Progress of Foreign Science. 407 



neutral in the following way:— Into an acid solution of indigo. 

 he put a certain quantity of wool ; when it had taken up all the 

 colour of which it was capable, he withdrew it, washed it with 

 cold water, to remove all the loosely-adhering indigo ; after 

 which he boiled it in water containing an excessively small 

 quantity of potash ; merely what he supposed necessary to sa- 

 turate the acid which the wool mi^ht still retain. In this 

 manner a neutral solution was obtained, whose proportion of 

 indigo could be ascertained ; for it is known from the very 

 exac°t experiments of M. Welter, that 100 parts of chlorine in 

 weight destroy the colour of 226 parts of indigo. Hence in 

 taking a solution of chlorine, whose proportions are known, we 

 can determine what is the quantity of "the solution of indigo 

 which a known portion of the first decomposes. It was in this 

 way that M. Bussy learned that the solution which served for 

 his trials, contained one thousandth of its weight of indigo. 



To try a charcoal with this solution, he took a certain quan- 

 tity, which he put into a phial, in contact with a known quantity 

 of "the charcoal ; he heated slightly, which hastens somewhat 

 the discoloration, and he added the test-liquor till the charcoal 

 ceased to discolour it. The discoloration also takes place in 

 the cold, but more slowly. 



In his second chapter he inquires what among the difFerent 

 substances, contained in charcoal, are those which act eflfica- 

 ciously in discoloration. He began by trying different species 

 of charcoal obtained by calcination in close vessels, at a heat 

 sufficiently strong to get rid as much as possible of the gaseous 

 bodies which it usually evolves. He thus calcined wood, starch, 

 gelatine, gum, blood, coal. All these species of carbon were 

 more or less hard, friable, brilliant, and did not sensibly dis- 

 colour the test-liquor. He next examined bone black, usually 

 employed for the clarification of sugar in Fiance ; and he ex- 

 amined particularly the charcoal, known under the name of 

 the charcoal of Prussian blue, which is produced by the cal- 

 cination of animal matters along with potash. To procure this 

 last product he took dried ox-blood, which he calcined twice 

 over with its own weight of subcarbonate of potash, at a tem- 

 perature a little under a dull red ; he obtained a spongy mass, 

 which, after being sufficiently washed with boiling water, to 

 separate every thing soluble which it contained, left a charcoal 

 of a dull black hue, excessively light and spongy, whose dis- 

 colouring power, compared to that of bone-black, was as 40 to 

 1. Astonished at so great a difference, he wished to see if the 

 charcoal had not contracted some new combination, or if it 

 owed this exaltation of property to the presence of a foreign 

 body, and particularly to potash. After having washed it as 

 well as possible in boiling water, which removed the ferro-prus- 

 »iale of potash, as well as the sniphuret and carbonate of thii 



