TIIK 7ALENCY AND SPECIFIC HEAT OF THE METALS 599 



the formation of a mixture of calcium oxide and carbonate (Rose). 

 But if the lime be slaked or taken in solution, then the absorption of 

 carbonic anhydride proceeds rapidly and to the end. These phenomena 

 are connected with the dissociation of calcium carbonate, studied by 

 Debray (1867) under the influence of the conceptions of dissociation 

 introduced into science by Henri Sainte-Claire Deville. Just as there is 

 no vapour tension for 11011- volatile substances, so there is no dissociation 

 tension of carbonic anhydride for calcium carbonate at the ordinary 

 temperature. Just as every volatile substance has a maximum possible 

 \,ipour tension for every temperature, so also calcium carbonate has 

 its corresponding dissociation tension ; thus at 770 (the boiling point 

 of cadmium) about 85 mm. (of the mercury column), and at 930 (the 

 boiling point of Zii) it is about 520 mm. As, if the tension be greater, 

 there will be no evaporation, so also there will be no decomposition. 

 Debray took crystals of calc spar, and could not observe the least change 

 in them at the boiling point of zinc (930) in an atmosphere of carbonic 

 anhydride taken at the atmospheric pressure (760 mm.), whilst, on the 

 other hand, calcium carbonate may be completely decomposed at a 

 much lower temperature if the tension of the carbonic anhydride be 

 less than the dissociation tension, which may be arrived at either by 

 directly pumping away the gas with an air-pump, or by mixing it with 

 some other gas that is, by diminishing the partial pressure of the 

 carbonic anhydride, 43 just as an object may be dried at the ordinary 

 temperature by removing the aqueous vapour or by carrying it off in 

 a stream of another gas. Thus it is possible to obtain calcium carbo- 

 nate from lime and carbonic anhydride at a certain temperature above 

 that at which dissociation begins, and conversely to decompose calcium 

 carbonate at the same temperature into lime and carbonic anhydride. 44 



following experiment. Lime, or barium oxide, is placed in a flask or retort having an 

 upper orifice and connected with a tube immersed in mercury. A funnel furnished with 

 a stopcock and filled with water is fixed into the upper orifice of the retort, which is then 

 filled with dry carbonic anhydride. There is no absorption. When the temperature of 

 equilibrium is arrived at, the unslaked oxide is made to absorb all the carbonic anhydride 

 by carefully letting in water. A vacuum is formed, as is seen by the mercury rising 

 up the neck of the retort. With water the absorption goes on to the end, whilst under 

 the action of heat there remains the dissociating tension of the carbonic anhydride. 

 Furthermore, we here see that, with a certain resemblance, there is also a distinction, 

 depending on the fact that at low temperatures calcium carbonate does not dissociate ; 

 this determines the complete absorption of the carbonic anhydride in the aqueous 

 solution. 



43 Experience has long shown that by moistening partially-burnt lime with water 

 and re-heating it, it is easy to drive off the last traces of carbonic anhydride from it, and 

 that, in general, by blowing air or steam through the lime, and even by using moist fuel, 

 it is possible to accelerate the decomposition of the calcium carbonate. The partial pres- 

 sure is decreased by these means. 



44 Before Deville's conception of dissociation, the modus operandi of decompo- 



