in the Cavities of Mimrals. 37 



13. SULPHATE OF IRON. The cavities in this salt are some- 

 times finely crystallized in the form of prisms, with double py- 

 ramids, and the sharpest truncations. In the same specimen 

 they are frequently oval, or imperfectly spherical. They some- 

 times contain apparent air-bubbles, and are often quite filled 

 with fluid. By the application of heat, these vacuities disappear 

 entirely, and reappear by cooling. 



14. SULPHATE or NICKEL. In this salt the cavities are 

 sometimes amorphous, and sometimes beautifully crystallized. 

 These vacuities frequently disappear by heat, and reappear by 

 the application of cold. 



15. SULPHATE OF COPPER. The air-bubbles move about in 

 this salt by the application of heat, but never vanish. By in- 

 creasing the heat, they diminish a little in size. 



16. ALUM. The air-bubbles in alum do not perceptibly 

 change their magnitude by heat. I have opened several cavi- 

 ties in this salt, but have never found the air to be either in a 

 state of dilatation or compression. The fluid seems to be pure 

 water. 



17. TARTRATE OF POTASH AND SODA. The cavities in this 

 salt are both crystallized and amorphous. A considerable vacui- 

 ty in a large cavity vanished completely with heat ; and in others, 

 where the vacuity was very large, it became extremely small 

 when heated. Two separate cavities, with separate vacuities, 

 became one, and united their vacuities. These phenomena, no 

 doubt, arose from the fluid having its dissolving power increased 

 by heat ; and it is probable that the disappearance of the large 

 vacuity arose from the dissolved salt occupying more space in its 

 fluid than in its solid state. 



