DR BREWSTER on the Refractive Powers and other 



cavities, or occupies the bottom of round ones, or fills the nar- 

 row necks or channels by which two or more cavities communi- 

 cate with one another. The expansible fluid, on the other hand, 

 occupies all the wide parts of the cavities, and in those which are 

 deep and round it lies above the dense fluid. 



If we now apply heat to a single deep cavity containing both 

 fluids, the elastic force exerted by the expansible one, after its 

 vacuity is filled up, will modify the form of the dense fluid, pres- 

 sing it out of some corners and into others, till the elastic force 

 of the one is in equilibrium with the capiUary attraction of the 

 other. 



But if there are two cavities, A, B, communicating with each 

 other, as in Fig. 10. Plate XIX., where the dotted part represents 

 the expansible fluid, then the dense fluid will be found in the neck 

 at m, n, and at the angles o, p, r, s. Let us now suppose that there 

 is a vacuity V only in the smaller cavity B, and that heat is ap- 

 plied to the specimen. It is obvious that the greater expansion 

 of the dotted fluid in A, which has no vacuity to fill, will force 

 the dense fluid m n towards V, where it will take up a new posi- 

 tion about b m c when the expansive forces are in (equilibria. But 

 if the cavity A is very large compared with B, the fluid m n will 

 be driven out of the neck b n, and will find its way to some of the 

 corners o, or p, from which, upon cooling, it will again return to 

 its position m n. 



Let us now suppose that the cavity A communicates with 

 other cavities which expand slowly into it, while it is expanding 

 into B ; then, at every expansion of A, the dense fluid m n will 

 be driven to a side, but it will immediately return, opening and 

 shutting like a valve. This effect is finely exhibited in an irre- 

 gular branching cavity of a specimen belonging to Mr SANDER- 

 SON ; but as the expansions and contractions are too numerous 

 and complicated, I shall describe them as existing in a cavity of a 

 more simple structure, represented in Fig. 9. Plate XIX, by A B 



