Mr. De la Rue on Cochineal. 475 



He affirms that he obtained by the action of hydrosulphuric 

 acid on the lead compounds of a great number of organic 

 colouring matters, substances bearing the same relation to 

 the original colouring matters as white indigo does to blue 

 indigo. In order to obtain the colourless modification of the 

 colouring matter of cochineal, he adds what he terms "hydrate 

 of oxide of lead " to an aqueous decoction of cochineal, the 

 fats being previously removed by aether. The colouring mat- 

 ter is entirely removed by the so-called oxide of lead, which 

 is nothing but a basic nitrate of lead, 2(3PbO, NO5) +3HO. 

 The lead compound suspended in water (hot?) was decom- 

 posed by a stream of hydrosulphuric acid ; the nearly colour- 

 less filtrate deposited on cooling needles of a pale yellow 

 colour, which became perfectly white by washing with aether 

 and pressing between bibulous paper ; these crystals, which, 

 according to his statement, are soluble in water and alcohol, 

 but less so in aether, assume in contact with the atmosphere 

 the purple-red of the colouring matter of cochineal. He more- 

 over asserts that his colourless modification gives a white lead 

 salt on adding acetate of lead to its aqueous solution, and that 

 this assumes a purple colour in contact with the air. 



He proposes to give the name carmine, hitherto applied to 

 the red colouring matter, to the white crystals, and to desig- 

 nate the red substance by the name " Carmeine." 



The statements of Preisser, generalizing most beautifully 

 under one head the chemical character of all colouring mat- 

 ters, making indigo as it were the prototype of them all, 

 could but induce other chemists to M^ork out more in detail 

 the relations cursorily pointed out in the memoir of this che- 

 mist. Unfortunately a careful repetition of these experiments 

 has not confirmed the basis on which his theory reposes. 



M. A. E. Arppe repeated Preisser's experiments on the 

 colouring matter of cochineal*. He found that by proceed- 

 ing in the manner described by Preisser that he could only 

 obtain a red solution, which on evaporation was converted 

 into white crystals of oxalic acid by the nitric acid derived 

 from the basic lead salt, 



Arppe now prepared a pure oxide of lead by precipita- 

 ting acetate of lead with potash. He found that this would 

 not take down the colouring matter in the cold, but by boil- 

 ing it is carried down as a blue lake, which he decomposed 

 by hydrosulphuric acid ; the supernatant liquor was nearly 

 colourless, the colouring matter he found adhered with great 

 obstinacy to the sulphuret of lead, from which water, alcohol 

 and ammonia failed to separate it ; but sulphuret of ammo- 

 * Liebig's Annalen, vol. J.v. p. 101. 



