198 Dr. Playfair on Transformations 



troducing a slip of indigo-blue calico, the portion in the clear 

 acid will be found to remain unaffected, while that in contact 

 with the insoluble oxide will be bleached in a few seconds. 

 That this decoloration of the indigo is due to the assistant 

 affinity of another body acting in the same direction, i. e. also 

 having a disposition to unite with oxygen, may perhaps best 

 be shown by the following experiment : — Warm nitric acid is 

 diluted to such extent that it just ceases to discharge indigo- 

 blue calico ; it is then divided into two portions, with slips of 

 coloured calico in each, and through one of these binoxide 

 of nitrogen is passed. In the latter the indigo becomes 

 quickly bleached, while it remains unaffected in the former, 

 the action obviously being due to the accessory affinity of the 

 nitric oxide for more oxygen. In the same w'ay indigo-blue 

 is discharged during the decomposition of a nitrate by heat, 

 other kinds of organic matter being oxidized under like cir- 

 cumstances ; in these instances the decomposition of the ni- 

 tric acid is much facilitated, — 1. by the affinity of the base for 

 oxygen ; 2. the affinity of the organic matter for oxygen, 

 which unites with it at the elevated temperature. There are 

 many similar instances of this kind, where the behaviour of 

 NO2 or NO4 as an assistant is too clearly contrasted with the 

 action of other bodies to permit mistake. Thus urine when 

 kept is unfit for the preparation of urea, that substance ha- 

 ving been converted into carbonate of ammonia during the 

 action of the air upon the mucus or colouring matter con- 

 tained in the fluid. Colourless nitric acid unites wdth urea 

 and may be heated with it without decomposition ; but nitric 

 acid containing any of the lower oxides of nitrogen, such as 

 NOg or NO4, immediately decomposes urea into carbonic acid 

 and ammonia*. We cannot conceive that a lower oxide can 

 more readily oxidize urea than a higher oxide, and hence we 

 can only view the NO4 as aiding the urea to oxidize itself, as 

 the mucus does in urine. In the same way, the action of 

 pure nitric acid on colourless uric acid is to form alloxan, if 

 the operation has been conducted so as to prevent the forma- 

 tion of nitrous acid (NO4) during the oxidation. But if NO4 

 has been evolved, or if the colouring matter of the urine be 

 still contained in the uric acid, the products are only carbo- 

 nate and oxalate of ammonia. The colouring matter of the 

 urine and NO4 are thus seen to possess a similar action, 

 which is exactly the same as that of protonitrate of manga- 

 nese on a mixture of starch and nitric acid, no oxalic acid 

 being formed in the presence of this salt, the only product of 



* A solution of urea in nitric acid is immediately decomposed with lively 

 effervescence when a little NOj is passed through it. 



