132 Dr Brewster on the Refractive Powers, and other 



temperature, the dense fluid appears at a and c, and slightly at 

 o and b, filling the narrow channel o b. By applying heat, the 

 expanding fluid in b V fills the vacuity V ; and, as the cavity 

 A a o c has no vacuity, a portion of its fluid is driven through 

 the neck a b into b V in small globules ; but, owing to the 

 narrowness of the neck at b, the phenomena are not easily ob- 

 served. Upon cooling, however, the retransference of the fluid 

 that had passed from A to b V, is finely seen. The contrac- 

 tion of the expanding fluid in A causes the dense fluid to ap- 

 pear as at m n o, in Fig. 11, and, in a short time, the curved 

 surface m n becomes more flat ; and, at last, a straight line, 

 as at m' n', Fig. 12. This indicates a pressure along the ca- 

 nal V ol, in the direction V o', and a bubble of the expansible 

 fluid instantly issues from of, as in Fig. 12, and, passing 

 through the dense fluid, joins the expansible fluid in A'. Af- 

 ter three or four of these have passed, the equilibrium is re- 

 stored. In this case, the capillary force exerted by the chan- 

 nel ol b' upon the dense fluid which it contains is too strong 

 to permit the little globule of the expansible fluid in b' V 7 to 

 displace it, as in Fig. 10, so that it passes very slowly in se- 

 parate globules. 



The fluid valves, as they may with propriety be called, 

 which thus separate the different branches of cavities, 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 appropriate ; but, in the 

 lesser functions of the same animals, and in almost all the mi- 

 croscopic 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. 

 We must leave it, however, to the physiologist to determine 

 the truth of this supposition. 



4. On the Condition of the Fluids when taken out of the 



Cavities. 

 I have already described so fully in a former paper the 

 singular movements into which the expansible fluid is thrown 



