THE VALENCY AND SPECIFIC HEAT OF THE METALS 001 



r;u mot take place, and thus all the most important phenomena with re- 

 spect to the behaviour of lime towards carbonic anhydride are explained 

 by starting from one common basis. 45 



Calcium carbonate, CaC0 3 which occurs, as has already been 

 mentioned, in limestone, marble, &c. is sometimes met with in nature 

 in a crystalline form, and it forms an example of the phenomenon 

 termed ft tin lu-jihixm that is, it appears in two crystalline forms. 

 \Vhen it exhibits combinations of forms belonging to the hexagonal 

 system (six-sided prisms, rhombohedra, Arc.) it is called calc spar. 

 Calc spar has a specific gravity of 2'7, and is further characterised by 

 ;i distinct cleavage along the planes of the fundamental rhombohedron 

 having an angle of 105. Perfectly transparent Iceland spar presents 

 a clear example of double refraction (for which reason it is frequently 



not been verified by experiment, nor has the calculated figure of 13 calories for the for- 

 mation of sodium hydride, Na^H, at 300 been verified ; but the figure of 4'1 calories 

 calculated ('20) by Moutier from Troost's data for the dissociation pressure of palladium 

 hydride, entirely agrees with the direct results of Favre, which showed that each gram of 

 hydrogen which combines with palladium at the ordinary temperature develops 4'174 

 calories, and therefore one and the same conception of the mechanical theory of heat 

 ix (tji/ilicdble to dissociation and evaporation. The heat evolved in the absorption of 

 hydrogen depends, naturally, not only on the physical process of the condensation of the 

 pis, but also on the formation of a new chemical compound, as is seen from the fact that 

 different metals in absorbing a gram of hydrogen develop a different amount of heat 

 namely, palladium 4 calories, potassium 10 calories, sodium 13 calories, platinum 20 

 calories and therefore in the concordance of calculation with fact we should see, 

 not an identity, but only a resemblance between the phenomena of dissociation and 

 evaporation. 



45 But still the question as to the formation of a basic calcium carbonate remains 

 undecided. According to certain data one would think that, with a rise of temperature, 

 not only calcium carbonate but also a basic salt may be formed, which, however, is 

 denied by certain investigators. Probably the presence of water complicates all the re- 

 lations between lime and carbonic anhydride, all the more as the existence of an attrac- 

 tion between calcium carbonate and water is seen from its being able to give a crystallo- 

 hydrate, CaCO3,5H 2 O (Pelouze, Salm-Horstmar), which crystallises in rhombic prisms of 

 sp. gr. about 1'77 and loses its water at 20. These crystals are obtained when a solution 

 of lime in sugar and water is left long exposed to the air and slowly attracts carbonic 

 in hydride from it, and also by the evaporation of such a solution at a temperature of 

 about 8. On the other hand, it is probable that an acid salt CaH 2 (CO r ,) 2 is formed in 

 u n aqueous solution, not only because water containing carbonic acid dissolves calcium 

 carbonate, but more especially judging by the researches of Schloesing (1872), which showed 

 that at lf> a litre of water in an atmosphere of earbonic anhydride (pressure 0'984 atmo- 

 sphere) dissolves 1'08(> grams of calcium carbonate and 1*778 grams of carbonic anhydride, 

 which corresponds with the formation of calcium hydrogen carbonate, and the solution 

 of carbonic anhydride in the remaining water. Caro showed that a litre of water is able 

 to dissolve as muoh as 3 grams of calcium carbonate if the pressure be increased to 4 and 

 more atmospheres. The calcium carbonate is precipitated when the carbonic anhydride 

 passes off hi the air or in a current of another gas ; this also takes place in many natural 

 springs. Tufa, stalactites, and other like formations from waters containing calcium 

 carbonate and carbonic acid in solution, are formed in this manner. The solubility of 

 calcium carbonate itself at the ordinary temperature does not exceed 13 milligrams per 

 litre of water. 



