Page:Experimental researches in chemistry and.djvu/44

1822.] power as the boracic acid, of altering the colouring matter of turmeric, for it is not probable there should be an actual separation of the elements of the salts by it, especially as they both wash out from it and leave it unaltered.

Hence also both acid and alkaline borates redden turmeric.

M. Desfosses proposes this effect of boracic acid as a test for its presence; for a very small quantity of it mixed with other acid has the power of reddening turmeric paper in consequence of these changes.

On the Changing of Vegetable Colours as an Alkaline Property, and on some Bodies possessing it.

changes produced by acids and alkalies on vegetable colours have long been considered as very distinctive and peculiar effects, and even sufficient of themselves to indicate the presence of these bodies. Since the introduction of substances before excluded, as of silica, various oxides, and vegetable substances, into the class of acids, and of oxide of lead, morphia, &c., into the class of alkalies, it becomes more important to substantiate any particular property as peculiar to those classes, or show its fallacy, by pointing out to what substances excluded from them it also belongs.

At present I shall detail the results of a few experiments made on the colouring matter of turmeric and rhubarb, comparing the changes produced on them by alkalies to those occasioned by some other bodies. Formerly, I mentioned the property possessed by muriatic acid gas and strong acids in general, of reddening or browning turmeric paper. I find that in general they have the same effect on rhubarb paper, and a very weak nitric acid gives a brown tint to it, exactly like that of an alkali: strong solution of muriatic acid does not affect it much, but sulphuric acid does.

At pp. 27, 28, &c. I have shown the effect of boracic acid in reddening turmeric paper. Mr. South, I believe, first showed that the sub acetate of lead reddened turmeric, and this has been considered sufficient evidence by many, that the oxide of lead merited, in some degree, the name of an alkali. I find on trial,