78 Meteorites and What they Teach us.—Hensoldt. 
ian motion. But the bubble-movement in our fluid cavities 
is due to the ever-varying temperature of the atmosphere. 
The temperature of the air which surrounds us is never con- 
stant, although we can not with our coarse instruments per- 
ceive very small differences. We only see the rise and fall of 
our thermometers after changes more or less considerable, but 
in reality there is a perpetual change of level in the quicksilver 
column. If we were to focus a high-power objective on the 
marginal level of that column we should see it constantly 
shifting and never observe it at rest. The same effect can be 
shown with a delicate spirit-level. If a spirit-level be placed 
on a table and so adjusted that the bubble is in the center, the 
holding of one's hand in the air within a foot's distance from 
the end of the tube suffices to cause a disturbance. The 
warmth of the hand drives the bubble slowly from its position 
which it will resume when the equilibrium is restored. 
The liquid imprisoned in the cavities of quartz and other 
minerals is generally water. Sometimes this water is strongly 
charged with chloride of sodium and in the cavities of many 
granites (notably in those of Cornwall) we frequently observe 
cubic salt-crystals floating about in the liquid. This would 
indicate a saturated solution which once doubtless filled the 
entire cavity, but in the course of ages some of the water 
either evaporated through the rock or a considerable lowering 
of temperature took place so that a corresponding quantity of 
the salt was precipitated. Occasionally the liquids are h3'dro- 
carbons, oily, petroleum-like substances. 
About twenty years ago Vogelsang and Geissler made the 
singular discovery that in many rocks the imprisoned fluid 
consists of carbon dioxide, liquified carbonic acid, and from 
experiments which the writer has made, he has come to the 
conclusion that the fluid contained in the cavities of the 
meteorites of Loutolaks, Noblesborough and Bustee is like- 
wise CO,. On warming the meteoric sections by means of a 
wire coiled around the slide and observing the temperature on 
a stage-thermometer, the writer invariably found that the 
bubbles suddenly vanished when a temperature of 
about 30° C. was reached, but returned again in cooling 
without any apparent diminution in size or moving- 
capacity. Now between 30° and 31° C. lies the so- 
called "critical point" of carbonic acid, that is, above 
