Progress of Foreign Science. 411 



ash from it, the charcoal which remains has a discolouring 

 power, expressed by 15 ; and this charcoal contains no foreign 

 body, for it burns away without leaving any residuum. Besides, 

 we cannot here suppose an alloy of charcoal with potassium, 

 since these kinds of alloys are decomposed by water ; whilst 

 the discolouring property, continues still in all its intensity, 

 after the action of muriatic acid. We are, however, led to con- 

 clude, that potash may at a high temperature act upon finely 

 divided charcoal, so as to change its physical constitution ; 

 and that this change is the sole source of the increase of its 

 blanching power, since it has contracted no combination with 

 it. M. Bussy attempted to substitute caustic potash for the 

 subcarbonate, to see if the effect would be greater ; but as the 

 caustic potash contains much water, the charcoal decomposed 

 this, and was thus entirely transformed into carburetted hydro- 

 gen gas, and into carbonic acid gas, which combined with the 

 potash. 



We now readily perceive how by calcining a vegetable sub- 

 stance with potash, we come to obtain a discolouring charcoal ; 

 and in fact, by calcining starch with four parts of potash, a 

 charcoal is obtained, whose blanching property was expressed 

 by 10. The charcoal resulting from the calcination of the 

 acetate of potash, possesses a discolouring power of 3. And 

 we see also, that if the discolouring force of these charcoals is 

 not so great as that of the charcoal obtained from the calcina- 

 tion of animal matters, this depends on the circumstance, that 

 in these last, there are two causes, which concur to augment the 

 blanching power; 1. the action of the potash on the charcoal 

 itself ; and 2. the abstraction which it effects of the foreign 

 matters, such as the iron and azote, whilst in the vegetable 

 matters, the first effect is the only one which occurs. 



Animal matters themselves do not all yield a charcoal equally 

 good for discolouring ; but it is so much the better, as the pot- 

 ash has combined with a larger quantity of the principles of 

 the animal substance ; hence, the more prussiate of potash a 

 body affords, when calcined with this alkali, the better will its 

 charcoal discolour. Gelatine and albumen, which yield much 

 less Prussian blue than the blood does, furnish a charcoal, 

 whose discolouring force is from 36 to 40, whilst that of blood 

 amounts even to 50. 



Let us now endeavour to explain the mode of action of bone- 

 charcoal, which is most in use, at the same time that it is one 

 of the most complex in its composition. This charcoal, such 

 as we find it in commerce, varies in its composition ; but in 

 general we find it to contain — 



