chiefly Chemical. 257 



obtained by burning the dried sugar-cane (after the expres- 

 sion of its saccharine juice), under the copper-pans in which 

 the juice is evaporated, in the common process in use in the 

 West Indies of making Muscovado sugar. After the ash 

 had been acted on by an acid, ammonia was added to the so- 

 lution formed. Owing to the presence of a little oxide of 

 copper derived from the pans, the copious precipitate pro- 

 duced, consisting chiefly of phosphate of lime, was coloured 

 bright-blue, as was also the solution after the addition of the 

 volatile alkali. Repeated washings of the precipitate, as in 

 the former example, only very slowly diminished the inten- 

 sity of its colour ; and after such washing, and standing co- 

 vered with water at least a fortnight, the precipitate con- 

 tinued coloured faintly blue, the supernatant incumbent 

 water being colourless. 



Do not such facts as the preceding, of the retention of co- 

 louring matter by certain precipitates, help to explain how 

 particular minerals are coloured, as opalite blue, imbedded 

 in colourless dolomite, and the various colours of quartz and 

 of the oriental aluminous gems, some, at least, of which we 

 know to be formed contemporaneously with the colourless 

 rock above mentioned \ In accordance with this, I may men- 

 tion, that carbonate of lime, when precipitated, as in the in- 

 stance brought forward of phosphate of lime, from a solution 

 containing a little copper, does not attach to it the colouring 

 matter, — a single washing on a filter is sufficient to render 

 it colourless. 



The same facts regarding the retention of one substance 

 adhering to the particles of another, shews the necessity of 

 great caution in deciding that a precipitate is pure, and free 

 from any of the fluid medium in which it has been thrown 

 down. In the instances noticed of the adhesion of one to an- 

 other, the precipitates were not only washed repeatedly in a 

 filter, but w^ere also taken from the filter repeatedly, and 

 agitated in large quantities of water. 



3. So delicate is the sense of touch, that it may be em- 

 ployed advantageously as an aid in chemical research. There 

 are fine clays and sandstones, very compound, and formed of 

 minute particles derived from disintegrated rocks. In the 



