340 LETTERS ON NATURAL MAGTC. 



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 an- 

 gular cavities, or filling the narrow necks by 

 which two cavities communicate. The expan- 

 sible 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 func- 

 tion of a fluid valve, opening and shutting itself 

 according to the expansions 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 organizations 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 ap- 

 propriate ; 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 

 crystallized and other bodies, sometimes trans- 

 parent crystals, sometimes black spicular crystals, 

 and sometimes black spheres, all of which are 

 moveable within the cavity. In some cavities the 

 two new fluids occur in an indurated state, and 



