This difficulty was at last entirely removed by the discovery of a cavity of the form shown in the annexed figure, where A, B, and C are three portions of the expansible fluid separated by the interposition of the second fluid D E F. The first portion A of the expansible fluid had four vacuities V, X, Y, Z, while the other two portions B, C, had no vacuity. In order to determine if the vacuities of the portions B, C, had passed over to A, I took an accurate drawing of the appearances at a temperature of 50°, as shown in the figure, and I watched the changes which took place in raising the temperature to 83°. The portion A gradually expanded itself till it filled up all the four vacuities V, X, Y, and Z, but as the portions B, C, had no vacuities, they could expand themselves only by pushing back the supposed second fluid D E F. This effect actually took place. The dense fluid quitted the side of the cavity at F. The two portions B, C, of the expansible fluid instantly united, and the dense fluid having retreated to the limit _m_ _n_ _o_, its other limit advanced to _p_ _q_ _r_, thus proving it to be a real fluid. This experiment, which I have often shown to others, involves one of those rare combinations of circumstances which nature sometimes presents to us in order to illustrate her most mysterious operations. Had the portions B, C, been accompanied, as is usual, with their vacuities, the interposed fluid would have remained immoveable between the two equal and opposite expansions; but owing to the accidental circumstance of these vacuities having passed over into the other branch A of the cavity, the fluid yielded to the difference of the expansive forces between which it lay, and thus exhibited its fluid character to the eye. [Illustration: _Fig. 84._] When we examine these cavities narrowly, we find that they are actually little laboratories, in which chemical operations are constantly going on, and beautiful optical phenomena continually displaying themselves. Let A B D C, for example, be the summit of a crystallized cavity in topaz, S S representing the dense, N N the expansible fluid, bounded by a circular line _a b c d_, and V V the vacuity in the new fluid, bounded by the circle _e f g h_. If the face A B D C is placed under a compound microscope, so that light may be reflected at an angle less than that of total reflexion, and if the observer now looks through the microscope, the temperature of the room being 50°, he will see the second fluid S S shining with a very feeble reflected light, the dense fluid N N with a light perceptibly brighter, and the vacuity V V with a light of considerable brilliancy. The boundaries _a b c d_, _e f g h_, are marked by a well-defined outline, and also by the concentric coloured rings of thin plates produced by the extreme thinness of each of the fluids at their edges.