470 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



fluid, or the elastic force of the vapour, bursts the cavity and often 

 causes the crystal to fly to pieces with great violence. When sub- 

 sequently examined with the microscope, it is seen that the fluid has 

 been expelled ; and in order to ascertain whether or not the fluid thus 

 given off is water, I adopt the following method. I have a glass tube 

 8 inches long and with a i-inch bore, closed at one end ; and, having 

 placed in it fragments of the mineral dried at 100° C, I fill the tube 

 with air dried by passing over chloride of calcium. The open end 

 is then closed with a well- dried cork, and the other passed through 

 two holes in the opposite sides of a small box containing a mixture 

 of pounded ice or snow and salt, so as to project about a couple of 

 inches. A sufficiently strong heat is then applied to the closed end, 

 containing the fragments, to expel the fluid from the cavities. If 

 they contained water, it is condensed as small crystals of ice on the 

 cold part of the tube ; and when the whole has cooled, it is with- 

 drawn from the box and placed in a strong solution of common salt, 

 at a temperature several degrees below the freezing-point of water, 

 and the form of the enclosed crystals examined with a magnifying 

 glass. By carefully noticing the rise of a thermometer as the solu- 

 tion of salt becomes warmer, the temperature at which they cease to 

 be solid and pass into a liquid is easily ascertained ; and if it be 

 found that the crystalline form and melting-point of the liquid thus 

 given off from the fluid-cavities are the same as those of water, I 

 think we may safely conclude that the liquid seen in the cavities is 

 water or some aqueous solution. 



§ 3. Minerals contained in Secondary Rocks, 

 a.. Rock-Salt, Calcite, fyc. 



In proceeding now to apply the above general conclusions to the 

 investigation of the circumstances under which natural crystals were 

 formed, it will be best to commence with rock-salt, since its peculiar 

 structure can be imitated artificially with perfect accuracy. It 

 often contains excellent fluid-cavities, which, besides a fluid, sometimes 

 enclose a variable, and often a considerable, quantity of mud. On 

 the whole, the specimens I have examined do not contain much of 

 this substance ; and in the solid parts of the crystals the cavities 

 are full of liquid. Hence the salt must have been deposited very 

 slowly from solution in more or less muddy water, at a heat not very 

 considerably, if at all, higher than the ordinary temperature of the 

 atmosphere, unless it was formed under a very great pressure. 

 Much the same conclusions apply to the selenite in gypseous marls ; 

 but most of the cavities have lost fluid by drying, — which need not 

 surprise us, since it contains combined water and has a very laminar 

 structure. 



When pure calcite containing no fluid-cavities is heated, it does 

 not decrepitate, and gives off no water ; but when it contains fluid- 

 cavities, the crystals fly to pieces and give off water, and I therefore 

 conclude that the fluid in the cavities is water. I have found many 

 excellent fluid-cavities in the calcite of modern tufaceous deposits, in 



