The usual invisible or sympathetic inks are made from cobalt nitrate or chloride, which in the hydrated condition (that is, containing water) are a pale pink, but become deep green by loss of water on heating. Writing upon paper with these inks is invisible at the ordinary temperature, but by warming the paper the marks appear very distinct, but fade away again after a short time. In hot climates the writing would not be invisible. Invisible writing may be done with a solution of tannic acid, and developed at any time by soaking in a dilute solution of ferric chloride. A true ink is then formed. Another method is to write with a solution of boiled starch, and develop the writing by damping the paper and holding it for a few minutes over a bottle containing iodine; the blue iodide of starch is then formed, and the writing becomes quite distinct for a time. It fades away again as the paper dries, but may be developed in the same manner several times. Another process is to write with a solution of lead acetate, and develop by moistening the paper and holding it over a bottle containing sulphuretted hydrogen; the writing then becomes permanently black, sulphide of lead being formed.