<>N WATKK AND ITS COMPOUNDS 101 



.Many compounds containing water of crystallisation are solid sub- 

 stances (when melted they are already solutions i.e., liquids) ; further- 

 more, they are capable of being formed from solutions, as is ice or 

 aqueous vapour. I propose calling them ///*/'/'/"-// //'//v/A-x. Inasmuch 

 as the direct presence of ice or aqueous vapour cannot be admitted in 

 solutions (for these are liquids), although the presence of water may 

 be, so also there is no basis for acknowledging the presence in solu- 

 tions of substances in an already -existing state of combination with 

 water of crystallisation, although they are obtained from solutions as 

 siu-h.' ::{ It is evident that such substances present one of the many 

 forms of equilibrium between water and a substance dissolved in it. 

 This form, however, reminds one, in all respects, of solutions that is, 

 aqueous compounds which are more or less easily decomposed, with 

 separation of water and the formation of a less aqueous or an anhydrous 

 compound. In fact, there are not a few crystals containing water 

 which lose a part of their water at the ordinary temperature. Of such 

 a kind, for instance, are the crystals of soda, or sodium carbonate, 

 which, when separated from an aqueous solution at the ordinary 

 Temperature, are quite transparent; but when left exposed to air, 

 lose a portion of their water, becoming opaque, and, in the process, 

 lose their crystalline appearance, although preserving their original 

 form. This process of the separation of water at the ordinary tempera- 

 ture is termed the efflorescence of crystals. Efflorescence takes place 

 more rapidly under the receiver of an air pump, and especially at a 

 gentle heat. This breaking up of a crystal is dissociation at the 

 ordinary temperature. Solutions are decomposed in exactly the same 

 manner. 64 The tension of the aqueous vapour, which is given off from 



63 Supersaturated solutions give an excellent proof in this respect. Thus a solution 

 of copper sulphate generally crystallises in penta-hydrated crystals, CuSC>4 + 5H 2 O, and 

 ii-- -uturated solution gives such crystals if it be brought into contact with the minutest 

 possible crystal of the same kind. But, according to the observations of Lecoq de Bois- 

 baudran, if a crystal of ferrous sulphate (an isomorphous salt, see note 55), FeSO 4 + 7H 2 O, 

 be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt, 

 ( 'uSO.j+7H 2 O, are obtained. It is evident that neither the penta- nor the hepta-hydrated 

 salt is contained as such in the solution. The solution presents its own particular liquid 

 form of equilibrium. 



64 Efflorescence, like every evaporation, proceeds from the surface. Inside crystals 

 which have effloresced there is usually found a non-effloresced mass, so that the majority 

 of effloresced crystals of washing soda show, in their fracture, a transparent nucleus 

 coated by an effloresced, opaque, powdery mass. It is a remarkable circumstance in this 

 respect that efflorescence proceeds in a completely regular and uniform manner, so that 

 the angles and planes of similar crystallographic character effloresce simultaneously, 

 and i)i this respect the crystalline form determines those part s of crystals where efflo- 

 rescence starts, and the order in which it continues. In solutions evaporation also 

 proceeds from the surface, and the first crystals which appear on its reaching the 

 required degree of saturation are also formed at the surface. After falling to the 

 bottom the crystals naturally continue to grow (see Chap. X.). 



