* Thus with Hermetic art. 1. 487. The sympathetic inks made by Zaffre dissolved in the marine and nitrous acids have this curious property, that being brought to the fire one of them becomes green, and the other red; but what is more wonderful, they again lose these colours, (unless the heat has been too great,) on their being again with∣drawn from the fire. Fire-screens have been thus painted, which in the cold have shewn only the trunk and branches of a dead tree, and sandy hills, which on their approach to the fire have put forth green leaves and red flowers, and grass upon the mountains. The process of making these inks is very easy, take Zaffre, as sold by the druggists, and digest it in aqua regia, and the calx of Cobalt will be dissolved; which solution must be diluted with a little common water to prevent it from making too strong an impression on the paper; the colour when the paper is heated becomes of a fine green-blue. If Zaffre or Regulus of Cobalt be dissolved in the same manner in spirit of nitre, or aqua fortis, a reddish colour is produced on exposing the paper to heat. Chemical Dictionary by Mr. Keir, Art. Ink Sympathetic.
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