BODIES GENERALLY EXISTING AS GASES. 



165 



Carbonic acid. — The solidification of carbonic acid by M. Thilorier is one of the 

 most beautiful expenraental results of modern times. He obtained the substance, as 

 is well known, in the form of a concrete white mass like fine snow, aggregated. 

 When it is melted and resolidified by a bath of low temperature, it then appears as a 

 clear, transparent, crystalline, colourless body, like ice ; so clear, indeed, that at times 

 it was doubtful to the eye whether anything was in the tube, yet at the same time 

 the part was filled with solid carbonic acid. It melts at the temperature of —70° or 

 —72° Fahr., and the solid carbonic acid is heavier than the fluid bathing it. The 

 solid or liquid carbonic acid at this temperature has a pressure of 5*33 atmospheres 

 nearly. Hence it is easy to understand the readiness with which liquid carbonic 

 acid, when allowed to escape into the air, exerting only a pressure of one atmosphere, 

 freezes a part of itself by the evaporation of another part. 



Thilorier gives —100° Cor — 148° Fahr. as the temperature at which carbonic acid 

 becomes solid. This however is rather the temperature to which solid carbonic acid 

 can sink by further evaporation in the air, and is a temperature belonging to a press- 

 ure, not only lower than that of 5-33 atmospheres, but even much below that of one 

 atmosphere. This cooling eflfect to temperatures below the boiling-point often ap- 

 pears. A bath of carbonic acid and ether exposed to the air will cool a tube con- 

 taining condensed solid carbonic acid, until the pressure within the tube is less than 

 one atmosphere ; yet, if the same bath be covered up so as to have the pressure of 

 one atmosphere of carbonic acid vapour over it, then the temperature is such as to 

 produce a pressure of 2*5 atmospheres by the vapour of the solid carbonic acid within 

 the tube. 



The estimates of the pressure of carbonic acid vapour are sadly at variance ; thus, 

 Thilorier* says it has a pressure of 26 atmospheres at —4° Fahr., whilst Addams-}- 

 says that for that pressure it requires a temperature of 30°. Addams gives the press- 

 ure about 27^ atmospheres at 32°, but Thilorier and myself :{: give it as 36 atmo- 

 spheres at the same temperature. At 50° Brunel§ estimates the pressure as 60 



* Annales de Chimie, 1835, Ix. 427, 432. 

 J Philosophical Transactions, 1823, p. 193. 



t Report of British Association, 1838, p. 70. 

 § Royal Institution Journal, xxi. 132. 



