28o SCIENTIFIC APPARATUS. 



rocks. The thin slices which he mounted on glass and placed 

 under the micioscope disclosed the presence of minute cavities in 

 various minerals, filled, some with fluid, some with gas or vapour, 

 some with glass or stone. From the nature of the substance 

 contained in these cavities he inferred whether the mineral or 

 rock had been formed from solution in water, from igneous fusion, 

 from sublimation, or from some combination of these. He further 

 pointed out that the fluid cavities were usually not quite full, but 

 contained a little bubble-like vacuity caused probably by the 

 cooling and subsequent contraction of the liquid which at first had 

 filled the cavity ; and he argued that either by experiment or cal- 

 culation it could be ascertained at what temperature and under what 

 pressure the cavities were originally filled. By these observations he 

 originated a new branch of geological research, which in recent 

 years has made great progress. One of the most valuable results 

 obtained by it has been the demonstration of the great part taken 

 by water in the production of the crystalline masses which enter 

 so largely into the structure of the earth's crust. Rocks, like 

 granite, at one time supposed to have originated from direct 

 igneous fusion, have been shown by the microscope to have 

 acquired their present characters under the combined action of 

 the earth's internal heat and subterranean water. Another im- 

 portant feature of the microscopical study of minerals and rocks 

 has been the light cast upon the processes whereby they have 

 been altered, and in some cases completely transformed. Thus it 

 has been ascertained that even in apparently solid and undecom- 

 posed rocks water has entered through the pores of the stone, and 

 has effected many changes far into the heart of the mass, intro- 

 ducing some substances, abstracting others, or re-arranging the 

 chemical combinations of the original materials. As this slow 

 percolation of water has been going on since the earliest times, 

 the older the rock is the more may it be expected to show proof 

 of this internal operation. The application of the microscope to 

 geological inquiry promises, therefore, to afford the most material 



