222 On the Microscopical Structure ["S'ourS. Ap^irFm 
excellent use of it. When slightly warmed the liquid expands with 
such extreme rapidity that the volume is doubled by a very moderate 
increase of heat, and at 31° 0. the expansion for one single degree 
centigrade is equal to no less that one-fourth of the bulk, as shown 
in the paper already cited.* After having been warmed so as to 
expand and fill the cavities, the liquid on cooling often suddenly 
boils violently ; and when these phenomena are observed under the 
microscope, they are extremely curious and interesting. Mr. Butler 
has contrived a very ingenious apparatus, which enabled him to show 
these facts to great advantage at the Soiree of the Koyal Society on 
March 6th. 
Eubies differ from sapphires in many particulars, which is pro- 
bably due to their occurring in a different class of rocks. Mr. David 
Forbes informs me that, as far as he can learn, rubies are derived 
from granitic rocks, but sapphires from metamorphic limestones. 
The fluid-cavities in rubies sometimes contain what seems to be 
water, and also the more expansible liquid met with in sapphire. 
These facts correspond to what I have seen in the quartz of some 
of the felspathic rocks of Scotland, which differ but shghtly from 
fine-grained granites. 
I have repeatedly mentioned in various papers that the bubbles 
in very minute fluid-cavities exhibit a curious and interesting 
spontaneous motion, analogous to that of all solid substances in 
a state of very fine division, when free to move about in liquids. 
The minute fluid-cavities in some rubies show this phenomenon to 
great perfection ; but unfortunately specimens containing them are 
rare. The rapidity of the movement depends on the size of the 
bubbles. It is not seen to advantage if their diameter is more than 
■ ,0^0 p th of an inch ; but when only twcto they move about in 
the most surprising manner, just as if they were minute animalcules 
trying to escape, and exploring every nook and corner of the 
cavities. 
In the paper by myself and Mr. Butler, there is an account of 
my experiments on the rate of expansion of the fluid in sapphires, 
which corresponds closely with that of liquid carbonic acid. There 
seemed every reason to conclude that it could be no other substance, 
and Mr. W. G. Lettsom has since told me that Geissler of Bonn 
has been led to the same conclusion by entirely different facts. He 
enclosed crystals containing the fluid in a tube, which, on exhaustion, 
showed the spectrum of atmospheric nitrogen, when electric sparks 
were passed through it. Arrangements had been made, so that 
the crystal could then be broken, and after that the tube gave the 
spectrum of carbonic acid. This fact, coupled with those described 
in the above-named paper, seems to prove beyond all doubt that the 
fluid is liquid carbonic acid; but, as was therein shown, it was 
* Proceedings of R.S., vol. xvii., p. 291. 
