THE VALENCY AND SPECIFIC HEAT OF THE METALS 609 



temperature. Just as every volatile substance has a maximum possible 

 vapour tension for every temperature, so also calcium carbonate has 

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

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

 boiling point of Zn) 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 

 kept below the dissociation tension, which may be done 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 carbon- 

 ate 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 

 At the ordinary temperature the reaction of the first order (combi- 

 nation) cannot proceed because the second (decomposition, dissociation) 



43 Experience has 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 pressure 

 is decreased by these means. 



41 Before the introduction of Deville's theory of dissociation, the modus operandi of 

 decompositions like that under consideration was understood in the sense that decompo- 

 sition starts at a certain temperature, and that it is accelerated by a rise of temperature, 

 but it was not considered possible that combination could proceed at the same temperature 

 as that at which decomposition goes on. Berthollet and Deville introduced the conception. 

 of equilibrium into chemical science, and elucidated the question of reversible reactions. 

 Naturally the subject is still far from being clear the questions of the rate and complete- 

 ness of reaction, of contact, &c., still intrude themselves but an important step has 

 been made in chemical mechanics, and we have started on a new path which promises 

 further progress, towards which much has been done not only by Deville himself, but more 

 especially by the French chemists Debray, Troost, Lemoine, Hautefeuille, Le Chatelier, 

 and others. Among other things those investigators have shown the close resemblance 

 between the phenomena of evaporation and dissociation, and pointed out that the amount 

 of heat absorbed by a dissociating substance may be calculated according to the law of 

 the variation of dissociation-pressure, in exactly the same manner as it is possible to 

 calculate the latent heat of the evaporation of water, knowing the variation of the tension 

 with the temperature, on the basis of the second law of the mechanical theory of heat. 

 Details of this subject must be looked for in special works on physical chemistry. Ont 

 and the same conception of the mechanical theory of heat is applicable to dissociation 

 and evaporation. 



