
Gypsum crystallizes in the monoclinic system of the mineralo- 
gists in the form of plates or prisms with pyramidal termina- 
tions. The relative lengths of the crystal axes, or the length 
of the inclined axis, which passes from front to back of the 
crystal, to the axis which passes from right to left, to the ver- 
tical axis, is represented by the formula 0.6891:1:0. 4156, 
with the angle of the inclined axis to the vertical, 81° 5. 
Twin or united crystals, as shown in Figure 8, are very com- 
mon, where the crystals are twinned on the ortho-pinacoidal 
face, which is the face at right angles to the vertical, and to the 
right and left axes. 
The typical forms of the crystals are shown in Figures 6 and 
7. The cleavage is almost perfect on the face b, which explains 
the plate-like character of gypsum found in the rocks. Cleav- 
age very often takes place on the face n, cutting across the first 
cleavage. The faces of the twin crystals are sometimes rounded, 
as shown in Figure 9. This is especially characteristic of many 
