When indigo is treated with eight times its weight of strong sulphuric acid, and the solution considerably diluted, indigotin monosulphonic acid separates as a fine blue precipitate, which dissolves with difficulty in water and alcohol, and is insoluble in dilute acids. In order to obtain the disulphonic acid 1 part of indigo is digested with 15 partsof sulphuric acid at 50° to 60° C. for several days. If fuming sulphuric acid be used the solution is effected in a shorter time. The solution is diluted with a large quantity of water, when a little monosulphonic acid separates; the clear solution is drawn off and a piece of wool immersed in it. The sulphonic acid is soon fixed by the animal fibre; the wool is washed with water and then treated with a solution of ammonium carbonate, when a deep blue solution of ammonium indigotin disulphonate is produced. This solution is evaporated at a low temperature and treated with strong alcohol, which leaves a residue of the pure disulphonate, which is dissolved in water, the solution precipitated with lead acetate and the lead indigotin disulphonate treated with sulphuretted hydrogen. The colourless solution becomes blue on evaporation; finally pure indigotin disulphonic acid is left as an amorphous deliquescent mass. The solution became colourless owing to the reducing action of the sulphuretted hydrogen; when this was driven off by heating the blue colour reappeared. CHAPTER LXI. INDIGO CARMINE. Indigo carmine is used in the laundry, for blue inks and as an artists’ pigment; in chemical composition it is a mixture of the sodium salts of the two indigotin sulphonic acids described in the last chapter. In working on the large scale it is particularly important that the indigo should be finely powdered and completely free from moisture. It is a laborious process to powder indigo in mortars, and loss always occurs through the formation of dust. Thus when large quantities of indigo are to be powdered it is advisable to use rotating cylinders. These are strong casks bound with iron hoops and provided with a well-fitting slide, through which the indigo and a number of iron balls are introduced. When these casks are rotated for a sufficient length of time the indigo is converted into the finest powder without any loss.