SOLUTION OF METALLIC OXIDES. 117 



less or differently coloured solution ; and xsometimes effervescence 

 (or bubbling due to formation of gas which escapes) is brought 

 about. 



Expt. 111. To dissolve Magnesia by Chemical Action. As 

 an illustration of the first of these kinds of complex solution, 

 where the chemical change is invisible, procure a little calcined 

 magnesia, and pour over it strong white vinegar, little by little, 

 stirring up well after each addition ; by and by the magnesia will 

 entirely dissolve, and in place of the white powder and the sour 

 fluid used you will have a tolerably clear liquid of disagreeable taste, 

 but not particularly sour (unless you have used too large a quantity 

 of vinegar). If you put the liquid into an evaporating basin and 

 boil it down, you will be unable to obtain from it the same calcined 

 magnesia that you originally employed, either by crystallisation or 

 cooling and standing, because a chemical change has taken place ; 

 the acid of the vinegar (acetic acid) and the magnesia have so 

 acted on one another that both are altered into something quite 

 different from either, viz., a compound called " acetate of magnesia, " 

 differing from calcined magnesia in being readily soluble in water, 

 and from vinegar in not being sour. 



Expt. 112. Another way of dissolving Magnesia. Instead of 

 acetic acid, use diluted sulphuric acid in quantity not quite sufficient 

 to dissolve the whole of the magnesia; filter the solution (Expt. 56), 

 and evaporate the clear liquid in an evaporating basin over a 

 lamp when it has become sufficiently concentrated, crystals will 

 form on allowing the fluid to cool. These crystals are sulphate of 

 magnesia (or magnesium sulphate, otherwise known as Epsom salts), 

 and are obviously different from the original magnesia, as they 

 will readily dissolve in pure water, giving a solution of somewhat 

 disagreeable saline taste, whereas the original calcined magnesia 

 would not dissolve at all in water, and had little or no taste of any 

 kind. Epsom salts derive their name from being contained in the 

 water of a spring at Epsom in such quantity that, when the water 

 was evaporated, this salt crystallised out. Many other mineral 

 waters contain magnesium sulphate, and owe a good deal of their 

 medicinal efficacy to this circumstance. 



Expt. 113. To dissolve Coloured Oxide of Lead or Oxide of 

 Mercury by Chemical Action, producing a Colourless Fluid. 

 In the last experiment a white powder was dissolved in a colourless 

 fluid (if white vinegar was used), yielding as result a colourless 

 solution. Litharge, or oxide of lead, when powdered is a brownish- 

 yellow substance ; if this is treated with dilated nitric acid* like 



* A few drops of strong nitric acid to a wine-glassful of water will 

 dissolve as much litharge as will lie on a sixpence. 



