86 Mr. Crwn on Indigo. [Feb. 



The tube, after the operation, had lost in weight 3*17 grains, 

 of which 1 grain being indigo, and 0*11 grain water from the 

 copper, 2*06 grains were oxygen supplied by the peroxide. 

 The gas in the receiver was = 6-46 cubic inches, mean temper- 

 ature and pressure. Of this, potash absorbed 5-82 cubic inches, 

 leaving a residue of 0-64 cubic inch. 



This residue consists of the common air of the apparatus, 

 together with the azote contained in the indigo. By filling the 

 mouth of the tube, as I did, with a round piece of glass, and 

 using a very small conducting tube, scarcely any empty space is 

 left in the apparatus ; but the peroxide of copper, being a very 

 loose substance, and merely shaken down into the tube, without 

 being pressed, encloses a quantity of air, which is very consider- 

 able. This, on the idea that the quantity was trifling, has 

 sometimes been neglected altogether; but more frequently as 

 much of it as possible has been driven out, by heating a part of 

 the tube, previous to the commencement of the experiment, and 

 the remainder neglected. A very simple and obvious method of 

 ascertaining with accuracy its amount, consists in making an 

 analysis of any vegetable substance, as sugar, which contains 

 no azote, in the same apparatus, and with the same quantity of 

 materials. Here nothing passes into the receiver but carbonic 

 acid, and the whole of the common air which the apparatus con- 

 tained. The former, being absorbed by means of potash, leaves 

 a quantity of common air, exactly equal to the common air in the 

 indigo experiment ; of course the difference between that, and 

 the total residue of gas obtained from indigo, is azote. 1 thu3 

 found that the apparatus which 1 employed contained 0*26 cubic 

 inch of common air; which, being deducted from the total 

 residue after absorption by potash 064, leaves 038 cubic 

 inch, or Ol 126 grain of azote, as the quantity of that substance 

 in a grain of indigo. This method, as well as that of estimating 

 the water in peroxide of copper, I communicated to Dr. Ure, 

 when 1 mentioned to him the existence of hydrogen in indigo. 



The 5'82 cubic inches carbonic acid gas + O08 which I 

 found to remain in the apparatus = 5-9 ; and this, allowing for 



aqueous vapour, is 5*9 — jjr = 5762 cubic inches dry car- 

 bonic acid gas, which consist of 0*7322 grain carbon, and 1-952 

 grain of oxygen. But 2-06 grains of oxygen were taken from 

 the peroxide of copper, which is 0-108 grain more than was 

 necessary to burn the carbon. This indicates 0*0135 grain of 

 free hydrogen.* The remainder is oxygen and hydrogen in the 



« The results of two careful experiments made at different times with loaf sugar, 

 exactly in the same manner as with the indigo, perfectly satisfied me as to the accuracy 

 of this part of the experiment. 'J hough the quantities of carbon were not precisely alike 

 in these two experiments, the loss of weight sustained by the peroxide, after deducting 

 the water it contained, was exactly equal to the respective quantities of oxygen in the 

 carbonic acid, calculated in the same manner as in the text. In one case, 1 obtained 



