A properly prepared dye-wood extract should dissolve in water without residue, and the solution when largely diluted should show the characteristic colour of the wood with no brownish shade, and when the colouring matter is precipitated from such a solution by a metallic salt the residual solution should be almost colourless. If the extract dissolves incompletely in water and the solution is brown after precipitation of the colouring matter, the extract has been burnt in the evaporation. * * * * * Logwood and logwood extract contain two substances of importance in dyeing and colour making. These are hæmatoxylin and hæmateïn. Hæmatoxylin is found in logwood in greatest amount shortly before the wood is cut. When pure it forms colourless crystals of a peculiar sweet taste, which are soluble in cold water with difficulty, more easily in hot, and readily soluble in alcohol or ether. The composition of hæmatoxylin is expressed by the formula C₁₆H₁₄O₆. Hæmatoxylin is not a colouring matter. It is important because from it is obtained the essential colouring matter of logwood—hæmateïn. When a trace of ammonia is added to the colourless solution of hæmatoxylin, the liquid at once becomes dark red owing to the formation of hæmateïn. When a larger quantity of ammonia is added the liquid acquires a deep red colour, and then contains only hæmateïn (its ammonia compound), which is formed according to the following equation:— C₁₆H₁₄O₆ + NH₃ + O = C₁₆H₉O₅.NH₄ + 2H₂O.