1822.] On Alkaline Colour Properties. 29 



power as the boracic acid, of altering the colouring matter of 

 turmeric, for it is not probable there should be an actual sepa- 

 ration 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 con- 

 sequence of these changes. 



On the Changing of Vegetable Colours as an Alkaline Pro- 

 perty, and on some Bodies possessing it*. 



THE 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 sub- 

 stances before excluded, as of silica, various oxides, and vege- 

 table substances, into the class of acids, and of oxide of lead, 

 morphia, &c., into the class of alkalies, it becomes more import- 

 ant 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, com- 

 paring the changes produced on them by alkalies to those occa- 

 sioned by some other bodies. Formerly t 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 subacetate 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, 



* Quarterly Journal of Science, xiii. 315. f Ibid. v. 125. 



