380 LETTERS ON NATURAL MAGIC. 



stances which nature sometimes presents to us in order to 

 illustrate her most mysterious operations. Had the por- 

 tions B, C been accompanied, as is usual, with their 

 vacuities, the interposed fluid would have remained 

 immoveable between the two equal and opposite expan- 

 sions ; but owing to the accidental circumstance of these 

 vacuities having passed over into the other branch A of 

 the cavity, the fluid yielded to the difference of the 

 expansive forces between which it lay, and thus exhibited 

 its fluid character to the eye. 



Fig. 79. 



When we examine these cavities narrowly, we find that 

 they are actually little laboratories, in which chemical ope- 

 rations are constantly going on, and beautiful optical phe- 

 nomena continually displaying themselves. Let A B D C, 

 for example, be the summit of a crystallized cavity in 

 topaz, S S representing the dense, N N the expansible 

 fluid, bounded by a circular line abed, and V V the 

 vacuity in the new fluid, bounded by the circle e f g h. 

 If the face A B D C is placed under a compound micro- 

 scope, so that light may be reflected at an angle 

 less than that of total reflection, and if the observer now 

 looks through the microscope, the temperature of the 

 room being 50, he will see the second fluid S S shining 



