the substance di>solved, and the sohent or compounds similar to them, 

 only in a liquid partly decomposed form. Kven the <<>?<>/// nf mlnt'tun* 

 may often conlirm this opinion. Copper sulphate forms crystals having 

 a blue colour and containing- water of crystallisation. If the water of 

 crystallisation be removed by heating the crystals to redness, a colour- 

 less anhydrous substance is obtained (a white powder). 1'Yom this it 

 may be seen that the blue colour belongs to the compound of the copper 

 salt with water. Solutions of copper sulphate are all blue, and con- 

 sequently "hey contain a compound similar to the compound formed by 

 the salt with its water of crystallisation. Crystals of cobalt chloride 

 when dissolved in an anhydrous liquid like alcohol, for instance <_nve 

 a blue solution, but when they are dissolved in water a red solution is 

 obtained, ('rystals from the aqueous solution, according to Professor 

 Potilit/in. contain six times as much "water (CoCl.,,f>H.,< )) for a ijiven 

 \\ eiu'lit of the salt, as t hose violet crystals (CoCb. II .,< > ) which are formed 

 by the evaporation of an alcoholic solution. 



That solutions contain particular compounds with water is further 

 shown bv the phenomena of supersaturated solutions, of so-called crvo- 

 hvdrates. of solutions of certain acids having constant boiling ]>oints. 

 and the properties of compounds containing water of crystallisation 

 whose data it i- indispensable to keep in view in tne consideration of 



-i i] Ut li i! IS. 



The phenomenon of supersaturated solutions consists in the follow- 

 ing : < Mi the refrigeration of a saturated solution of certain salts,-' 1 

 if the liuuid be brought tinder certain conditions, the excess of the solid 

 ina\' -omctiiiies remain in solution and not separate out. A "Teat 

 number of substances, and especially sodium sulphate, Na._,S(),, or 

 ( Haulier's sab. ea-ilyform supersaturated solution-. If boiling water 

 be saturated \\ith tin- salt, and the solution be poured ot]' from any 

 reniainiiiLf undis-olved salt, and. the boiling beinif still continued, the 

 Vessel holding the solution be \\-ell closed by cot toll wool, or by fusing up 

 the vcs>cl. or by covering t he sol ut ion with a layer of oil. i hen it will he 

 found that this saturated solution does not separate out anv (dauber's 

 ^alt whatever on cooling do\\ ii to the ordinary or even to a much 

 lower temperature : a It lioii^h \\iihout the abo\e precautions a salt 

 M -pa rate- out on eoolinir. in the form of crystals \\ hidi contain water of 



'I , ., , ,!< v.hirli c}..irate ..Hi witli \siiti-r !' cry-tiilli-iitii.il t'l.nn sii|M-rsiitunitf<l 



n-r.,11- p- -i-.ir.-li. - hasc |.r(.\ci| that -ii| n-r-at urat.'il snlut ii.ns !M imt 

 i ,,| , i- ,- -.-in ial |in.|.i-rt , -. 'i lie \ ariation M!' 



, M i',,cin.il "ii "i i, ... A ,-.. take |iliici. an Mi-.lin- ti. tlic i.rilimiry 



