190 MR. J. B. DANCER ON CRYSTALS CONTAINING FLUID. 



British Association^ entitled '^ On a new Method of deter- 

 mining the Temperature and Pressure at which various 

 Rocks and Minerals were formed/^ states that " When 

 crystals are artificially formed from solution in water, they 

 catch up and hermetically enclose in their solid substance 

 small quantities of that liquid, so as to produce fluid- 

 cavities, which, from the nature of the circumstances 

 under which they originate, are just full of the liquid at 

 the temperature and pressure at which they are formed. 



*' This fluid is also afiected by changes of temperature 

 and pressure, only that, of course, the actual amount of the 

 change of dimensions, and the laws connecting it with the 

 temperature and pressure, are not the same as in the case 

 of air. If, then, a crystal be formed at an elevated tem- 

 perature, but under no very great pressure, when it cools 

 down to the ordinary heat of the atmosphere, the fluid in 

 these cavities contracts, so as to leave a vacuity, the rela- 

 tive size of which must of course depend upon the height 

 of the original temperature. 



^' Such fluid-cavities are easily seen with a suitable mag- 

 nifying-power, and the relative size of the vacuity can be 

 measured by means of a micrometer. 



" Applying these principles to the study of natural crys- 

 tals, it is found that, whilst some indicate a temperature not 

 materially higher than that of the atmosphere, many must 

 have been formed at a heat rising upwards to that of dull 

 redness, which is specially the case with igneous and me- 

 tamorphic rocks. If, however, the crystals were formed 

 under very great pressure, of course the above conclusions 

 would be invalidated, and the calculated temperature would 

 be too low ; but if we could form some approximation to 

 the actual temperature, we could deduce from the relative 

 size of the vacuities in the fluid-cavities the pressure under 

 which the crystals were generated. 



" When the fluid-cavities in granite rocks are studied in 



