348 On Alkanet as a substitute for 



its inferior hardness, and its infusibility before the blow 

 pipe. It corresponds in other respects with the descrip- 

 tions of that mineral given in the books, and possesses in a 

 hi»;h degree that peculiar tenacity which is so characteris- 

 tic of the different varieties of Jade. 



Art. XVIIl, — Letter from Robert Hare, M D. Profes- 

 sor of Chemistry in the University of Pennsylvania, on 

 Alkanet as a substitute for Litmus or Turmeric, ^c. also 

 on preparincr pure nitrate of silver and on JVitraie of Am- 

 monia for JVitrous Oxide. 



Having infused some Alkanet roots in Alcohol, I was sur- 

 prized at nndidg the infusion blue instead of red. Re- 

 collecting that the Alcohol employed had stood over Pearl- 

 ash, 1 tried some of the roots in pure Alcohol, when a red 

 tincture resulted which was rendered blue by a drop of any 

 alkaline solution. 



In our chemical compilations I never have met with any 

 account of this habitude. No notice is taken of it under 

 the article Alkanet in Ure's or Aikin's Dictionary. On the 

 contrary the broad assertion is constantly repeated, that 

 acids redden vegetable blues, while alkalies make them 

 green. Yet, as litmus is not converted to a green, and al- 

 kanet is made blue by alkalies, it is evident that they not on- 

 ly fail in rendering some blue infusions green, but may ren- 

 der red infusions blue. From the process of manufactur'- 

 ing Litmus we may infer, that its colour is developed by an 

 alkali. 



Alkanet roots may be used in place of Litmus, produ- 

 cing the same phenomena in a reversed order. The Alka- 

 net infusion must be made blue by an alkali and restored by 

 an acid, instead of being as in the case of litmus, reddened 

 by an acid and restored by an alkali. Thus as the one is 

 indirectly a test for alkalies, so is the other for acids. In 

 making the infusion of alkanet blue for this purpose, the 

 smallest quantity of alkali should be used, which will ac- 

 complish the change, as in that case less acid will be requi- 

 site to restore the colour, and thus manifest its presence in 

 any solution to be tested. 



