NEW FLUIDS IN MINERALS. 383 



cence in the sulphuric, nitric, and muriatic acids. The 

 residue of the expansible fluid is volatilized by heat, and 

 is dissolved, but without effervescence, in the above-men- 

 tioned acids. The refractive power of the dense fluid is 

 about 1*295, and of the expansible one 1/131. 



The particles of the dense fluid have a very powerful 

 attraction for each other and for the mineral which con- 

 tains them, while those of the expansible fluid have a 

 very slight attraction for one another, and also for the 

 substance of the mineral. Hence the two fluids never 

 mix, the dense fluid being attracted to the angles of angu- 

 lar cavities, or filling the narrow necks by which two 

 cavities communicate. The expansible fluid, on the other 

 hand, fills the wide parts of the cavities, and in deep and 

 round cavities it lies above the dense fluid. 



When the dense fluid occupies the necks which join two 

 cavities, it performs the singular function of a fluid 

 valve, opening and shutting itself according to the expan- 

 sions or contractions of the other fluid. The fluid valves 

 thus exhibited in action may suggest some useful hints to 

 the mechanic and the philosopher, while they afford 

 ground of curious speculation in reference to the functions 

 of animal and vegetable bodies. In the larger organiza- 

 tions of ordinary animals, where gravity must in general 

 overpower, or at least modify, the influence of capillary 

 attraction, such a mechanism is neither necessary nor 

 appropriate ; but, in the lesser functions of the same 

 animals, and in almost all the microscopic structures of 

 the lower world, where the force of gravity is entirely 

 subjected to the more powerful energy of capillary forces, 

 it is extremely probable that the mechanism of immiscible 

 fluids and fluid valves is generally adopted. 



In several cavities in minerals I have found crystal- 

 lised and other bodies, sometimes transparent crystals, 

 sometimes black spicular crystals, and sometimes black 



