236 ESSAY XXII. 



perfectly calcined calx of iron (which I prepared for this 

 purpose from vitriolated iron, by boiling it in nitrous acid, 

 and precipitating with caustic alkali), no part of it is 

 dissolved ; for if the lixivium be afterwards supersaturated 

 with acid, and vitriolated iron added to it, no Prussian blue 

 is obtained. \ The same thing happens if such a solution of 

 perfectly calcined iron be precipitated with lixivium sanguinis, 

 and some acid be afterwards added to it. Hence it likewise 

 appears how much the small quantity of phlogiston which 

 the calx of iron retains in the vitriol contributes to the 

 fixing of the colouring matter. 



SECTION III. 



(a) In order now to learn whither the colouring matter had 

 gone in the experiments (Sec. I. (a), (&), (c)), I poured some 

 lixivium sanguinis into a glass vessel filled with aerial acid ; 

 it was kept well corked during the night, and the next day 

 I fixed to the cork a piece of paper that had been dipped 

 in a solution of vitriolated iron, and then pencilled it over 

 with a couple of drops of a solution of alkali in water. 

 The piece of paper was soon covered with precipitated iron. 

 A couple of hours afterwards I took the paper again out of 

 the vessel, and besmeared it with some muriatic acid, when, 

 to my great surprise, I saw it immediately covered with the 

 most beautiful Prussian blue. (&) The same experiment was 

 repeated with lixivium sanguinis, supersaturated with vitriolic 

 acid. This mixture was put into a glass vessel, and the 

 piece of paper treated as in the last-mentioned experiment, 

 (a). I here likewise observed that the air was filled with the 

 colouring matter ; for the piece of paper became blue on 

 applying muriatic acid to it. (c) Though acids expel this 

 matter from alkali, a considerable quantity of it nevertheless 



