602 PRINCIPLES OF CHEMISTRY 



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

 occurs in crystals belonging to the rhombic system, and it is then 

 called aragonite ; its specific gravity is 3*0. If calcium carbonate is 

 artificially produced by slow crystallisation at the ordinary temperature, 

 then it appears in the rhombohedral form, but if the crystallisation be 

 aided by heat then it appears as aragonite. One may suppose, there- 

 fore, that calc spar presents the form corresponding with a low temper- 

 ature, and aragonite with a higher temperature during crystallisation. 46 

 Calcium sulphate in combination with two equivalents of water, 

 CaSO 4 ,2H 2 0, is very widely distributed in nature, and is known as 

 yypsum. Gypsum loses its two equivalents of water at a moderate 

 temperature, 47 and anhydrous or burnt gypsum is then obtained, which 

 is also known as plaster of Paris, and is employed in large quantities 

 for modelling. 48 This use depends on the fact that burnt and finely- 

 divided and sifted gypsum forms a paste when mixed with water ;. 

 after a certain time this paste becomes slightly heated and solidifies, 

 owing to the fact that the anhydrous calcium sulphate, CaSO 4 , again 

 combines with water. When the plaster of Paris and water are first 

 made into a paste they form a mechanical mixture, but when the mass 



46 When calcium carbonate separates out from solutions, it at first has a gelatinous 

 appearance, which leads one to think that this salt appears in a colloidal state. It only 

 crystallises with the progress of time. The colloidal state of calcium carbonate is parti- 

 cularly clear from the following observations made by Prof. Fammtsin, who showed that 

 when it separates from solutions it is obtained under certain conditions in the form of 

 grains having the peculiar paste-like structure proper to starch, which fact has not only 

 an independent interest, but presents an example of a mineral substance being obtained 

 in a form until then only known in the organic substances elaborated in plants. This 

 shows that the forms (cells, vessels, &c.) in which vegetable and animal substances- 

 occur in organisms do not present in themselves anything peculiar to organisms, 

 but are only the result of those particular conditions in which these substances are 

 formed. Traube and afterwards Monnier and Vogt (1882) obtained formations which, 

 under the microscope, were in every respect identical in appearance with vegetable cells, 

 by means of a similar slow formation of precipitates (by reacting on sulphates of 

 different metals with sodium silicate or carbonate). Owing to its insolubility in water,, 

 calcium carbonate may be easily obtained from any other soluble calcium salt by the 

 addition of a solution of an alkali carbonate ; for example, ammonium carbonate. 



"7 According to Le Chatelier (1888), lH 2 O is lost at 120 that is, H 2 O,2CaSO 4 

 is formed, but at 194 all the water is expelled. According to Shenstone and Cundali 

 (1888) gypsum begins to lose water at 70 in dry air. The semi-hydrated compound,, 

 HoO,2CaSO4, is also formed when gypsum is heated with water in a closed vessel at 150 

 (Hoppe-Seyler). 



48 For stucco-work it is usual to add lime and sand, as the mass is then harder and 

 does not solidify so quickly. For imitating marble, glue is added to the plaster, and the 

 mass is polished when thoroughly dry. Keburnt gypsum cannot be used over again, as 

 that which has once solidified is, like the natural anhydride, not able to re-combine with 

 water. It is evident that the structure of the molecules in the crystallised mass, or in 

 general in any dense mass, exerts an influence on the chemical action, as is particularly 

 clearly seen in metals in their different forms (powder, crystalline, rolled, &c.). 



