Chemical Test Papers.

Manufacturer and builder 10, 1881

Among the most useful, though at the came time the most simple, auxiliaries in the lands of the analytical chemist, are "test papers." These are small strips of bibulous paper which have been saturated with certain vegetable tinctures, and indicate, by an immediate change of color, the presence of an acid or an alkali in a given solution. They are made up in the form of little books, and strip can be readily torn off when required. Blue litmus paper has been steeped in a tincture made from a kind of lichen (Rocella tinctoria) growing abundantly in the Canary and Cape Verd Islands. This paper turns red when dipped into a liquid having an acid reaction. Red litmus paper, which had been reddened by an acid, is used as a feet for alkalies, which restores the blue color. An amusing experiment may be performed by placing a drop of some acid at the bottom of a tall vessel, and pouring in a solution of blue litmus. The blue color is immediately changed to red. The experiment may be reversed by pouring the reddened liquid into another vessel containing a drop of ammonia. The blue color will reappear. These changes appear extraordinary to the uninitiated. Yellow turmeric paper has been steeped in a tincture made from the roots of the Curcuma longa, a plant growing in all parts of Bengal. Its color is changed by alkalies to brown. It must not bo forgotten that this property is possessed by some substances that are not, strictly speaking, alkalies. For instance, carbonate of soda is a salt, but it changes red litmus to blue; we can, therefore, only say with correctness that it has an alkaline reaction. There are other test papers, such as ozone papers, for detecting ozone in the atmosphere. These are impregnated with starch paste and iodide of potassium, and turn blue when acted upon by minute quantities of ozone. Then we have lead-papers, which turn black on exposure to sulphuretted hydrogen.