FROSTED WINDOWS. 67 



represents a block of alum crystals slowly deposited from a large 

 bulk of fluid. Many other substances may be used instead of 

 alum to produce the same kind of effect; thus the salt called 

 sulphate of copper will coat the objects with beautiful blue crystals, 

 and ferrocyanide of potassium (the salt used in Expt. 57 as a test 

 for iron) with pretty yellow ones. 



Expt. 60. To frost Glass Windows. A peculiar frosted 

 appearance is often given to windows and other glass objects by 

 brushing over the glass a hot solution of some salt that will 

 crystallise easily, such as Epsom salts, or alum ; as the solution 

 cools the crystals form, giving a frosted appearance to the glass ; 

 and for the same reason, glass frosted in winter time is covered 

 over with fine ice crystals (Expts. 18 and 44). If a little size be 

 added to the liquid, the crystals are less easily rubbed off when 

 dry ; whilst a little whiting (tinted to fancy with prussian blue 

 or other pigment) makes the frosting more opaque, preventing 

 people from seeing into the room from outside. 



Expt. 61. To obtain a Colourless Solution of a Yellow Sub- 

 stance and Yellow-coloured Crystals from a White Solution. 

 As a general rule, if a colourless substance is dissolved in a colourless 

 solvent, unless some chemical change takes place (Chapter X.), a 

 colourless solution results ; and conversely, if a substance crystallise 

 from a colourless solution, the crystals are generally also colourless ; 

 whilst if a coloured substance be dissolved in water or other 

 appropriate solvent, the solution is usually of the same colour as 

 the solid substance dissolved. Thus blue copper sulphate, when 

 dissolved in water, forms a blue solution; yellow ferrocyanide of 

 potassium produces a yellow fluid; whilst colourless common salt, 

 alum, and loaf sugar give rise to colourless solutions in each case. 

 There are, however, some exceptions to these general rules ; thus 

 iodide of lead is a yellow solid,* but on boiling this with the right 

 quantity of water it dissolves, forming a clear colourless fluid. On 

 allowing this fluid to cool again, small shining bright yellow crystals 

 of solid iodide of lead form, like minute spangles in the liquid, these 

 redissolve on heating to a colourless fluid, and reappear again on 

 cooling, the changes being capable of alternate repetition as often 

 as you please, provided that a little water be added from time to 

 time to replace that lost by evaporation during the heating. 



* Iodide of lead is easily obtained by putting in a flask some clear filtered 

 solution of nitrate of lead or sugar of lead, and then adding some clear 

 solution of iodide of potassium; a chemical change takes place, one result of 

 which is that iodide of lead is formed as a solid substance, gradually subsiding 

 as a yellow " precipitate " Add some distilled water and boil, adding more 

 water if the whole of the yellow precipitate does not dissolve at first on boil- 

 ing. Compare Expt. 11. 



