Progress of Foreign Science. 409 



on the contrary, if we conduct the heat properly, and repeat the 

 calcination with the potash a sufficient number of times, the re- 

 siduum will contain but a very minute quantity of iron. 



The cause of this difference is easily perceived. We know 

 that when we treat animal matters, and especially blood, with 

 potash, Ave obtain ferro-prussiate of potash, since on this product 

 is founded,the manufacture of Prussian Blue ; but ferro-prussiate 

 of potash contains hydrogen, carbon, iron, and azote. The three 

 last substances are derived from the elements of the blood, 

 which combine together so at to give birth to the ferro-prussic 

 acid ; consequently the more we shall favour the formation of 

 the ferroprussiate of potash, the more iron we shall carry off 

 from the blood. 



But if we apply all at once a too violent heat, then no more 

 ferro-prussiate of potash is formed ; for this salt is decomposable 

 at a very high temperature, and gives as products of its decom- 

 position a little hydrocyanate of potash, and a solid compound 

 of iron and charcoal. In this case the azote of the blood is dis- 

 sipated, and the iron remains entire, combined with the char- 

 coal. M. Bussy succeeded by several successive calcinations of 

 blood with potash, to deprive it of all the iron which it contained. 

 The greatest difficulty experienced to attain this point depends 

 upon this, that the azote abandons more easily the charcoal 

 than the iron does, and that when we have arrived at the point 

 where the charcoal contains no more azote, potash is then with- 

 out action on the iron. But if at this period we add blood- 

 charcoal, or any animal substance containing much azote, as an 

 empyreumatic animal oil, then we succeed readily in carrying 

 off the residuary portion of iron, and we obtain a charcoal which 

 burns without residuum. Albumen and gelatine, substances 

 which contain much less iron than blood, are very readily 

 brought to the point of containing none of it. For this pur- 

 pose it is sufficient to dry them, to reduce them into powder, 

 and to calcine them with the potash of commerce. We must 

 likewise take care after every calcination to treat the residuum 

 at first with water, then with muriatic acid, which removes every 

 time a small portion of iron, which is probably not combined 

 with charcoal. 



Since in all these successive treatments to which we subject 

 charcoal to deprive it of foreign substances, it loses none of its 

 discolouring power, we must conclude that this property re- 

 sides essentially in charcoal, but that it is developed in virtue 

 of the physical circumstances in which it is placed. This con- 

 clusion which rigorously follows from the experiments just ad- 

 duced, is supported by a multitude of others, of which a few 

 may be detailed; and wlijch were made chiefly with the intention 

 of shewing that the discolouring property of charcoal varied ac- 

 cording as the substances with which we calcine it, act with more 

 or less energy on it, and on the foreign bodies which it contains. 



