CRYSTALLISATION. 65 



directed, the paper will be reddened by the acid present. Also 

 obtain a little of the yellow salt termed "yellow prussiate of 

 potash " (more accurately ferrocyanide of potassium\ and dissolve 

 it in water : the solution or test-liquor thus obtained serves as a 

 test for iron, for when mixed with a liquid containing iron in 

 solution (such as the solution of perchloride of iron employed as 

 above) double decomposition takes place, and a precipitate is 

 formed, consisting of a beautiful blue substance termed prussian 

 blue ; so that if a little of this test-liquor be added to a portion of 

 your compound solution (of salt, hydrochloric acid, and perchloride 

 of iron) in a wine glass, the liquor will become blue like blue ink, 

 depositing on standing a blue powder consisting of precipitated 

 prussian blue. 



Now boil down the compound solution to a small bulk in an 

 evaporating basin, so as to obtain a considerable proportion of the 

 salt in the form of separated crystals ; let the whole cool, and then 

 filter these crystals as directed in the last experiment. Sprinkle 

 a few drops of water over the crystals, so as to wash out most of 

 the adhering mother-liquor, and then dissolve the crystals in pure 

 distilled water by the aid of heat. On testing the solution thus 

 obtained with the litmus test-paper, it will be found that the 

 paper is either not reddened at all, or only very slightly, in com- 

 parison with the effect produced by the original compound solution ; 

 and on testing it with the ferrocyanide of potassium test-liquor, 

 either no blue colour at all will result or only a comparatively 

 faint one. On the other hand, the mother-liquor that has run 

 through the filter will give a stronger acid reaction with the litmus 

 paper than the original compound solution, and will react more 

 powerfully with the ferrocyanide test-liquor producing prussian 

 blue, for the obvious reason that the boiling down has concen- 

 trated the fluid. It is thus evident that the salt crystals that 

 formed have been nearly, if not completely, separated from the 

 hydrochloric acid and perchloride of iron. By boiling down the 

 solution of these crystals obtained as above, until it deposits crystals 

 for the second time, and collecting them on a second clean paper- 

 filter as before, you will obtain a more complete separation, if the 

 first batch of crystals were not quite pure. 



This process of crystallisation and re-crystallisation (or dissolving 

 the first crystals in fresh water and crystallising afresh) is most 

 extensively employed for the purification of chemical products 

 manufactured for various purposes in the arts, as being at once the 

 cheapest and most effective way of separating impurities. 



Expt. 58. To make Sugar Candy. When a saturated solution 

 made boiling is allowed to cool somewhat the same thing happens 



E 



