496 M. Deville on the Formation.of Staurotide and Zircon. 
acid is passed through a U-tube dipping in the’ammonia, a small 
portion of it liquefies. 
By increasing the pressure to some extent, considerable quan- 
tities of carbonic acid may be readily solidified. About 150 
cubic centims. of liquid ammonia are introduced into an inverted 
bell-jar provided with a collar, on which a plate perforated with 
two apertures is hermetically fitted. In the central aperture there 
is a tube closed at one end and reaching to the bottom of the 
jar; the other aperture serves to connect the apparatus with the 
air-pump. ‘The carbonic acid is produced -by heating dried bi- 
carbonate of soda to reduess in a copper flask. This flask is con- 
nected with the tube dipping in the liquid ammonia, and also 
with a small air-manometer. All the air having been expelled 
from the apparatus, and the temperature of the liquid ammonia 
reduced to near solidification, the flask is heated until the mano- 
meter indicates a pressure of 3 to 4 atmospheres. Crystals of 
carbonic acid soon begin to form on the inside of the tube, 
and in half an hour about 25 grammes of solid carbonic acid 
are obtained, forming a thick layer on the inside of the tube 
which dips in the liquid ammonia. 
This solid carbonic acid is a colourless mass, as transparent as 
glass ; it may be detached from the tube by touching it with a 
glass rod, and is seen to consist of small cubical crystals. Iix- 
posed to the air, these crystals slowly evaporate without leaving 
any residue; they may be placed on the hand without producing 
any sensation either of heat or of cold: they can be scarcely 
seized between the fingers. Mixed with ether and exposed to 
the air, they form a freezing mixture, the temperature of which 
is —81° C. 
The temperatures were observed by MM. Loir and Drion, by 
means of an alcohol thermometer on which two fixed points 
had been marked; that is, 0° the temperature of melting ice, 
and —40° the temperature of melting mercury. The liquid 
ammonia was prepared by Bussy’s method*, of passing gaseous 
ammonia into a flask surrounded by liquid sulphurous acid, the 
evaporation of which was promoted by the air-pump. In this 
way 6 to 7 fluid ounces may be obtained without difficulty in 
the course of two hours. 
The following experiments by Deville} throw considerable 
light on the formation of some native minerals. 
When fluoride of silicon was passed over calcined alumina 
heated to whiteness in a porcelain tube, fluoride of aluminium 
was disengaged, and staurotide formed analogous in all its pro- 
* Phil. Mag. vol. xx. p. 202. 
+ Comptes Rendus, April 22, 1861. 
