Page:Experimental researches in chemistry and.djvu/43

28 Turmeric paper dipped into a solution of pure boracic acid very speedily receives a slight tint of brownish-red, which, when the paper is dry, is very marked, and resembles that produced by a weak alkali. In this state the properties of the colouring matter are entirely different to what they were before: sulphuric, nitric, muriatic, and phosphoric acids, even when very dilute, produce a bright red colour on this paper, and a strong solution of oxalic acid also reddens it. Alkalies, on the contrary, make it blue, gradually passing to shades of purplish blue, yellowish-red, &c. As long as the acids or alkalies remain on the paper, if not so strong as to destroy the colouring matter, the new colour remains, but a slight washing removes them, and then the boracic acid tint returns, and the paper has its Brst peculiar properties. When altered by muriatic acid or ammonia, the mere volatilization restores the paper to its first state; with the ammonia the restoration is very ready and perfect; with the acid it is longer and not so complete. If the paper reddened by boracic acid be heated, the yellow of the turmeric is almost restored, and then it takes from acids a weaker red tinge, and from alkalies a more purplish colour than before.

Turmeric, thus altered by boracic acid, is readily restored to its original state by washing; altered turmeric paper when put in water for two or three hours resumes its original properties, and acts, as at first, in testing the alkalies.

When the altered paper is placed in sunlight a few days, the colour is soon destroyed as with turmeric alone, and then neither acid nor alkali will affect it.

When turmeric paper is dipped into neutral or slightly alkaline borate of ammonia, it soon becomes of the red tint produced by boracic acid, and is in every respect as if altered by boracic acid alone; when this paper is made blue by ammonia, the ammonia easily washes out and the blue tint disappears, and afterwards the boracic acid or borates will wash out and leave the paper as at first.

Borax itself at first reddens turmeric paper because of the excess of alkali, but as the colouring matter becomes altered by the presence of the boracic acid, the tint becomes of a dirty bluish colour, and then the paper is changed by acids or alkalies,just as if it had been altered by boracic acid.

Hence it is probable that the neutral borates have the same