84 Mr* Grunt on Indigo. [Feb. 



plates, much broader than the needles, and extremely thin ; 

 twisted sometimes almost into tubes. These appear to the 

 naked eye perfectly opaque. I was not a little pleased, how- 

 ever, with their unexpected appearance, when seen through the 

 microscope. Viewed obliquely, they appear still opaque, and 

 copper coloured like the needles; but when held perpendicularly 

 to the rays of light, they are seen to be transparent, and of a 

 beautiful blue colour, exactly similar to a dilute solution of indigo 

 which has been acted upon by sulphuric acid. Their intensity 

 varies, according to the thickness of the plate, from a blue just 

 distinguishable from while, to one almost black. The bronze 

 colour which these crystals assume when in heaps is obviously 

 a mixture of the copper colour with this blue. 



The vapour of indigo is transparent, and of a must beautiful 

 reddish-violet colour, resembling a good deal the vapour of 

 iodine, but sufficiently distinguished from it by the shade of red. 

 The sublimation takes place at the heat of about 550° Fahren- 

 heit ; for the vapour rises at a heat lower than that of melting 

 lead, and requires more than the melting heat of bismuth. 

 Upon the rough bright surface of the lead, I observed some of 

 the crystals melt while the vapour was rising ; but 1 have in no 

 other circumstances remarked any thing like fusion, till the sub- 

 stance was decomposed. Hence the melting point of indigo, 

 its point of volatilization, and that at which it is decomposed, 

 are remarkably near each other. 



The specific gravity of sublimed indigo is T35. 



These crystals sublime when heated in open vessels, leaving 

 no residue. In close vessels, the vapour is at first reddish- 

 violet, as in the open air ; but as the heat advances, it acquires 

 a tinge of scarlet; and before it is entirely decomposed, becomes 

 deep scarlet, and then orange coloured : a quantity of charcoal 

 is at the same time deposited. 



Action of Oils. — Of the essential oils, oil of turpentine dis- 

 solves at its boiling heat as much indigo as gives it the fine 

 violet colour of the vapour of indigo, but a slight reduction of 

 temperature is sufficient to precipitate the whole of it. The 

 fixed oils and fatty substances, as they may be heated to a higher 

 degree, exert a more powerful action upon indigo. None of 

 them that I have tried act upon it at the heat of boiling water ; 

 but when the heat is increased, they gradually dissolve it, acquir- 

 ing the colour of its vapour much more deep than oil of turpen- 

 tine does. If the solution be cooled at this period, the indigo 

 precipitates blue. As the heat is continued, more of the indigo 

 is dissolved, but tha colour of the solution begins to change; it 

 gradually inclines to crimson, and has then begun to be 

 destroyed. It is now green when cooled. Heated still more, 

 the solution is of a strong crimson ; then becomes orange ; and 

 at last, when entirely decomposed, it is yellow, which colour it 

 retains when cold. 



