PRU 



PRU 



part of sulphate of iron, and two parts of 

 alum, and to this the liquor obtained 

 from the calcined blood and alkali is 

 added, as long 1 as any precipitate is 

 formed. This precipitate is of a green 

 colour, but by washing it with a little 

 dilute muriatic acid, it becomes of a dark 

 rich blue colour. This forms the Prussian 

 blue of commerce. The property of 

 forming the colour depends on a peculiar 

 principle combined with the alkali ; that 

 in the formation of the Prussian blue, this 

 is transferred to the iron, and that it may 

 be again abstracted from it, by boiling 

 the blue in an alkaline solution ; the pro- 

 perties of the alkali are thus changed, 

 and it acquires the power of again form- 

 ing the precipitate of Prussian blue from 

 a solution of sulphate of iron. The reason 

 the precipitate is thrown down green is, 

 that the alkali is not entirely saturated 

 with the colouring principle ; the ex- 

 cess of alkali throws down, therefore, a 

 portion of yellow oxide of iron from the 

 sulphate, Which mingling with the blue 

 precipitate, renders it green, and the 

 muriatic acid gives the deep blue colour, 

 by dissolving, and of course removing 

 this oxide. See Pnussic acid. 



PRUSSIATES, in chemistry, salts 

 formed with the prussic acid. These salts 

 have not been attentively examined, on 

 account of their want of permanency, 

 unless they are united with some metallic 

 oxide ; but the prussiate of potash and 

 iron, which is a triple salt, has been used 

 by chemists as the best combination of 

 prussic acid for detecting the existence 

 of iron. In chemistry and mineralogy this 

 is a very important substance, as it is 

 capable of indicating whether most me- 

 tallic substances be present in any solu- 

 tion whatever, and of pointing out the 

 particular metal, and of ascertaining its 

 quantity. This is done by precipitating 

 the metals from their solution, in conse- 

 quence of the insoluble compound which 

 it forms with them. The colour of the 

 precipitate indicates the particular metal, 

 while its quantity enables us to judge of 

 the proportion of metallic oxide con- 

 tained in any solution. This salt has 

 obtained, at different times, the names of 

 Prussian alkali, phlogisticated alkali, 

 Prussian test, &c. This salt, though of 

 great importance as a test, is of no use 

 whatever, if it be not quite pure. There 

 are two ways in which this test may be 

 rendered impure, besides the introduc- 

 tion of foreign ingredients, which it is 

 needless to mention, because it is ob- 

 vious that it must be guarded against. 



1. There may be a superabundance of 

 alkali present, or, which is the same 

 thing, there may be mixed with the 

 Prussian test a quantity of pure alkali ; 

 or, 2. There may be contained in it a 

 quantity of yellow prussiate of iron, for 

 which prussiate of potash has also a con- 

 siderable affinity. If the Prussian test 

 contain a superabundance of alkali, two 

 inconveniences follow. This superabund- 

 ant quantity will precipitate those earthy 

 salts which are liable to contain an ex- 

 cess of acid, and which are only soluble 

 by that excess. Hence alumina and bary- 

 tes will be precipitated. Another in- 

 convenience arising from the supera- 

 bundance of alkali in the Prussian test 

 is, that it gradually decomposes the blue 

 prussiate which the test contains, and 

 converts it into a yellow prussiate. In 

 what manner it does this will be under- 

 stood, after what has been said, without 

 any explanation. On the other hand, 

 when the Prussian alkali contains a 

 quantity of yellow prussiate of iron, as 

 great inconveniences follow. This yel- 

 low prussiate has an affinity for prussic 

 acid, which, though inferior to that of the 

 potash, is still considerable ; and, on the 

 other hand, the potash has a stronger 

 affinity for every other acid than for the 

 prussic. When, therefore, the test is 

 exposed to the air, the carbonic acid, 

 which the atmosphere always contains, 

 assisted by the affinity between the yel- 

 low prussiate and the prussic acid, 'de- 

 composes the prussiate of potash in the 

 test, and the yellow prussiate is precipi- 

 tated in the form of Prussian blue, and 

 every other acid produces the same ef- 

 fect. A test of this kind would indicate 

 the presence of iron in every mixture 

 which contains an acid (for a precipita- 

 tion of Prussian blue would appear), and 

 could not therefore be employed with 

 any confidence. 



PRUSSIC add, in chemistry and the 

 arts, is one of the most important of the 

 acids. It was discovered by accident, 

 about the beginning of the last century, 

 by Diesbach, a chemist of Berlin. This 

 gentleman, wishing to precipitate a decoc- 

 tion of cochineal with an alkali, got some 

 potash, on which he had distilled several 

 times his animal oil, and as there was some 

 sulphate of iron in the decoction, the liquor 

 instantly exhibited a beautiful blue in the 

 place of a red precipitate. Hence he saw 

 the method of producing the same sub- 

 stance at pleasure, and it soon became an 

 object of commerce, and obtained the 

 name of Prussian blue, from the place 



