<>.\ VTATKK AND ITS COMPOUNDS 93 



temperature at which the water can no longer hold the former quantity 

 of the substance in solution. If this separation, by cooling a saturated 

 solution or by evaporation, take place slowly, cryxtal* of the substance 

 dissolved an- in many cases formed ; and this is the method by which 

 crystals of soluble salts are usually obtained. Certain solids very 

 easily separate out from their solutions in perfectly-formed crystals, 

 which may attain very large dimensions. Such are nickel sulphate, 

 alum, sodium carbonate, chrome-alum, copper sulphate, potassium ferri- 

 cvanidc, and a whole series of other salts. The most remarkable circum- 

 stance in this is that many solids in separating out from an aqueous 

 solution retain a portion of water, forming crystallised solid substances 

 which contain water. A portion of the water previously in the solution 

 remains in the separated crystals. The water which is thus retained 

 is called the water of crystallisation. Alum, copper sulphate, Glauber's 

 salt, and magnesium sulphate contain such water, but neither sal- 

 ammoniac, nor table salt, nor nitre, nor potassium chlorate, nor silver 

 nitrate, nor sugar, contains any water of crystallisation. One and the 

 same substance may separate out from a solution with or without water 

 of crystallisation, according to the temperature at which the crystals are 

 formed. Thus common salt in crystallising from its solution in water 

 at the ordinary or a higher temperature does not contain water of 

 crystallisation. But if its separation from the solution takes place at 

 a low temperature, namely below 5, then the crystals contain 38 

 parts of water in 100 parts. Crystals of the same substance which 

 separate out at different temperatures may contain different amounts 

 of water of crystallisation. This proves to us that a solid dissolved in 

 water may form various compounds with it, differing in their properties 

 and composition, and capable of appearing in a solid separate form like 

 many ordinary definite compounds. This is indicated by the numerous 

 properties and phenomena connected with solutions, and gives reason 

 for thinking that there exist in solutions themselves such compounds of 



note 24), so these substances do not separate from their saturated solutions on cooling 

 but on heating. Thus a solution of manganese sulphate, saturated at 70, becomes cloudy 

 on further heating. The point at which a substance separates from its solution with a 

 change of temperature gives an easy means of determining the co-efficient of solubility, 

 and this was taken advantage of by Prof . Alexeeff for determining the solubility of many 

 substances. The phenomenon and method of observation is here essentially the same 

 as in the determination of the temperature of formation of ice. If a solution of a sub- 

 stain < which separates out on heating be taken (for example, the sulphate of calcium 

 or manj_ r ane>ei. then at a certain fall of temperature ice will separate out from it, and at 

 a certain rise of temperature the salt will separate out. From this example, and from 

 general considerations, it is clear that the separation of a substance dissolved from a 

 solution should present a certain analogy to the separation of ice from a solution. In 

 both cases, a heterogeneous system of a solid and a liquid is formed from a homogeneous 

 (liquid) system. 



