130 Dr Brewster on the Refractive Powers, and other 



position when the equilibrium of the adjacent portions was 

 destroyed by heat. The particles of the dense fluid have a 

 very powerful attraction for themselves, like those of water, 

 and they are also powerfully attracted by the mineral which 

 contains it. The particles of the expansible fluid have, on 

 the contrary, a very slight attraction for one another, and al- 

 so for the mineral which incloses them. Hence it follows, 

 that, as the two fluids never in the slightest degree mix 

 with one another, the dense fluid is either attracted to the 

 angles of angular cavities, or occupies the bottom of round 

 ones, or fills the narrow necks or channels by which two or 

 more cavities communicate with one another. The expan- 

 sible fluid, on the other hand, occupies all the wide parts of 

 the cavities, arid 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, pressing it out of some corners and into others, till the 

 elastic force of the one is in equilibrium with the capillary at- 

 traction of the other. 



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

 each other* as in Fig. 9, 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 sup- 

 pose that there is a vacuity V only in the smaller cavity B, 

 and that heat is applied to the specimen. It is obvious that 

 the greater expansion of the expansible 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 position about b m c when the expan- 

 sive forces are in cequilibrio. 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 po- 

 sition m n. 



Let us now suppose that the cavity A communicates with 

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

 ing into B ; then, at every expansion of A, the dense fluid 

 m n will be driven to a side, but it will immediately return, 



