610 PRINCIPLES OF CHEMISTRY 



cannot 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, CaCCX, is sometimes met with in nature in a 

 crystalline form, and it forms an example of the phenomenon termed 

 dimorphism that is, it appears in two crystalline forms. When it 

 exhibits combinations of forms belonging to the hexagonal system (six- 

 sided prisms, rhombohedra, &c.) it is called calc spar. Calc spar has a 

 specific gravity of 2*7, and is further characterised by a distinct cleav- 

 age 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 employed in 

 physical apparatus). The other form of calcium carbonate occurs in 

 crystals belonging to the rhombic system, and it .is then called aragon- 

 ite ; its specific gravity is 3*0. If calcium carbonate be artificially 

 produced by slow crystallisation at the ordinary temperature, it appears 

 in the rhombohedral form, but if the crystallisation be aided by heat it 

 then appears as aragonite. It may therefore be supposed that calc spar 

 presents the form corresponding with a low temperature, and aragonite 

 with a higher temperature during crystallisation. 46 



45 But the question as to the formation of a basic calcium carbonate with a rise of 

 temperature still remains undecided. The presence of water complicates all the relations 

 between lime and carbonic anhydride, all the more as the existence of an attraction 

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

 hydrate, CaC0 3 ,5H 2 (Pelouze), which crystallises in rhombic prisms of sp. gr. about 

 I'll 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 anhydride 

 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 (CO3). 2 , is formed in an aqueous 

 solution, not only because water containing carbonic acid dissolves calcium carbonate, 

 but more especially in view of the researches of Schloesing (1872), which showed that 

 at 16 a litre of water in an atmosphere of carbonic anhydride (pressure 0'984 atmo- 

 sphere) dissolves T086 gram of calcium carbonate and T778 gram of carbonic anhydride, 

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

 of carbonic anhydride in the remaining water. Cai'o showed that a litre of water is able 

 to dissolve as much 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 in 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. 



46 Dimorphous bodies differ from true isomers and polymers in that they do not 

 differ in their chemical reactions, which are determined by a difference in the distribu- 

 tion (motion) of the atoms in the molecules, and therefore dimorphism is usually 

 ascribed to a difference in the distribution of similar molecules, building up a crystal. 

 Although such a hypothesis is quite admissible in the spirit of the atomic and molecular 

 theory, yet, as in such a redistribution of the molecules a perfect conservation of the 

 distribution of the atoms in them cannot be imagined, and in every effort of chemical 



