Dr. Ure on Priissic Acid. 315 



density, necessary for decomposing the above cyanide, will be 

 immediately found; bearing in mind, that 32.5— the prime 

 equivalent of salt, corresponds to 9 of chlorine. 



In Dr. Thomson's System of Chemistry, it is said, that the 

 red precipitate of mercury, is " somewhat soluble in water*." 

 This statement, were it true, would militate against the use of 

 that substance, as a test of prussic acid. But on digesting 

 water with heat, on the mercurial peroxide, and passing a cur- 

 rent of sulphuretted hydrogen through the filtered liquid, I 

 could find no traces of mercury. 



Art. IX. On the Changing of Vegetable Colours as an Alka 

 line Fioperty, and on some Bodies possessing it. By M. 

 Faraday, Chem. Assist., Royal Institution. 



The changes produced by acids and alkalies on vegetable 

 colours, have long been considered as very distinctive and pe- 

 culiar effects, and even sufficient of themselves to indicate the 

 presence of these bodies. Since the introduction into those 

 classes of substances before excluded, as of silica, various 

 oxides, and vegetable substances, into the class of acids ; and 

 of oxide of lead, morphia, Sfc, 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 experiment* 

 made on the colouring matter of turmeric and rhubarb, com- 

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

 casioned by some other bodies. At Vol. V. p. 125, 1 mentioned 

 the property possessed by muriatic acid gas and strong acids 

 jn 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 lint to it, exactly like 

 that of an alkali : strong solution of muriatic acid does not affect 

 it much, but sulphuric acid does. 



* 6ih Edition, Vol. I. page 482. 



