422 DR BREWSTER on the Refractive Powers and other 



A very curious modification of these actions is seen in a ca- 

 vity of the specimen shewn in Plate XXI., which I have repre- 

 sented separately in Fig. 1 1 . of Plate XIX. The branch b V has 

 always, at common temperatures, a vacuity V, and the cavity A, 

 connected with it by the filamentous channel o b, has no vacuity. 

 At the ordinary temperature, the dense fluid appears at a c, and 

 slightly at o and b, filling the narrow channel o b. By applying 

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

 cavity A a o c has no vacuity, a portion of its fluid is necessarily 

 driven through the neck a b into b V in small globules ; but, ow- 

 ing to the narrowness of the neck at b, the phenomena are not 

 easily observed. Upon cooling, however, the retransference of the 

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

 traction of the expanding fluid in A causes the dense fluid to 

 appear as at m n o, in Fig. 10., 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 canal b' o', in 

 the direction b' o', and a bubble of the expansible fluid instantly 

 issues from o', as in Fig. 12., and, passing through the dense 

 fluid, joins the expansible fluid in A'. After three or four of 

 these have passed, the equilibrium is restored. In this case, the 

 capillary force exerted by the channel o' b' upon the dense fluid 

 which it contains is too strong to permit the little globule of the 

 expansible fluid in b' V to displace it, as in Fig. 9., so that it 

 passes very slowly in separate 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 organisations of ordi- 

 nary animals, where gravity must in general overpower, or at 

 least modify, the influence of capillary attraction, such a me- 

 chanism is neither necessary nor appropriate ; but, in the lesser 

 functions of the same animals, and in almost all the microscopic 



