ON THE SOLIDIFICATION OP CARBONIC ACID. 307 
L. At the end of ten minutes the communication with the 
generator may be cut off — when about eight fluid ounces of 
liquid acid at 32° F. will be found in the receiver. 
By letting this liquid into the box, 0, through the pipe, G, 
a large part of it is instantly expanded into gas, which escapes 
through the tube, P. The coldness consequent on the enor- 
mous expansion freezes another part of the liquid, which falls 
to the bottom of 0. About one drachm of solid matter is 
thus formed for each ounce of liquid. 
The porosity and volatile character of the solid renders its 
specific gravity of difficult ascertainment. When recently form- 
ed, it is about the weight of carbonate of magnesia; and when 
strongly compressed by the fingers, its density is nearly dou- 
bled. Solid carbonic acid is of a perfect whiteness, and of a 
soft and spongy texture, very like slightly moistened and 
aggregated snow. It evaporates rapidly, becoming thereby 
colder and colder, but the coldness produced seems to steadily 
lessen the evaporation, so that the mass may be kept for some 
time. A quantity weighing 346 grains lost from 3 to 4 grains 
per minute at first, but did not entirely disappear for 3 hours 
and a half. The natural temperature was 76° — 79°. The 
solid is most easily kept when compressed and rolled up in 
cotton or wool. Its temperature when newly formed is not 
exactly ascertainable, because it is immediately lowered by 
evaporation. Thillorier seems to have entertained the opinion 
that the greatest degree of cold was created at the time of the 
formation of the solid. In my experiments a constant de- 
crease of temperature was observed; which was accelerated 
by a current of air, or any other means of augmenting evapo- 
ration. At its formation, the carbonic snow depresses the 
thermometer to about — 85°. If it be confined in wool or 
raw cotton, its cooling influence is retarded; if it be exposed 
to the air, especially when in motion, the thermometer de- 
scends much more rapidly; and under the receiver of an air 
pump, the effect is at its maximum. The greatest cold pro- 
duced by the solid carbonic acid in the air was — 109°, under 
