82 Mr. Crum on Indigo. [Feb. 



over a common fire, defending the flask from the contact of the 

 fire." 



To prepare these flowers, Chevreul directs* that eight grains 

 of common indigo in powder, in a covered crucible, should be 

 placed upon burning coals; in which case the colouring matter 

 will crystallize in the middle part of the crucible, from which it 

 is to be detached with a feather when cold. I. have never been 

 able to obtain any quantity of it in this way ; part of it is always 

 discoloured, and the whole of it is condensed upon the ashes, 

 which are in the state of a fine powder, and from which it cannot 

 be detached without contamination. 



I succeeded, however, by another method to procure as much 

 as I needed for my experiments, perfectly pure. I used the 

 covers of two platina crucibles, nearly three inches in diameter, 

 of such a form, that, when placed with their concave sides 

 inwards, they were about three-eighths of an inch distant in the 

 middle. I placed thinly about the centre of the lower one ten 

 grains of precipitated indigo, not in powder, but in small lumps 

 of about a grain in weight ; then, having put on the cover, I 

 applied the flame of a spirit-lamp beneath the indigo. In a 

 short time, this substance partially decomposed, begins to melt, 

 and the purple vapour to be disengaged, which is known by the 

 hissing noise that accompanies it. The heat is continued till 

 this noise nearly ceases, when the lamp is withdrawn, and the 

 apparatus allowed to cool. Then, on removing the cover, the 

 sublimed indigo will be found planted upon its inner surface, 

 with sometimes a few long needles upon the bottom of the appa- 

 ratus, which are easily removed from the button of coaly matter 

 that remains. In this way I have generally obtained 18 to 20 

 per cent, of the indigo employed; a small quantity unavoidably 

 escapes, but I am persuaded that very little more can, by any 

 means of this kind, be obtained. I have made several attempts, 

 by using different forms of apparatus, to prepare this substance 

 on a greater scale ; but have found none that answers so well 

 as that which 1 have described. When the cover is kept cool, 

 as by means of a wet cloth, none of the indigo condenses upon 

 it. The interior of the apparatus being then comparatively cold, 

 the vapour is condensed before it reaches the cover, falling back, 

 or rather crystallizing upon the cinder. The same effect is pro- 

 duced when ashes are formed instead of a fused coal ; and pure 

 indigo when pounded, and common indigo in any shape, always 

 leave a quantity of loose matter. 



Those who cannot readily procure precipitated indigo for the 

 purpose of subliming, may find it convenient to combine with my 

 method, one lately given in the Journal de Pharmacie, by MM. 

 Le Rover and Dumas. It consists in spreading about 30 grains 

 of common indigo in coarse powder upon an open silver capsule, 



* Annalei de C'hiraie, torn. lsvi. Avril, 1808. 



