1823.] Mr. Crum on Indigo. 91 



transparent. Water in a wine glass containing th of its 



weight of this substance is distinctly blue coloured. 



The saturated solution is precipitated by spring water, and 

 by every liquid that I have tried, except distilled water. From 

 this it appears that the mere presence of any foreign substance 

 in pure water greatly diminishes its solubility. 



If the solution be diluted with 20 parts of pure water, it is 

 still precipitated by solutions of the salts of potash and soda, 

 lime, barytes, strontian, lead, and mercury. An addition of 

 sulphuric or muriatic acid does not redissolve them. But nei- 

 ther ammonia nor any of its salts precipitate this weak solution. 

 None of the salts of magnesia, zinc, or copper, nor the solutions 

 of alum, sulphate of manganese, permuriate of tin, protosulphate 

 or persulphate of iron, or nitrate of silver, decompose it. It is 

 not precipitated by any of the acids, by infusion of galls, or 

 by pure gelatine. Alcohol and ether do not precipitate the weak 

 aqueous solution, though they do not dissolve any of the dry 

 substance. Ceruleo-sulphate of potash dissolves readily in con- 

 centrated sulphuric acid, but not in concentrated muriatic acid. 



When chloride of tin is mixed with the solution of this sub- 

 stance, its colour is immediately changed to yellow. This 

 yellow product is not very soluble in water ; it becomes blue 

 again on the addition of any substance, as a salt of copper, capa- 

 ble of imparting oxygen to it. 



When heat is applied to the blue substance, it does not melt ; 

 no purple vapour is given off, and in consequence of its being 

 defended by the saline matter, a strong heat long applied is 

 necessary for its being reduced to ashes. 



When luminous objects, as the sun or moon, or the flame of a 

 candle, are viewed through the blue solution of this substance, 

 of the proper degree of intensity, they appear of a fine rich 

 scarlet colour. It is worthy of remark, that a single drop of 

 nil i ate or sulphate of copper, mixed with a quantity of this 

 solution, makes the same objects appear blue through it, although 

 the general appearance of the liquid is not in the least degree 

 altered. Zinc produces the same effect, though not so power- 

 fully. Any acid restores to these mixtures the property of mak- 

 ing luminous objects appear red, unless when a large quantity 

 of copper has been added, which makes the liquid itself green. 



Sir H. Davy* was the first to discover that a vegetable sub- 

 stance had the power of precipitating in combination with 

 certain neutral salts, which are themselves abundantly soluble in 

 water. The carbonates of potash, soda, and ammonia, and the 

 chlorides of tin and of iron, are among the substances which 

 that philosopher found undecomposed in combination with 

 tannin, in the precipitates formed by these salts in an infusion of 



» On Astringent Vegetables, Phil. Trans. 1803. 



