92 ■ SALINES. [IV. 



alkaline carbonates. To decide between these, a little blue- 

 vitriol solution, added to tlie water, gives a bluisb precipitate 

 when bicarbonate of lime is present ; in the same case, 

 chloride of calcium gives no precipitate, but gives a milkiness 

 or precipitate if alkaline carbonates are in solution. 



Solution. — Salts being obtained and purified from solution, 

 we may here allude to a general principle in the purification 

 of these and other substances, organic or inorganic, and then 

 to the decomposition of salts, &c., by filtration, especially 

 filtration through charcoal. After a salt or other substance 

 has been obtained from solution by crystallization, it retains 

 a portion of other salts, either crystallized with it, or in the 

 mother liquor, enclosed in cavities in the crystals. Although 

 these impurities may in general be removed by repeated crys- 

 tallization, yet the same end may often be attained more 

 readily, by washing the crystals with a saturated solution of 

 the same substance in a pure state ; for being saturated, it 

 will dissolve no more of that substance, but will dissolve por- 

 tions of others. To efi"ect this, the crystals to be washed 

 should be small, either made so by disturbing the liquor during 

 crystallization or by crushing. Thus salt, nitre, sugar, &c. 

 are obtained pure by washing them respectively with saturated 

 solutions of pure salt, nitre, sugar, &c. 



The other point, of decomposition by charcoal, we have 

 touched upon at the commencement of hydrometallurgy, 

 ■where it was shown that a metallic oxide could be brought to 

 a lower state of oxidation by carbon ; but this is probably at- 

 tended with the formation of carbonic acid, whereas in the 

 instances to be cited, the porosity of the coal separates a 

 substance from solution, and sunders a base more or less per- 

 fectly from its acid. 



Morfit and Highway, in repeating Lebourdai's process for 

 the preparation of alkaloids by means of animal charcoal, 

 found that the bone-black used for decolorizing the solutions, 

 always retained a portion of the precipitated alkaloid. They 

 extended their investigations to the refuse black of chemical 

 factories, and obtained from that which had been used in the 



