Note 23 — LECTURE VI. THE CORRELATION OF THE PHYSICAL FORCES.
Reading edition
Page 162. Sulpho-indigotic Acid.—A mixture of one part of indigo and fifteen parts of concentrated oil of vitriol. It is bleached on the side at which hydrogen gas is evolved, in consequence of the liberated hydrogen withdrawing oxygen from the indigo, thereby forming a colourless deoxidised indigo. In making the experiment, only enough of the sulpho-indigotic acid must be added to give the water a decided blue colour.