400 
DR. BENCE JONES ON THE OXIDATION OF 
series of experiments was made to determine what was the smallest amount of nitrate 
of potash that could be detected in the urine when nitre was dissolved in the water 
after it had passed out of the body. 
The indigo test for nitric acid being more delicate than the protosulphate of iron 
test, it was chiefly employed. But far more delicate than either test was a mixture of 
starch with one drop or two of a solution of hydriodate of potash, specific gravity ] 052, 
and very dilute hydrochloric acid, specific gravity 1005. This test Dr. Price suggested 
to me, and the precautions necessary in its use and its advantages he will more fully 
describe elsewhere. Depending on the production of a deep colour, and not on the 
removal of colour, it gave evidence when the indigo test afforded none ; and if the 
quantity of iodide of potassium was very small, and if the hydrochloric acid was very 
dilute, comparative experiments showed that reliance could be placed upon this test. 
In the examination of the urine the method was this. From four to eight ounces 
were taken, and if a larger quantity was passed, it was evaporated to about this quan- 
tity in a water-bath, and then mixed with half an ounce of strong and pure sulphuric 
acid, perfectly free from every trace of nitric acid. Distillation was carried on in a 
retort until at least two thirds of the fluid had passed into the receiver, when the 
distillate was neutralized with pure carbonate of potash and evaporated to a very 
small bulk. From a drop to half the residue was added to the mixture of starch, 
hydriodate of potash and dilute hydrochloric acid. Another portion was placed in a 
bason ; a very small quantity of indigo was added with an excess of sulphuric acid, 
and heat was applied for some minutes. The greenish colour of the indigo with the 
residue of the distillate arose from a small quantity of yellow colouring matter which 
came over in the distillation and grew darker by evaporation. If too much indigo 
was used, slight traces of nitric acid could not be detected. Occasionally the proto- 
sulphate of iron was also used as a test. 
Beginning at first with lOgrs. of nitrate of potash added to 10 ounces of urine, 
it was found at last that as little as 1 gr. of nitrate to 10 ounces of urine could be 
detected by this process when the starch test was used, with the greatest certainty 
and clearness. This quantity could not be detected as surely by the indigo and sul- 
phuric acid test. 
Healthy urine was first examined to see if it contained nitric acid. 
(1.) One hundred and twenty ounces, the produce of five persons, were evaporated 
to a small bulk ; on distillation with sulphuric acid no nitric acid could be found. 
(2.) Thirty ounces, the produce of two other persons, were examined, and no nitric 
acid was found. 
(3.) Twenty ounces of urine, passed by a healthy man, were concentrated and 
distilled, but no nitric acid was found. 
The same person was then made the subject of the following experiments: — 
