THE VALENCY AND SPECIFIC HEAT OF THE METALS 611 



Calcium sulphate in combination with two equivalents of water, 

 CaS0 4 ,2H 2 O, is very widely distributed in nature, and is known as 

 gypsum. Gypsum loses one and a half and 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, CaS0 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 solidifies, then a compound of the calcium sulphate with two 

 molecules of water is produced ; and this may be regarded as derived 

 from S(OH) 6 by the substitution of two atoms of hydrogen by one atom 

 of bivalent calcium. Natural gyps am sometimes appears as perfectly 

 colourless, or variegated, marble-like, masses, and sometimes in perfectly 

 colourless crystals, selenite, of specific gravity 2-33. The semi-trans- 

 parent gypsum, or alabaster, is often carved into small statues. Besides 



reaction there must take place a certain motion among the atoms ; so in my opinion there' 

 is no firm basis for distinguishing dimorphism from the general conception of isomerism, 

 under which the cases of those organic bodies which are dextro and Isevo rotatory (with 

 respect to polarised light) have recently been brought with such brilliant success. When' 

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

 which leads to the supposition that this salt appears in a colloidal state. It only crys- 

 tallises with the progress of time. The colloidal state of calcium carbonate is par- 

 ticularly clear from the following observations made by Prof. Famintzin, 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 meana 

 of a similar slow formation of precipitates (by reacting on sulphates of different metals 

 with sodium silicate or carbonate). 



< 7 According to Le Chatelier (1888), liH 2 O is lost at 120 that is, H 2 0,2CaSp 4 is 

 fprmed, but at 194 all the water is expelled. According to Shenstone and Cundall 

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

 H 2 O,2CaSO 4 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. Re burnt gypsum cannot be used over again, as 

 that which has once solidified is, like the natural anhydride, not able to recombine 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, which is more 

 particularly evident in inetals in their different forms (powder, crystalline, rolled, <fcc.) 



