As all forms of indigo extract are regular articles of commerce, details for their preparation will not be given here. It will suffice to say that indigo is heated with strong sulphuric acid until test samples show that the indigo has been completely dissolved, and it is then diluted with water and filtered. Sometimes it is sold in this condition under the term "chemic," but if this be used in dyeing wool it gives rather unsatisfactory results. When "sour extract" is required, the liquor filtered out is next treated with salt until all the colour has been precipitated out, when it is filtered off, drained, pressed and sold. Should "neutral" or "sweet" extract be required, then the acid liquor is neutralised with soda, and the product is salted out as before, drained and pressed to a suitable consistence. It is then sold as "indigo extract," or dried, at 150 deg. F., to a powder, which is known as "indigo carmine". All forms of indigo extract are dyed on wool from baths of (p. 151) Glauber's salt and sulphuric acid, and therefore they can be classed with the acid-dyeing coal-tar colours. Indigo extract is notable for its level dyeing and penetrative properties, but it is not fast to light or milling. Messrs. Read Holliday & Sons have a powder form of indigo extract which will be found very useful and to give better shades than the usual run of paste extract, while it only takes about one-fifth the quantity to give a similar shade. Working at the boil should be avoided with indigo extract, as tending to make the shades greenish in tone; from 170 deg. to 180 deg. F. will usually be found hot enough to dye good shades. Indigo extract is not much used by itself in dyeing blues on wool, but it is extensively employed along with other dye-stuffs to produce an immense variety of shades--drabs, greens, fawns, greys, lilacs, etc., of which some examples will be given later on. _Indigo Blue_.--Prepare a bath with 10 lb. indigo extract, 5 lb. sulphuric acid, and 10 lb. Glauber's salt. Work just under the boil to shade. _Sky Blue_.--The dye-bath contains 1 lb. indigo extract, 2 lb. sulphuric acid, and 10 lb. Glauber's salt. Work at about 160 deg. F. to shade.