336 LETTEKS ON NATURAL MAGIC. 



the supposed second fluid DEF. This effect 

 actually took place. The dense fluid quitted the 

 side of the cavity at F. The two portions B, C, 

 of the expansible fluid instantly united, and the 

 dense fluid having retreated to the limit m n n 0, 

 its other limit advanced to p q r, thus proving it 

 to be a real fluid. This experiment, which I 

 have often shown to others, involves one of those 

 rare combinations of circumstances which nature 

 sometimes presents to us in order to illustrate her 

 most mysterious operations. Had the portions 

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

 vacuities, the interposed fluid would have re- 

 mained immoveable between the two equal and 

 opposite expansions ; 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. 84. 



When we examine these cavities narrowly, we 

 find that they are actually little laboratories, in 



