in the Cavities of Minerals. 25 



As the heat increases, the vacuity V advances to the summit 

 AB, and disappears at 79i, exhibiting several curious pheno- 

 mena, which we have not room to describe. One of these, how- 

 ever, is so singular that it deserves to be particularly noticed. 

 After V has disappeared entirely, a brown spot comes from the 

 summit AB, and takes its station in the centre of the ring of the 

 new fluid abed. This brown tint sometimes rises to higher or- 

 ders of colours ; but disappears by the application of heat. That 

 the coloured rings formed within VV are vapour, and not a film 

 of the fluid itself, may be inferred from its never mixing with 

 the fluid with which it is in immediate contact. It might, how- 

 ever, be a fluid substance, arising either from the decomposition 

 of the fluid itself, or from the condensation of gaseous matter 

 within the vacuity ; though this is not very probable, from its 

 constant disappearance when it has accumulated to a certain de- 

 gree, and its constant reproduction while the temperature re- 

 mains the same. 



These views respecting the vaporisation of the expansible 

 fluid, have been fully confirmed by the discovery of the cavities 

 already noticed, in which the expansible fluid occupies only one- 

 third or one-fourth of the cavity. Cavities of this kind are repre- 

 sented in Fig. 26., where AB is the cavity, V the vacuity in the 

 expansible fluid m n op, and A.mn, Bpo the second fluid. When 

 heat is applied to this cavity, the vacuity V does not contract, as 

 in ordinary cases, but expands, till its circumference coincides 

 with the boundary mnop. This unexpected effect might have 

 arisen from the expansible fluid occupying the lower part of the 

 cavity below V, as in the section, Fig. 27. In this case cefd 

 might have been the vacuity, and the surface of the fluid ef 

 might have risen by heat, and gradually filled the vacuity V, while 

 its boundary cd retired to m and n as the surface ef ascended. 

 In order to determine if this supposition was true, I placed AB 



VOL. x. P. i. D 



