228 Intelligence and Miscellaneous Articles. 



ON THE COMBINATION OF CARBONIC ACID AND WATER. 

 BY S. WROBLEWSKI. 



If carbonic acid be sufficiently compressed in contact with water, 

 in a space kept at zero, the portion of the acid not absorbed liquefies, 

 and two distinctly separate liquids are obtained : — water, more or 

 less saturated, below ; and liquid carbonic acid, lighter, above. 

 The pressure being slowly lessened, the carbonic acid volatilizes, 

 and the whole returns to the orighial state. But if we compress 

 the carbonic acid almost to liquefaction and then allow the gas to 

 expand a little, so as to produce a trace of solid substance either in 

 the water or on the sides of the tube, the following phenomenon is 

 observed : — Every time that the pressure while being augmented 

 passes through a determined point (which I will call the critical 

 pressure), the tube becomes covered with an opaque rime ; and 

 every time the same point is passed through in diminishing the 

 pressure, the rime disappears. The critical point, at zero tempe- 

 rature, is at the pressure of 12-3 atmospheres. The phenomenon 

 can be reproduced at pleasure, if the pressure be not too much 

 diminished, and consequently the ice does not completely disappear. 

 The latter can be preserved as long as we please, if the pressure 

 remains greater than that above indicated. 



The phenomenon is likewise produced at higher temperatures 

 than zero. The value of the critical pressure rises with the tem- 

 perature. At * 



atm. atm. atm. 



0-48 it is 12-7 3-6 .... 17-9 6-1 .... 23-3 



2-7 .... lfi-7 5-3 ... . 21-8 6-8 .... 26-1 



~So such result is obtainable by experimenting with water and 

 air. At zero, with the strongest expansion possible with my appa- 

 ratus, a little ice is seen to form in the w T ater, which soon melts 

 without producing any rime. 



The explanation of the above-described phenomenon appears to 

 me necessarily to involve the existence of a hydrate of carbonic 

 acid, readily dissociable, and capable of being formed by pressure, 

 like M. Ogier's chlorhydrate of phosphuretted hydrogen. The 

 critical pressure would be the dissociation-tension of the formed 

 hydrate. The proportion of water and acid seeming to have no 

 influence on the value of the critical tension, it is probable, from 

 M. Debray's researches, that there is only one hydrate of this acid, 

 containing equal volumes of carbonic acid gas and aqueous vapour. 

 In order that it may readily crystallize, a portion of substance 

 already crystallized must be present ; the expansion of the gas, 

 which determines a local lowering of temperature, serves to pro- 

 duce this effect. — Comptes Rendus, Jan. 30, 1882, p. 212. 



* The numbers are the means of the pressures at which the rime begins 

 to disappear and to form. 



