14 SIR DAVID BREWSTER ON THE EXISTENCE OF CRYSTALS 
analogous kind ; but as it appeared unexpectedly, and was instantly followed by 
the explosion of the crystal, I could neither observe it accurately, nor confirm 
what I did observe, by a repetition of the experiment. I have, therefore, some 
satisfaction in describing a similar phenomenon, seen frequently, and under more 
favourable circumstances, not only from its intrinsic interest, but because a dis- 
tinguished philosopher had treated with an air of incredibility an observation 
which I had made of a similar kind. There can be no higher testimony to the 
novelty and importance of a scientific fact, than when a competent judge raises 
it to the supernatural. 
I come now to describe a property of the dense fluid, so new and remarkable 
that it cannot fail to excite the attention of chemists. This fluid occupies the 
whole of a large cavity ABCD E, Fig. 10, with the exception of a bubble at A, 
which must be either a vacuum, as it is in all cavities containing only this fluid, 
or a bubble of the expansible fluid, or the vapour of the dense fluid, or some gase- 
ous body. It cannot be a vacuum; because it expands with heat, in place of being 
filled up by the expansion of the fluid. It cannot be the expansible fluid; because 
cold would contract it, and produce a vacuity. It cannot be the vapour of the 
expansible fluid; because there is no expansible fluid to throw it off, and it has 
not the optical properties of its vapour. It cannot be the vapour of the fluid 
in the cavity ; for it does not disappear by the application of cold, and does not 
become a vacuity, which fills up by the expansion of the fluid. It is, therefore, 
an independent gas, which exhibits the following phenomena. 
When heat is applied, the bubble A expands, not by the degradation of its 
circular margin passing into vapour, as in the vapour cavities described in a former 
paper, but by the rapid enlargement of its area. When it attains a certain size, 
it throws off a secondary bubble B, which passes over a sort of ridge or weir mno, 
in the bottom of the cavity, and settles at B. If the heat is continued, these two 
bubbles increase in size ; but it was instantly withdrawn when B had begun to 
swell. As the topaz began to cool, both the bubbles A and B quickly contracted. 
The primary bubble A returned gradually to its original condition, and B, when 
reduced to a single speck, would have disappeared, had the cooling not been stop- 
ped. This speck swelled again by the application of heat, and so did the bubble A. 
When the speck at B was allowed to vanish, which it did on the spot which the 
bubble occupied, the fresh application of heat did not revive it at that spot, but 
merely expanded the primary bubble A, which again threw off a secondary 
bubble B, which exhibited by heat and cold the same phenomena as before. These 
experiments I repeated many times with the same result. It will naturally be 
asked, what was the condition of the fluid itself which has the property of expanding 
by heat; and what became of it while a part of the space which it occupied was 
appropriated by the bubble B, and the addition to the bubble A? An accidental 
circumstance enables me to answer this question, which would have been 
