40 



of Lead exhibiting a purplish lustre, whilst the Tin appears 

 like silver. 



We, in this manner, obtain crystals of the metals Lead 

 and Tin by simply placing pieces of Zinc in contact with 

 their solutions, and it is worthy of remark that an interchange 

 of metals goes on in each flask, so that, if time permitted, 

 all the Lead and all the Tin would be withdrawn from the 

 solutions. Zinc taking their place, (i) finally containing only 

 Zinc Acetate, (2) Zinc Chloride. 



The possibility of thus depositing one metal on another 

 is the basis of the operation of Electro-plating, and the metal 

 is said to be obtained by Electro-deposition. 



(4.) Solution. 



This may be called the general method of obtaining 

 crystals, and the liquid in general use as a solvent is Water. 



A very large percentage of substances which we have in 

 common use will dissolve in water, and if we place salt, 

 sugar, or alum in contact with cold pure water, in suitable 

 vessels, we shall obtain solutions of them. 



Every substance which dissolves in water does so in 

 obedience to a definite fixed law, which may be enunciated 

 as follows : — 



(i) Of one and the same substance a fixed amount is 

 always dissolved by water at the same tempera- 

 ture. 

 (2) For nearly all substances the amount dissolved 

 is greater as the temperature of the water is 

 higher. 



We unconsciously demonstrate the second law by 

 boiling any substance with water, when we wish to dissolve 

 a large quantity of it : this increased solution holds good for 

 most solid substances, but there are some which dissolve to 

 a greater extent in cold than in hot water, good instances 

 being Lime and Strontium Sulphate. 



By the first law, it follows that if we take some crystals 

 of Alum, powder them, and place them in a beaker, and cover 

 with cold water, they will continue to dissolve until we reach 



